Category: Class 12 Chemistry

NCERT Solutions For Class 12 Chemistry Chapter 14 Biomolecules

Topics and Subtopics in NCERT Solutions for Class 12 Chemistry Chapter 14 Biomolecules:

Section Name	Topic Name
14	Biomolecules
14.1	Carbohydrates
14.2	Proteins
14.3	Enzymes
14.4	Vitamins
14.5	Nucleic Acids
14.6	Hormones

NCERT INTEXT QUESTIONS

14.1. Glucose or sucrose are soluble in water but cyclohexane and benzene (simple six membred ring compounds) are insoluble in water Explain.
Ans: The .solubility of a solute in a given solvent follows the rule ‘ Like dissolves like’.Glucose contains five and sucrose contains eight -OH groups. These -OH groups form H-bonds with water. As a result of this extensive intermoleeular H-bonding, glucose and sucrose are soluble in water.On the other hand, benzene and cyclohexane do not contain -OH bonds and hence do not form H-bonds with water. Moreover, they are non-polar molecules and hence do not dissolve in polar water molecules.

14.2. What are the expected products of hydrolysis of lactose?
Ans: Lactose being a disaccharide gives two molecules of monosaccharides Le. one molecule each of D-(+) – glucose and D-(+)-galactbse.

14.3. How do you explain the absence of aldehyde group in the pentaacetate of D-glucose?
Ans: The cyclic hemiacetal form of glucose contains an -OH group at C-l which gets hydrolysed in aqueous solution to produce open chain aldehydic form which then reacts with NH₂OH -to form corresponding oxime. Thus, glucose contains an aldehydic group. However, when glucose is reacted with acetic anhydride, the -OH group at C-l along with the other -OH groups at C-2, C-3, C-4 and C-6 form a pentaacetate.
Since the penta acetate of1 glucose does not contain a free -OH group at C-l, it cannot get hydrolysed in aqueous solution to produce open chain aldehydic form and hence glucose pentaacetate does not react with NH₂OH to form glucose oxime. The reactions are shown as:

14.4. The melting points and solubility in water of a-amino acids are generally higher than those of corresponding haloacids. Explain.
Ans: a-amino acids as we all know, are dipolar in nature (\(\overset { + }{ N }\)H₃-CHR-COO^– ) and have strong dipolar interactions. As a result, these are high melting solids. These are also involved in intermolecular hydrogen bonding with the  molecules of water and are therefore, water soluble. On the contrary, the haloacids RCH(X)COOH are not dipolar like a-amino acids. Moreover, only the carboxyl group of haloacids are involved in hydrogen bonding with the molecules of water and not the halogen atoms. These have therefore, comparatively less melting points and are also soluble in water to smaller extent.

14.5. Where does the water present in the egg go after boiling the egg?
Ans: When egg is boiled, proteins first undergo denaturation and then coagulation and the water present in the egg gets absorbed in coagulated protein, probably through H- bonding

14.6. Why cannot Vitamin C be stored in our body?
Ans: Vitamin C cannot be stored in the body because it is water soluble and is, therefore, easily excreted in urine.

14.7. Which products would be formed when a nucleotide from DNA containing thymine is hydrolysed?
Ans: Upon hydrolysis, nucleotide from DNA would form 2-deoxyribose and phosphoric acid along-with thymine.

14.8. When RNA is hydrolysed, there is no relationship among the quantities of different bases obtained. What does this fact suggest about the structure of RNA?
Ans: A DNA molecule has two strands in which the four complementary bases pair each other, i.e., cytosine (C) always pair with guanine (G) while thymine (T) always pairs with adenine (A). Thus, when a DNA molecule is hydrolysed, the molar amounts of cytosine is always equal to that of guanine and that of adenine is always equal to thymine.In RNA, there is no relationship between the quantities of four bases (C, G, A and U) obtained, therefore, the base pairing principle, i.e. A pairs with U and C pairs with G is not followed. Therefore, unlike DNA, RNA has a single strand.

NCERT EXERCISES

14.1. What are monosaccharides ?
Ans: Monosaccharides are carbohydrates Which cannot be hydrolysed to smaller molecules.Their general formula is (CH₂O)n Where n=3-7 These are of two types: Those which contain an aldehyde group (-CHO) are called aldoses and those which contain a keto (C=O) group are called ketoses.
They are further classified as trioses , tetroses ,pentoses , hexoses and heptoses according as they contain 3,4,5,6, and 7 carbon atoms respectively.For example.

14.2. What are reducing sugars?
Ans: Reducing sugars are those which can act as reducing agents. They contain in them a reducing group which may be aldehydic (-CHO) or ketonic (>C=0) group. The characteristic reactions of reducing sugars are with Tollen’s reagent and Fehling solution. Non-reducing sugars donot give these reactions. For example, glucose, fructose, lactose etc. are reducing sugars. Sucrose is regarded as a non-reducing sugar because both glucose and fructose are linked through their aldehydic and ketonic groups by glycosidic linkage. Since these groups are not free, sucrose is a non-reducing sugar.

14.3. Write two main functions of carbohydrates in plants.
Ans: Two major functions of carbohydrates in plants are following
(a)Structural material for plant cell walls: The polysaccharide cellulose acts as the chief structural material of the plant cell walls.
(b)Reserve food material: The polysaccharide starch is the major reserve food material in the plants. It is stored in seeds and act as the reserve food material for the tiny plant till it is capable of making its own food by photosynthesis.

14.4. Classify the following into monosaccharides and disaccharides. Ribose, 2-deoxyribose, maltose, galactose, fructose and lactose.
Ans: Monosaccharides: Ribose, 2-deoxyribose, galactose and fructose. Disaccharides: Maltose and lactose.

14.5. What do you understand by the term glycosidic linkage?
Ans: The ethereal or oxide linkage through which two monosaccharide units are joined together by the loss of a water molecule to form a molecule of disaccharide is called the glycosidic linkage. The glycosidic linkage in maltpse molecule is shown below:

14.6. What is glycogen? How is it different from starch?
Ans: The carbohydrates are stored in animal body as glycogen. It is also called animal starch and its structure is similar toamylopectin which means that it is a branched chain polymer of α-D-glucose units in which the chain is formed by C₁ – C₄ glycosidic linkage whereas branching occurs by the formation of C₁– C₆ glycosidic linkage. One main difference between glycogen and amylopectin is the length of the chain. In amylopectin, the chain consists of 20 – 25 α – D – glucose molecules whereas in glycogen, there are 10 -14 molecules of α – D – glucose present. Glycogen is more branched than amylopectin. It is present mainly in liver, muscles and also in brain. Glycogen gets converted into glucose when the body needs it with the help of certain enzymes present in the body. Glycogen has also been found to be present in yeast and fungi.

Starch is a major source of carbohydrates which are very much essential to the human body since they supply energy to the body. It occurs as granules mainly in seeds, fruits, tubers and also in the roots of the plants. The chief commercial sources of starch are wheat, maize, rice, potatoes etc.

14.7. What are the hydrolysis products of (i) sucrose, and (ii) lactose?
Ans: Both sucrose and lactose are disaccharides. Sucrose on hydrolysis gives one molecule each of glucose and fructose but lactose on hydrolysis gives one molecule each of glucose and galactose.

14.8. What is the basic structural difference between starch and cellulose?
Ans: Starch consists of amylose and amylopectin. Amylose is a linear polymer of α-D-glucose while cellulose is a linear polymer of β -D- glucose. In amylose, C -1 of one glucose unit is connected to C – 4 of the other through α-glycosidic linkage. However in cellulose, C – 1 of one glucose unit is connected to C-4 of the other through β – glycosidic linkage. Amylopectin on the other hand has highly branched structure.

14.9. What happens when D-glucose is treated with . the following reagents.
(i) HI
(ii) Bromine water
(iii) HNO₃
Ans:

14.10. Enumerate the reactions of D-glucose which cannot be explained with open chain structure. (C.B.S.E. Delhi 2008, C.B.S.E. Sample Paper 2011)
Ans:
Open chain structure of D-glucose contains a free aldehydic group (- CHO). However, it does not give the following reactions:

• D(+) glucose does not react with 2, 4 D.N.P.
• D(+) glucose does not react with NaHSO₃.
• D(+) glucose does not restore the pink colour to Schiff’s reagent.
• Penia acetyl glucose formed by carrying acetylation with acetic anhydride does not react with hydroxyl amine
  (NH₂OH) which is the characteristic reaction of all aldehydes.
• D( +) glucose is found to exist in two different crystalline forms which are named as α and β. Both these forms have actually been isolated. For example, α form with m.p. 419 K is obtained by the crystallisation of the saturated solution of glucose prepared at 303 K. Similarly, β-form with m.p. 423 K is isolated by carrying out the crystallisation of the saturated solution of glucose prepared at 371 K. Apart from that the a-form has a specific rotation (α) equal to + 112° while the β- form has specific rotation (α) equal to + 19°.

In the light of the limitations stated above, Tollen stated that an open chain structure for D(+) glucose is probably not practicable. He proposed a cyclic structure which is a hemiacetal structure. In this structure, the aldehydic (CHO) group
is involved in the form of a ring with the -OH group attached to C₅ carbon. It is a six membered ring, often called ô-
oxide ring. The ring structure accounts for the two isomeric forms a and shown below.

14.11. What are essential and non-essential amino acids? Give two examples of each type.
Ans: α-Amino acids which are needed for good health and proper growth of human beings but are not synthesized by the human body are called- essential amino acids. For example, valine, leucine, phenylalanine, etc. On the other hand, α-amino acids which are needed for health and growth of human beings and are synthesized by the human body are called non-essential amino acids. For example, glycine, alanine, aspartic acid etc.

14.12. Define the following as related to proteins:
(i) Peptide linkage
(ii) Primary structure
(iii) Denaturation
Ans: (i) Peptide bond: Proteins are condensation polymers of α-amino acids in which the same or different α-amino acids are joined by peptide bonds. Chemically, a peptide bond is an amide linkage formed between – COOH group of one α-amino acid and -NH-, group of the other α-amino acid by lo;ss of a molecule of water. For example,

(ii) Primary structure: Proteins may contain one or more polypeptide chains. Each . polypeptide chain has a large number of α-amino acids which are linked to one another in a specific manner. The specific sequence in which the various amino acids present in a protein linked to one another is called its primary structure. Any change in the sequence of α-amino acids creates a different protein.

(iii) Denaturation: Each protein in the biological system has a unique three-dimensional structure and has specific biologicalactivity. This is called native form of a protein. When a protein in its native form is subjected to a physical change such as change in temperature or a chemical change like change in pH, etc., hydrogen bonds gets broken. As a result, soluble forms of proteins such as globular proteins undergo coagulation or precipitation to give fibrous proteins which are insoluble in water. This coagulation also results in loss of biological activity of the proteins and this loss in biological activity, is called denaturation. During denaturation, 2° and 3° structures of proteins are destroyed but 1° structure remains intact.
The most common example of denaturation of proteins is the coagulation of albumin present in the white of an egg. When the egg is boiled hard, the soluble globular protein present in it is denatured and is converted into insoluble fibrous protein.

14.13. What are the common types of secondary structure of proteins?
Ans:
Secondary structure of protein refers to the shape in which a long polypeptide chain can exist. These are found to exist in two types :

• α-helix structure
• β-pleated sheet structure.

Secondary Structure of Proteins:
The long, flexible peptide chains of proteins are folded into the relatively rigid regular conformations called the
secondary structure. It refers to the conformation which the polvpeptide chains assume as a result of hydrogen bonding
between the > C= O and > N-H groups of different peptide bonds.
The type of secondary structure a protein will acquire, in general depends upon the size of the R-group. If the size of the
R-groups are quite large, the protein will acquire ct-helix structure. If on the other hand, the size of the R-groups are relatively
smaller, the protein will acquire a β – flat sheet structure.

(a) α-Helix structure: If the size of the R-groups are quite large, the hydrogen bonding occurs between > C = O group
of one amino acid unit and the > N-H group of the fourth amino acid unit within the same chain. As such the polypeptide
chain coils up into a spiral structure called right handed ct- helix structure. This type of structure is adopted by most of the
fibrous structural proteins such as those present in wool, hair and muscles. These proteins are elastic i.e., they can be
stretched. During this process, the weak hydrogen bonds causing the a – helix are broken. This tends to increase the length of
the helix like a spring. On releasing the tension, the hydrogen bonds are reformed, giving back the original helical shape.

(b) β—Flat sheet or β—Pleated sheet structure: If R-groups are relatively small, the peptide chains lie side by side in a zig
zag manner with alternate R-groups on the same side situated at fixed distances apart. The two such neighbouring chains are held together by intermolecular hydrogen bonds. A number of such chains can be inter-bonded and this results in the formation of a flat sheet structure These chains may contract or bend a little in order to accommodate moderate sized R-groups. As a result, the sheet bends into parallel folds to form pleated sheet structure known as β – pleated sheet structure. These sheets are then stacked one above the other like the pages of a book to form a three dimensional structure. The protein fibrion in silk fibre has a β – pleated sheet structure. The characteristic mechanical properties of silk can easily be explained on the basis of its β – sheet structure. For example, silk is non-elastic since stretching leads to pulling the peptide covalent bonds. On the other hand, it can be bent easily like a stack of pages because during this process, the sheets slide over each other.

14.14. What types of bonding helps in stabilising the α-helix structure of proteins?
Ans: α-helix structure of proteins is stabilised through hydrogen bonding. (a) α -Helix structure. If the size of the R-groups are quite large, the hydrogen bonding occurs between > C = O group of one amino acid unit and the > N- H group of the fourth amino acid unit within the same chain. As such the polypeptide chain coils up into a spiral structure called right handed a—helix structure. This type of structure is adopted by most of the fibrous structural proteins such as those present in wool, hair and muscles. These proteins are elastic i.e., they can be stretched. During this process, the weak hydrogen bonds causing the α-helix are broken. This tends to increase the length of the helix like a spring. On releasing the tension, the hydrogen bonds are reformed, giving back the original helical shape.

14.15: Differentiate between globular and fibrous proteins.
Ans. (i) Fibrous proteins: These proteins consist of linear thread like molecules which tend to lie side by side (parallel) to form fibres. The polypeptide chains in them are held together usually at many points by hydrogen bonds and some disulphide bonds. As a result,intermolecular forces of attraction are very’ strong and hence fibrous proteins are insoluble in water. Further, these proteins are stable to moderate changes in temperature and pH. Fibrous proteins serve as the chief structural material of animal tissues.For example, keratin in skin, hair, nails and wool, collagen in tendons, fibrosis in silk and myosin in muscles.
(ii) Globular proteins: The polypeptide chain in these proteins is folded around itself in such a way so as to give the entire protein molecule an almost spheroidal shape. The folding takes place in such a manner that hydrophobic (non-polar) parts are pushed inwards and hydrophilic (polar) parts are pushed outwards. As a result, water molecules interact strongly with the polar groups and hence globular protein are water soluble. As compared to fibrous proteins, these are very sensitive to small changes of temperature and pH. This class of proteins include all enzymes, many hormones such as insulin from pancreas, thyroglobulin from thyroid gland, etc.

14.16. How do you explain the amphoteric behaviour of amino acids?
Ans: Amino acids contain an acidic (carboxyl group) and basic (amino group) group in the same molecule. In aqueous solution, they neutralize each other. The carboxyl group loses a proton while the amino group accepts it. As a result, a dipolar or zwitter ion is formed.

In zwitter ionjc form, a-amino acid show amphoteric behaviour as they react with both acids and bases.

14.17. What are enzymes?
Ans: Enzymes are biological catalyst. Each biological reaction requires a different enzyme. Thus, as compared to conventional catalyst enzymes are very specific and efficient in their action. Each type of enzyme has its own specific optimum conditions of concentration, pH and temperature at which it works best.

14.18. What is the effect of denaturation on the structure of proteins?
Ans: Denaturation of proteins is done either by change in temperature (upon heating) or by bringing a change in the pH of the medium. As a result, the hydrogen bonding is disturbed and the proteins lose their biological activity i.e., their nature changes. During the denaturation, both the tertiary and secondary structures of proteins are destroyed while the primary structures remain intact.

14.19. How are vitamins classified? Name the vitamin responsible for the coagulation of blood.
Ans: Vitamins are classified into two groups depending upon their solubility in water or fat: (i) Water soluble vitamins: These include vitamin B-complex (B₁, B₂, B₅, i.e., nicotinic acid,B₆, B₁₂, pantothenic acid, biotin, i.e., vitamin H and folic acid) and vitamin C.
(ii) Fat soluble vitamins: These include vitamins A, D, E and K. They are stored in liver and adipose (fat storing) tissues. Vitamin K is responsible for coagulation of blood.

14.20. Why are vitamin A and vitamin C essential to us? Give their important sources.
Ans: Vitamin A is essential for us because its deficiency causes xerophthalmia (hardening of cornea of eye) and night blindness.
Sources: Fish liver oil, carrots, butter, milk, etc. Vitamin C is essential for us because its deficiency causes scurvy (bleeding of gums) and pyorrhea (loosening and bleeding of teeth). Sources: Citrous fruits, amla, green leafy vegetables etc.

14.21. What are nucleic acids ? Mention their two important functions.
Ans: Nucleic acids are biomolecules which are found in the nuclei of all living cell in form of nucleoproteins or chromosomes (proteins contains nucleic acids as the prosthetic group).

Nucleic acids are of two types: deoxyribonucleic acid (DNA) and ribonucleic acid.(RNA).
The two main functions of nucleic acids are:
(a) DNA is responsible for transmission of hereditary effects from one generation to another. This is due to its unique property of replication, during cell division and two identical DNA strands are transferred to the daughter cells.
(b) DNA and RNA are responsible for synthesis of all proteins needed for the growth and maintenance of our body. Actually the proteins are synthesized by various RNA molecules (r-RNA, m-RNA) and t-RNA) in the cell but the message for the synthesis of a particular protein is coded in DNA.

14.22. What is the difference between a nucleoside and a nucleotide?
Ans: A nucleoside contains only two basic components of nucleic acids i.e., a pentose sugar and a nitrogenous base. It is formed when 1- position of pyrimidine (cytosine, thiamine or uracil) or 9-position of purine (guanine or adenine) base is attached to C -1 of sugar (ribose or deoxyribose) by a β-linkage. Nucleic acids are also called polynucleotides since the repeating structural unit of nucleic acids is a nucleotide.
A nucleotide contains all the three basic . components of nucleic acids, i.e., a phosphoric acid group, a pentose sugar and a nitrogenous base. These are obtained by esterification of C₅, – OH group of the pentose sugar by phosphoric acid.

14.23. The two strands in DNA are not identical but are complementary. Explain.
Ans: The two strands in DNA molecule are held together by hydrogen bonds between purine base of one strand and pyrimidine base of the other and vice versa. Because of different sizes and geometries of the bases, the only possible pairing in DNA are G (guanine) and C (cytosine) through three H-bonds, (i.e.,C = G) and between A (adenine) and T (thiamine) through two H-bonds (i.e., A = T). Due to this base -pairing principle, the sequence of bases in one strand automatically fixes the sequence of bases in the other strand. Thus, the two strands are complimentary and not identical.

14.24. Write the important structural and functional differences between DNA and RNA.
Ans:

14.25. What are the different types of RNA found in the cell?
Ans: There are three types of RNA:
(a) Ribosomal RNA (r RNA)
(b) Messenger RNA (m RNA)
(c) Transfer RNA (t RNA)

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NCERT Solutions For Class 12 Chemistry Chapter 16 Chemistry in Everyday Life

Topics and Subtopics in NCERT Solutions for Class 12 Chemistry Chapter 16 Chemistry in Everyday Life:

Section Name	Topic Name
16	Chemistry in Everyday Life
16.1	Drugs and their Classification
16.2	Drug-Target Interaction
16.3	Therapeutic Action of Different Classes of Drugs
16.4	Chemicals in Food
16.5	Cleansing Agents

NCERT INTEXT QUESTIONS

16.1. Sleeping pills are recommended by doctors to the patients suffering from sleeplessness but it is not advisable to take its doses without consultation with the doctor. Why?
Ans: Most of drugs taken in doses higher than recommended may produce harmful effects and act as poison and cause even death. Therefore, a doctor must always be consulted before taking the drug.

16.2. “Ranitidine is an antacid” With reference to which classification, has this statement been given?
Ans: Ranitidine is labelled as antacid since it is quite effective in neutralising the excess of acidity in the stomach. It is sold in the market under the trade name Zintac.

16.3. Why do we require artificial sweetening agents?
Ans: To reduce calorie intake and to protect teeth from decaying, we need artificial sweeteners.

16.4. Write the chemical equation for preparing sodium soap from glyceryl oleate and glyceryl palmitate. Structures of these compounds are given below:
(i) (C₁₅H₃₁COO)₃C₃H₅-Glyceryl palmitate
(ii) (C₁₇H₃₂COO)₃C₃H₅-Glyceryl oleate
Ans:

16.5. Following type of non-ionic detergents are present in liquid detergents, emulsifying agents and wetting agents. Label the hydrophilic and hydrophobic part in the molecule. Identify the functional group (s) present in the molecule.

Ans:

Functional groups present in the detergent molecule are:
(i)ether
(ii)1°alcoholic group

NCERT EXERCISES

16.1. Why do we need to classify drugs in different ways?
Ans: Drugs are classified in following different ways:
(a) Based on pharmacological effect.
(b) Based on action on a particular biochemical process.
(c) Based on chemical structure.
(d) Based on molecular targets.
Each classification has its own usefulness.
(а) Classification based on pharmacological effect is useful for doctors because it provides them the whole range of drugs available for the treatment of a particular disease.
(b) Classification based on action on a particular biochemical proc*ess is useful for choosing the correct compound for designing the synthesis of a desired drug.
(c) Classification based on chemical structure helps us to design the synthesis of a number of structurally similar compounds having different substituents and then choosing the drug having least toxicity.
(d) Classification on the basis of molecular targets is useful for medical chemists so that they can design a drug which is most effective for a particular receptor site.

16.2. Explain the following as used in medicinal chemistry
(a) Lead compounds
(b) Target molecules or drug targets.
Ans:
(a) Lead compounds are the compounds which are effective in different drugs. They have specific chemical formulas and may be extracted either from natural sources (plants and animals) or may be synthesised in the laboratory.

(b) Target molecules or drug targets. An enzyme (E) functions by combining with the reactant (called substrate) denoted as ‘S’ to form an activated complex known as enzyme-substrate complex (E-S). The complex dissociates to form product and releases the enzyme for carrying out further activity.

16.3. Name the macro molecules that are chosen as drug targets.
Ans: Proteins, carbohydrates, lipids and nucleic acids are chosen as drug targets.

16.4. Why the medicines should not be taken without consulting doctors?
Ans: No doubt medicines are panacea for most of the body ailments. But their wrong choice and overdose can cause havoc and may even prove to be fatal. Therefore, it is of utmost importance that the medicines should not be given without consulting doctors.

16.5. Define the term chemotherapy.
Ans: It is the branch of chemistry that deals with the treatment of diseases by using chemicals as medicines.

16.6. Which forces are involved in holding the drugs to the active site of enzymes?
Ans: The following forces are involved in holding the drugs to the active site of enzymes:
(a) Hydrogen bonding
(b) Ionic bonding
(c) Dipole-dipole interactions
(d) van der Waals interactions

16.7. Antacids and antiallergic drugs interfere with the function of histamines but do not interfere with the function of each other. Explain.
Ans: They donot interfere with the functioning of each other because they work on different receptors in the body.Histamine stimulates the secretion of pepsin and hydrochloric acid in the stomach. The drug cimetidine (antacid) was designed to prevent the interaction of histamine with the receptors present in the stomach wall. This resulted in release of lesser amount of acid. Antacid and antiallergic drugs work on different receptors.

16.8. Low level of noradrenaline is the cause of depression. What type of drugs are needed to cure this problem? Name two drugs.
Ans: In case of low level of neurotransmitter, . noradrenaline, tranquilizer (antidepressant) drugs are required because low levels of noradrenaline leads to depression. These drugs inhibit the enzymes which catalyse the degradation of noradrenaline. If the enzyme is inhibited, noradrenaline is slowly metabolized and can activate its receptor for longer periods of time thereby reducing depression. Two important drugs are iproniazid and phenylzine.

16.9. What is meant by the term broad spectrum antibiotics? Explain.
Ans: Broad spectrum antibiotics are effective against several different types or wide range of harmful bacteria. For example, tetracycline, chloramphenicol and of loxacin. Chloramphenicol can be used in case of typhoid, dysentry, acute fever, urinary infections, meningitis and pneumonia.

16.10. How do antiseptics differ from disinfectants ? Give one example of each.
Ans: Many times, the same substance can act as an antiseptic as well as disinfectant by changing the concentration of the solution used. For example, a 0.2 per cent solution of phenol acts as an antiseptic and its 1 percent solution is a disinfectant. Chlorine is used in India for making water fit for drinking at a concentration of 0.2 to 0.4 ppm (parts per million). Low concentration of sulphur dioxide is used for sterilizing squashes for preservation. A few points of distinction between antiseptics and disinfectants are listed.

Antiseptics	Disinfectants
1. Can kill or prevent the growth of micro-organisms.	1.Can kill micro-organisms.
2. Do not harm the living tissues. Therefore, these can be applied to the skin.	2. Toxic to the living tissues. Therefore, these cannot be applied to the skin.
3. These are used for the dressing of wounds, ulcers and in the treatment of diseased skin.	3. These are used for disinfecting floors, toilets, drains, instruments etc.

16.11. Why are cimetidine and ranitidine better antacids than sodium hydrogencarbonate or magnesium or aluminium hydroxide?
Ans: If excess of NaHCO₃ or Mg(OH)₂ or Al(OH)₃ is used, it makes the stomach alkaline and thus triggers the release of even more HCl which may cause ulcer in the stomach. In contrast, cimetidine and ranitidine prevent the interaction of histamine with the receptor cells in the stomach wall and thus release of HCl will be less as histamine stimulates the secretion of acid.

16.12. Name a substance which can be used as an antiseptic as well as disinfectant.
Ans: 0.2% solution of phenol acts as antiseptic while 1% solution acts as a disinfectant.

16.13. What are the main constituents of dettol?
Ans: Chloroxylenol .and α-terpineol in a suitable solvent.

16.14. What is tincture of iodine? What is its use?
Ans: 2-3% solution of iodine in alcohol and water is called tincture of iodine. It is a powerful antiseptic. It is applied on wounds.

16.15. What are food preservatives?
Ans: Preservation has a major role in the food products. Chemically preserved squashes and crushes can be kept for a fairly long time even after opening the seal of bottle.
A preservative may be defined as the substance which is capable of inhibiting or arresting the process of fermentation, acidification or any other decomposition of food. Salting i.e. addition of table salt is a well known method for food preservation and was applied in ancient times for preserving raw mangoes, tamarind, meat, fish etc. Sugar syrup can also act as a preservative. Vinegar is a useful preservative for pickles. Apart from these, sulphur dioxide and benzoic acid can be employed for the preservation of food. The major source of sulphur dioxide is potassium metabisulphite (K₂S₂O₅). It is fairly stable in neutral and alkaline medium but gets decomposed by weak acids such as carbonic, citric, tartane and malic acids. Benzoic acid is used either as such or in the form of sodium benzoate. However, sulphur dioxide has a better preservative action than sodium benzoate against bacteria and moulds. It also retards the development of yeast in juice but fails to arrest their multiplication once the number has reached a high value. Sorne salts of sorbic acid and propionic acid are also being used these days for the preservation of the food.
The use of preservatives must be properly controlled as their indiscriminate use is likely to be harmful. The preservative should not be injurious to health and should be also non-irritant.

16.16. Why is the use of aspartame limited to cold foods and drinks?
Ans: This is because it decomposes at baking or cooking temperatures and hence can be used only in cold foods and drinks as an artificial sweetener

16.17. What are artificial sweetening agents? Give two examples.
Ans: Artificial sweeteners are chemical substances which are sweet in taste but do not add any calories to our body. They are excreted as such through urine. For example, saccharin, aspartame, alitame etc.

16.18. Name the sweetening agent used in the preparation of sweets for a diabetic patient.
Ans: Saccharine, aspartame or alitame may be used in the preparation of sweets for a diabetic patient.

16.19. What problem arises in using alitame as artificial sweetener?
Ans: Alitame is a high potency artificial sweetener.Therefore, it is difficult to control the sweetness of the food to which it is added.

16.20. How are synthetic detergents better than soaps?
Ans: They can be used in hard water as well as in acidic solution. The reason being that sulphonic acids and their calcium and magnesium salts are soluble in water thus they do not form curdy white precipitate with hard water but the fatty acids and their calcium and magnesium salts of soaps are insoluble. Detergents also works in slightly acidic solution due to formation of soluble alkyl hydrogen sulphates. Soaps react with acidic solution to form insoluble fatty acids.

16.21. Explain the following terms with suitable examples:
(i) cationic detergents (ii) anionic detergents and (iii) non-ionic detergents
Ans: (i) Cationic detergents: These are quaternary ammonium salts, chlorides, acetates, bromides etc containing one or more long chain alkyl groups. For example, cetyltrimethyl ammonium chloride.
(ii) Anionic detergents are called so because a large part of their molecules are anions. ‘These are of two types:
(a) Sodium alkyl sulphates: For example, sodium lauryl sulphate, C₁₁H₂₃CH₂OSO₃ Na⁺.
(b) Sodium alkylbenzenesulphonates.Vor example, sodium 4-(l-dodecyl) benzenesu Iphphonate (SDS).

(iii) Neutral or non-ionic detergents: These are esters of high molecular mass alcohols with fatty acids. These can also be obtained by treatment of long chain alcohols by with excess of ethylene oxide in presence of a base. For example, polyethylene glycol stearate,CH₃(CH₂)₁₆COO (CH₂CH₂O)₁₁ CH₂CH₂OH Polyethylene glycol stearate.

16.22. What are biodegradable and non-biodegradable detergents? Give one example of each.
Ans: Detergents having straight chain hydrocarbons are easily degraded (or decomposed) by microorganisms and hence are called biodegradable detergents while detergents containing branched hydrocarbon chains are not easily degraded by the microorganisms find hence are called non-biodegradable detergents. Consequently, non-biodegradable detergents accumulate in rivers and water ways thereby causing severe water pollution. Examples of biodegradable detergents are sodium lauryl sulphate, sodium 4-(-l-dodecyl) benzenesulphonate and sodium 4-(2-dodecyl) benzenesulphonate.
Examples of non-biodegradable detergents is sodium 4-(1, 3,5,7 – tetramethyloctyl) benzene sulphonate.

16.23. Why do soaps not work in hard water? (C.B.S.E. Outside Delhi 2009, 2011)
Ans: Soaps are water soluble sodium or potassium salts of higher fatty acids like palmitic acid (C₁₅H₃₁COOH), oleic acid (C₁₇H₃₃COOH) and stearic acid (C₁₇H₃₅COOH). Hard water contains certain calcium and magnesium salts which combine with soaps to form corresponding magnesium compounds. These being insoluble, get separated as curdy white precipitates resulting in wastage of soap.

16.24. Can you use soaps and synthetic detergents to check the hardness of water?
Ans: Soaps get precipitated as insoluble calcium and magnesium soaps in hard water but detergents do not. Therefore, soaps but not synthetic detergents can be used to check the hardness of water.

16.25. Explain the cleansing action of soaps.
Ans: Cleansing action of soaps : Soaps contain two parts, a large hydrocarbon which is a hydrophobic (water repelling) and a negative charged head, which is hydrophillic (water attracting). In solution water molecules being polar in nature, surround the ions & not the organic part of the molecule. When a soap is dissolved in water the molecules gather together as clusters, called micelles. The tails stick inwards & the head outwards. The hydrocarbon tail attaches itself to oily dirt. When water is agitated, the oily dirt tends to lift off from the dirty surface & dissociates into fragments. The solution now contains small globules of oil surrounded detergent molecules. The negatively charged heads present in water prevent the small globules from coming together and form aggregates. Thus the oily dirt is removed from the object.

16.26. If water contains dissolved calcium hydrogencarbonate, out of soaps and synthetic detergents, which one will you use for cleaning clothes?
Ans: Calcium hydrogencarbonate makes water hard. Therefore, soap cannot be used because it gets precipitated in hard water. On the other hand, a synthetic detergent does not precipitate in hard water because its calcium salt is also soluble in water. Therefore, synthetic detergents can be used for cleaning clothes in hard water.

16.27. Label the hydrophilic and hydrophobic parts in the following compounds.
(i)cCH₃(CH₂)₁₀CH₂OSO₃ ^–Na⁺
(ii) CH₃(CH₂)₁₅ -N+(CH₃)₃Br^–
(iii) CH₃(CH₂)₁₆C00(CH₂CH₂O)₁₁CH₂CH₂OH
Ans:

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NCERT Solutions For Class 12 Chemistry Chapter 8 The d and f Block Elements

Topics and Subtopics in NCERT Solutions for Class 12 Chemistry Chapter 8 The d and f Block Elements:

Section Name	Topic Name
8	The d – and f – Block Elements
8.1	Position in the Periodic Table
8.2	Electronic Configurations of the d-Block Elements
8.3	General Properties of the Transition Elements (d-Block)
8.4	Some Important Compounds of Transition Elements
8.5	The Lanthanoids
8.6	The Actinoids
8.7	Some Applications of d – and f -Block Elements

NCERT Solutions CBSE Sample Papers ChemistryClass 12 Chemistry

NCERT IN TEXT QUESTIONS 

8.1. Silver atom has completely filled d orbitals (4d¹⁰) in its ground state. How can you say that it is a transition element?
Ans: The outer electronic configuration of Ag (Z=47) is 4d¹⁰5s¹. It shows+1 and + 2 O.S. (in AgO and AgF₂). And in + 2 O.S., the electronic configuration is d⁹ i.e, d-subshell is incompletely filled. Hence, it is a transition element.

8.2. In the series Sc(Z = 21) to (Z = 30), the enthalpy of atomisation of zinc is the lowest i.e., 126 kJ mol^-1. Why?
Ans: The enthalpy of atomisation is directly linked with the stability of the crystal lattice and also the strength of the metallic bond. In case of zinc (3d¹⁰4s² configuration), no electrons from the 3d-orbitals are involved in the formation of metallic bonds since all the orbitals are filled. However, in all other elements belonging to 3d series one or more d-electrons are involved in the metallic bonds. This means that the metallic bonds are quite weak in zinc and it has therefore, lowest enthalpy of atomisation in the 3d series.

8.3. Which of the 3d series of the transition metals exhibits the largest number of oxidation states and why?
Ans: Manganese (Z = 25) shows maximum number of O.S. This is because its outer EC is 3d⁵4s². As 3d and 4s are close in energy, it has maximum number of e^-1 s to loose or share. Hence, it shows O.S. from +2 to +7 which is the maximum number.

8.4.

Ans.

8.5. How would you account for the irregular variation of ionisation enthalpies (first and second) in the first series of the transition elements?
Ans:  There is a irregularity in the IE’s of 3d-series due to alternation of energies of 4s and 3d orbitals when an e^-1 is removed. Thus, there is a reorganisation energy accompanying ionization. This results into release of exchange energy which increases as the number of e^-1 s increases in the dn configuration. Cr has low 1st IE because loss of 1 e- gives stable EC (3d⁶). Zn has very high IE because e~ has to be removed from 4s orbital of the stable configuration (3d¹⁰ 4s²) After the loss of one e^–, removal of 2nd e^–, becomes difficult. Hence, 2nd IE’s are higher and in general, increase from left to right. However, Cr and Cu show much higher values because 2nd e^– has to be removed from stable configuration of Cr⁺ (3d⁵) and Cu⁺ (3d¹⁰)

8.6. Why is the highest oxidation state of a metal exhibited by its fluoride and oxide only? (C.B.S.E. Delhi 2010)
Ans: Both fluorine and oxygen have very high electronegativity values. They can oxidise the metals to the highest oxidation state. As a result, the highest oxidation states are shown by the fluorides and oxides of the metals; transition metals in particular.

8.7.Which is a stronger reducing agent Cr²⁺ or Fe²⁺ and why?
Ans: Cr²⁺ is a stronger reducing agent than Fe²⁺. This is because E°(Cr³⁺/Cr²⁺) is negative (- 0.41V) whereas E°(Fe³⁺/Fe²⁺) is positive (+ 0.77 V). Thus, Cr²⁺ is easily oxidised to Fe³⁺ but Fe²⁺ cannot be easily oxidised to Fe³⁺.

8.8.Calculate the ‘spin only’ magnetic moment of M²⁺(aq) ion (Z = 27).
Ans:

8.9.Explain why Cu⁺ ion is not stable in aqueous solutions?
Ans: Cu⁺ (aq) is not stable, while Cu²⁺ (aq) is stable. This is becuase Δ_hydH of Cu²⁺(aq) is much higher than that of Cu+(aq) and hence it compensates for the 2nd IE of Cu. Thus, many Cu(I) compounds are unstable in aqueous solution and undergo disproportionation as follows :
2 Cu⁺ —–> Cu²⁺ + Cu

8.10. Actinoid contraction is greater from element to element than lanthanoid contraction. Why? (C.B.S.E. Sample Paper 2011, Jharkhand Board 2010)
Ans: The decrease or contraction in atomic radii, as well as ionic radii in actinoid elements (actinoid contraction), is more as compared to lanthanoid contraction because 5/ electrons have more poor shielding effect as compared to 4f electrons. Therefore, the effect of increased nuclear charge leading to contraction in size is more in case of actinoid elements.

NCERT EXERCISES

8.1. Write down the electronic configuration of (i) Cr³⁺ (ii) Pm³⁺ (iii) Cu⁺ (iv) Ce⁴⁺(v) Co²⁺ (vi) Lu²⁺(vii) Mn²⁺ (viii) Th4+.
Sol: (i) Cr³⁺ = [Ar]¹⁸3d³
(ii)Pm³⁺ = [Xe]⁵⁴ 4f⁴
(iii)Cu⁺ = [Ar]¹⁸ 3d¹⁰
(iv)Ce⁴⁺ = [Xe]⁵⁴
(v)Co²⁺ = [Ar]¹⁸ 3d⁷
(vi)Lu²⁺ = [Xe]⁵⁴ 4f¹⁴ 5d¹
(vii) Mn²⁺ = [Ar]¹⁸ 3d⁵ (viii)Th⁴⁺= [Rn]⁸⁶

8.2. Why are Mn²⁺ compounds more stable than Fe²⁺ towards oxidation to their+3 state?
Sol: Electronic configuration of Mn²⁺ is 3d⁵. This is a half-filled configuration and hence stable. Therefore, third ionization enthalpy is’very high, i. e., third electron cannot be lost easily. Electronic configuration of Fe²⁺ is 3d⁶. It can lose one electron easily to achieve a stable configuration 3d⁵.

8.3. Explain briefly how+2 state becomes more and more stable in the first half of the first row transition elements with increasing atomic number?
Sol: Here after losing 2 electrons from j-orbitals, the 3d-orbital gets gradually occupied with increase in atomic number. Since the number of unpaired electrons in 3d orbital increases, the stability of the cations (M²⁺) increases from Sc²⁺ to Mn²⁺.

8.4. To what extent do the electronic configurations decide the stability of oxidation states in the first series of the transition elements? Illustrate your answer with examples.
Sol: In the first series of transition elements, the oxidation states which lead to exactly half-filled or completely filled d-orbitals are more stable. For example, Mn (Z = 25) has electronic configuration [Ar] 3d⁵ 4 s². It shows oxidation states + 2 to + 7 but Mn (II) is most stable because of half-filled configuration [Ar] 3d⁵. Similarly Sc³⁺ is more stable then Sc+ and Fe³⁺ is more stable than Fe²⁺ due to half filled it f-orbitals.

8.5. What may be the stable oxidation state of the transition element with the following delectron configurations in the ground state of their atoms: 3d³,3d⁵, 3d⁸ and 3d⁴?
Sol: (a) 3d³ 4s¹ = + 5.
(b) 3d⁵ 4s² = + 2, + 7,3d⁵ 4s¹ =+6.
(c)3d⁸4s² = + 2.
(d)3d⁴4s² = 3d⁵ 4s¹ = + 6(and + 3).

8.6. Name the oxometal anions of the first series of the transition metals in which the metal exhibits the oxidation state equal to its group number.
Sol: Cr₂0₇^2- and Cr0₄^2- (Group number = Oxidation state of Cr = 6).
Mn0₄^–  (Group number = Oxidation state of Mn = 7).

8.7. What is lanthanoid contraction? What are the consequences of lanthanoid contraction?
Sol: Lanthanoid Contraction : In the lanthanoids , the electrons are getting filled in the 4f-subshell. On moving from left to right, the nuclear charge increases and this increase is expected to be compensated by the increase in the magnitude of shielding effect by the 4 f- electrons However,
the f-electrons have very poor shielding effect. Consequently, the atomic and ionic radii decrease from left to right and this is knwon as lanthanoid contraction.
Consequences of lanthanoid Contraction
(a)Separation Lanthanoids: All the lanthanoids have quite similar properties and due to this reason they are difficult to separate.
(b)Variation in basic strength of hydroxides: Due to lanthanoid contraction, size of M³⁺ ions decreases and thus there is a corresponding increase in the covalent character in M—OH bond. Thus basic character of oxides and hydroxides decreases from La(OH)₃ to Lu(OH)₃.
(c)Similarity in the atomic sizes of the elements of second and third transition series present in the same group. The difference in the value of atomic radii of Y and La is quite, large as compared to the difference in the value of Zr and Hf. This is because of the lanthanoid contraction.
(d)Variation in standard reduciton potential: Due to lanthanoid contraction there is a small but steady increase in the standard reduction potential (E°) for the reduction process.
M³⁺ (aq) + 3e^– —–> 4 M(aq)
(e)Variation in physical properties like melting point, boiling point, hardness etc.

8.8. What are the characteristics of the transition . elements and why are they called transition elements? Which of the d-block elements may not be regarded as the transition elements?
Sol: General characteristics of transition elements.
(i)Electronic configuration – (n -1) d^1-10 ns^1-2
(ii)Metallic character – With the exceptions of Zn, Cd and Hg, they have typical metallic structures.
(iii)Atomic and ionic size-ions of same charge in a given series show progressive decrease in radius with increasing atomic number.
(iv)Oxidation state-Variable; ranging from+2 to +7.
(v)Paramagnetism – The ions with unpaired electrons are paramagnetic.
(vi)Ionisation enthalpy – Increases with increase in charge.
Formation of coloured ions – Due to presence of unpaired electrons.
(viii) Formation of complex compounds – Due to small size and high charge density of metal ions.
(ix)They possess catalj^c properties – Due to
their ability to adopt multiple oxidation states. .
(x)Formation of interstitial compounds.
(xi)Alloy formation.
They are called transition elements due to their incompletely filled d-orbitals in ground state or in any stable oxidation state and they are placed between s and p- block elements. Zn, Cd and Hg have fully filled d- orbitals in their ground state hence may not be regarded as the transition elements.

8.9. In what way are the electronic configuration of the transition elements different from non-transition elements?
Sol: Electronic configuration of transition elements : (n – 1)d^1-10 ns^1-2. Electronic configuration of non-transition elements : ns^1-2 or ns²np^1-6. From comparison, it is quite evident that the transition elements have incomplete d-orbitals (s- orbitals in some cases) while the non-transition elements have no d-orbitals present in the valence shells of their atoms. This is responsible for the difference in the characteristics of the elements belonging to these classess of elements.

8.10. What are the different oxidation states exhibited by the lanthanoids?
Sol: Lanthanides exhibits + 2, + 3 and + 4 oxidation states. The most common oxidation state of lanthanoids is +3.

8.11. Explain giving reasons:
(i)Transition metals and many of their compounds show paramagnetic behaviour.
(ii)The enthalpies of atomisation of the transition metals are high.
(iii)The transition metals generally form coloured compounds.
(iv)Transition metals and their many compounds act as good catalyst
Sol: (i) Magnetic properties: Transition elements and many of their compounds are paramagnetic, i.e., they are weakly attracted by a magnetic field. This is due to the presence of unpaired electrons in atoms, ions or molecules. The paramagnetic character increases as the number of . unpaired electrons increases. The paramagnetic character is measured in terms of magnetic moment and is given by
\(\mu =\sqrt { n(n+2) }\) where n – number of unpaired electrons.
(ii) Because of large number of unpaired electrons in d-orbitals of their atoms they have stronger interatomic intefactions and hence stronger metallic bonding between atoms resulting in higher enthalpies of atomisation.
(iii) Formation of coloured compounds (both in solid state as well as in aqueous solution) is another very common characteristics of transition metals. This is due to absorption of some radiation from visible light to cause d-d transition of electrons in transition metal atom. The d-orbitals do not have same energy and under the influence of ligands, the d-orbitals split into two sets of orbitals having different energies; transition of electrons can take place from one set of d-orbitals to another set within the same sub-shell. Such transitions are called d-d transitions. The energy difference for these d-d transitions fall in the visible region. When white light is incident on compounds of transition metals, they absorb a particular frequency and remaining colours are emitted imparting a characteristic colour to the complex. Zn²⁺ and Ti⁴⁺ salts are white because they do not absorb any radiation in visible region.
(iv)Catalytic properties: Many of transition metals and their compounds act as catalyst in variety of reactions, e.g., finely divided iron in manufacture of NH₃ by Haber’s process, V₂O₅ or Pt in manufacture of H₂S0₄ by Contact process, etc.). The catalytic activity is due to following two reasons.
(a)The ability of transition metal ion to pass ” easily from one oxidation state to another
and thus providing a new path to reaction with lower activation energy.
(b)The surface of transition metal acts as very good adsorbent and thus provides increased concentration of reactants on their surface causing the reaction to occur.

8.12. What are interstitial compounds? Why are such compounds well known for transition metals?
Sol: Transition metals form large number of interstitial compounds. They are able to entrap small atoms of elements like H, G, N, B, etc., in their crystal lattice and even can make weak bonds with them.
Due to formation of interstitial compounds, their malleability and ductility decreases and tensile . strength increases. Steel and cast iron are hard in comparison to wrought iron due to the presence of trapped carbon atoms in interstitial spaces.

8.13. How is the variability in oxidation states of transition metals different from that of the non-transition metals? Illustrate with examples.
Sol: The transition metals show a number of variable oxidation states due to the participation of (n – 1) d electrons in addition to ns electrons in the bond formation. They therefore, exhibit a large number of variable oxidation states. On the other hand, the non-transition metals generally belonging to s-block do not show variable oxidation states because by the loss of valence s-electrons, they acquire the configuration of the nearest noble gas elements.

In the p-block the lower oxidation states are favoured by the heavier members (due to inert pair effect), the opposite is true in the groups of d-block. For example, in group 6, Mo(VI) and W(VI) are found to be more stable than Cr(VI). Thus Cr(VI) in the form of dichromate in acidic medium is a strong oxidising agent, whereas MoO₃ and WO₃ are not.

8.14. Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH on a solution of potassium dichromate?
Sol: Potassium dichromate is prepared from chromate, which in turn is obtained by the fusion of chromite ore (FeCr₂O₃) with sodium or potassium carbonate in free excess of air. The reaction with sodium carbonate occurs as follows:

The yellow solution of sodium chromate is filtered and acidified with sulphuric acid to give a solution from which orange sodium dichromate, Na₂Cr,0₇.2H₂0 can be crystallised.

Sodium dichromate is more soluble than potassium dichromate. The latter is therefore, prepared by treating the solution of sodium dichromate with potassium chloride.

Orange crystals of potassium dichromate crystallise out. The chromates and dichromates depending upon p^H of the solution. If p^H of potassium dichromate is increased it is converted to yellow potassium chromate.

8.15. Describe the oxidising action of potassium dichromate and write the ionic equations for its reaction with:
(i)iodide
(ii)iron (II) solution and
(iii)H₂S
Sol: K₂Gr₂0₇is a powerful oxidising agent. In dilute sulphuric acid medium the oxidation state of Cr changes from+6 to + 3. The oxidising action can be represented as follows:

8.16. Describe the preparation of potassium permanganate. How does the acidified permanganate solution react with (i) iron (II) ions (ii) S0₂ and (iii) oxalic acid? Write the ionic, equations for the reactions.
Sol: Potassium permanganate (KMn04) is prepared by the fusion of a mixture of pyrolusite (Mn0₂),potassiufn hydroxide and oxygen, first green coloured potassium manganate is formed. 2MnO₂ + 4KOH + 0₂ —> 2K₂Mn0₄+2H₂0 The potassium manganate is extracted by water, which then undergoes disproportionation in neutral or acidic solution to give potassium permanganate.

8.17. For M²⁺/M and M³⁺/M²⁺ systems the E° values
for some metals are as follows:
Cr²⁺/Cr   –> -0.9 V
Mn²⁺/Mn  –> -1.2V
Fe²⁺/Fe     –> -0.4 V
Cr³⁺/Cr²⁺  –> -0.4 V
Mn³⁺/Mn²⁺   –>+ 1.5V
Fe³⁺/Fe²⁺   –>+ 0.8V
(ii)the ease with which iron can be oxidised as compared to a similar process for either chromium or manganese metal.
Sol: (i) Cr³⁺/Cr²⁺ has negative reduction potential. Hence, Cr³⁺ cannot be reduced to Cr²⁺. Mn³⁺/Mn²⁺ has a large positive reduction potential. Hence, Mn³⁺ can be easily reduced to Mn²⁺. Fe³⁺/Fe²⁺ has small positive reduction potential. Hence, Fe³⁺ is more stable than Mn³⁺ but less stable than Cr³⁺.
(ii)From the E° values, the order of oxidation of the metal to the divalent cation is : Mn > Cr > Fe.

8.18. Predict which of the following will be coloured in aqueous solution?
Ti³⁺, V³⁺, Cu⁺, Sc³⁺, Mn²⁺, Fe³⁺, Co²⁺.
Sol: Only those ions will be coloured which have incomplete d-orbitals. The ions with either empty or filled d-orbitals are colourless. Keeping this in view, the coloured ions among the given list are :
Ti³⁺(3d¹), V³⁺(3d²), Mn²⁺(3d⁵), Fe³⁺(3d⁵), Co²⁺ (3d⁷)
Sc³⁺ (3d°) and Cu⁺ (3d¹⁰) ions are colourless.

8.19. Compare the stability of +2 oxidation state for the elements of the first transition series.
Sol: In general, the stability of +2 oxidation state in first transition series decreases from left to right due to increase in the sum of first and second ionisation energies. However Mn²⁺ is more stable due to half filled d-orbitals (3d⁵) and Zn²⁺ is more stable due to completely filled d-orbitals (3d¹⁰).

8.20. Compare the chemistry of actinoids with that of the lanthanoids with special reference to
(i)electronic configuration,
(ii)atomic and ionic sizes and
(iii)oxidation state
(iv)chemical reactivity.
Sol: (i) Electronic configuration: The general electronic configuration of lanthanoids is [Xe]⁵⁴ 4f^1-14 5d^0-1  6s² and that of actinoids is [Rn]⁸⁶ 5f^0-14 6d^0-1  7s², lanthanoids . belong to 4 f series whereas actinoids belong to 5f-series.
(ii) Atomic and ionic sizes: Both lanthanoids and actinoids show decrease in size of their atoms or ions in + 3 oxidation state as we go from left to right. In lanthanoids, the decrease is called lanthanoid contraction whereas in actinoids, it is called actinoid contraction. The contratibn is greater from element to element in actinodes due to poorer shielding by 5f electrons.
(iii)Oxidation state: Lanthanoids show limited oxidation states (+ 2, + 3, + 4) out of which + 3 is most common whereas actinoids show +3, +4, +5, +6, +7 oxidation states.This is because of large energy gap between 4f 5d and 6s orbitals. However, actinoids show a large number of oxidation states because of small energy ap- between 5f 6d and Is orbitals.
(iv) Chemical reactivity: The earlier members
of the lanthanoids series are quite reactive similar to calcium but, with increase in atomic number, they behave more like aluminium. The metals combine with hydrogen when . gently heated in the gas. Carbides, Ln₃C, Ln₂C₃ and LnC₂ are formed when the metals are heated with carbon. They liberate hydrogen from dilute acid and burn in halogens to form halides. They form oxides M₂0₃ and hydroxides M(OH)₃.
Actinoids are highly reactive metals, especially when finely divided. The action of boiling water on them gives a mixture of oxide and hydride and combination with most non-metals take place at moderate temperatures. HCl attacks all metals but most are slightly affected by nitric acid owing to the formation of protective oxide layers, alkalis have no action. Actinoids are more reactive than lanthanoids due to bigger atomic size and lower ionisation energy.

8.21. How would you account for the following:
(i) Of the d⁴ species, Cr²⁺ is strongly reducing while manganese (III) is strongly oxidizing.
(ii) Cobalt (II) is stable in aqueous solution but in the presence of complexing reagents it is easily oxidised.
(iii) The d¹ configuration is very unstable in ions.
Sol: (i) E° value for Cr³⁺/Cr²⁺ is negative (-0-41 V) whereas E° values for Mn³⁺/Mn²⁺is positive (+1.57 V). Hence, Cr²⁺ ion can easily undergo oxidation to give Cr³⁺ ion and, therefore, act as strong reducing agent whereas Mn³⁺ can easily undergo’ reduction to give Mn²⁺ and hence act as an oxidizing agent.
(ii) Co (III) has .greater tendency to form coordination complexes than Co (II). Hence, in the presence of ligands, Co (II) changes to Co (III), i.e., is easily oxidized.
(iii) The ions with dx configuration have the tendency to lose the only electron present in d-subshell to acquire stable d° configuration. Hence, they are unstable and undergo oxidation or disproportionation.

8.22. What is meant by disproportionation? Give two examples of disproportionation reaction in aqueous solution
Sol: Disproportionation reactions are those in which the same substance undergoes oxidation as well as reduction, i.e., oxidation number of an element increases as well as decreases to form two different products.

8.23. Which metal in the first transition metal series exhibits +1 oxidation state most frequently and why?
Sol: Cu with configuration [Ar] 4s¹3d¹⁰ exhibits +1 oxidation state and forms Cu⁺ ion because by losing one electron, the cation or positive ion acquires a stable configuration of d-orbitals (3d¹⁰).

8.24. Calculate the number of unpaired electrons in the following gaseous ions : Mn³⁺, Cr³⁺, V³⁺ and Ti³⁺. Which one of these is the most stable in aqueous solution.
Sol: Mn³⁺ = 3d¹ = 4 unpaired electrons, Cr³⁺ = 3d³ = 3 electrons,V³⁺ = 3d² = 2 electrons, Ti³⁺=3d¹ = l electron.Out of these, Cr³⁺ is most stable in aqueous solution because of half-filled t_2g level.

8.25. Give examples and suggest reasons for the following features of the transition metal chemistry:
(i) The lowest oxide of transition metal is basic the highest is amphoteric/ acidic.
(ii) A transition metal exhibits highest oxidation state ih oxides and fluorides.
(iii) The highest oxidation state is exhibited in oxoanions of a metal.
Sol: (i) The lower oxide of transition metal is basic because the metal atom has low oxidation state whereas higher once are acidic due to high oxidation state. For example, MnO is basic whereas Mn₂O₇is acidic. Oxides in lower oxidation state are ionic hence basic. Oxides in higher oxidation state are covalent hence acidic
(ii) A transition metal exhibits higher oxidation states in oxides and fluorides because oxygen and fluorine are highly electronegative elements, small in size and strongest oxidising agents. For example, osmium shows an oxidation states of + 6 in O₅F₆and vanadium shows an oxidation states of + 5 in V₂O₅.
(iii) Oxo metal anions have highest oxidation state, e.g., Cr in Cr₂0₇^2- has an. oxidation state of + 6 whereas Mn in Mn0₄^– has an oxidation state of + 7. This is again due to the combination of the metal with oxygen, which is highly electronegative and oxidizing agent.

8.26. Indicate the steps in the preparation of:
(i)K₂Cr₂0₇from chromite ore
(ii)KMn0₄ from pyrolusite ore.
Sol:

8.27. What are alloys? Name an alloy which contains some lanthanoid metals. Mention its uses.
Sol: An alloy is a homogeneous mixture of different metals or metals and non-metals.
Misch metal is an alloy of cerium (Ce). lanthanum (La), neodymium (Nd), iron (Fe) and traces of carbon, sulphur, aluminium etc. It is used in making parts of jet engines.

8.28. What are inner transition elements? Decide which of the following atomic numbers are the atomic numbers of the inner transition elements: 29,59,74,95,102,104.
Sol: The f-block elements in which the. last electron enters into f-sub shell-are called inner-transition elements. These include lanthanoids (Z=58 to 71) and actinoids (Z=90 to 103). Thus, the elements with atomic numbers 59,95 and 102 are the? inner transition elements.

8.29. The chemistry of the actinoid elements is not so smooth as that of the lanthanoids. Justify this statement by giving some examples from the oxidation state of these elements.
Sol: Lanthanoids show limited number of oxidation state, viz, + 2, + 3 and + 4 (out of which + 3 is most common). This is because of large energy gap between 4f 5d and 6s subshells. The dominant oxidation state of actinoids is also + 3 but they show a number of other oxidation states also. For example, uranium (Z=92) and plutonium (Z – 94), show + 3, + 4, + 5 and + 6, neptunium (Z = 94) shows + 3, +4, + 5 and + 7, etc. This is because of the small energy difference between. 5f, 6d and 7s orbitals of the actinoids.

8.30. Which is the last element in the series of the actinoids? Write the electronic configuration of this element. Comment on the possible oxidation state of this element
Sol: Last actinoid=Lawrencium (Z = 103)
Electronic configuration = [Rn]⁸⁶ 5f¹⁴ 6d¹ 7s² Possible oxidation state = + 3.

8.31 Use Hund’s rule to derive the electronic configuration of Ce³⁺ ion, and calculate its magnetic moment on the basis of ‘spin-only’ formula.
Sol.

8.32. Name the members of the lanthanoid series which exhibit +4 oxidation state and those which exhibit +2 oxidation state. Try to co-relate this type of behaviour with the electronic configuration of these elements.
Sol: +4 oxidation state : ₅₈Ce, ₅₉Pr, ₆₅Tb
+ 2 oxidation state : ₆₀Nd, ₆₂Sm, ₆₃Eu, ₆₉Tm, ₇₀Yb.
In general +2 oxidation state is exhibited by the elements with configuration 5d⁰6s² so that two electrons may be easily lost. Similarly +4 oxidation state is shown by the elements which after losing four electrons acquire configuration either close to 4f⁰ or 4f⁷.

8.33. Compare the chemistry of actinoids with that of lanthanoids with reference to:
(i)Electronic configuration
(ii)Oxidation states
(iii)Chemical reactivity
Sol: (i)Electronic configuration : In lanthanoids 4f- orbitals are progressively filled whereas in actinoids 5f-orbitals are progressively filled.
(ii)Oxidation states : Lanthanoids shows +3 oxidation state. Some elements shows +2 and +4 oxidation state also. Actinoids shows +3, +4, +5 +6, +7 oxidation states. Although +3 and +4 are most common.
(iii)Chemical reactivity : Actinoids are more reactive than lanthanoids due to bigger atomic size and lower ionisation energy.

8.34. Write the electronic configurations of the elements with the atomic numbers 61,91,101 and 109.
Sol: Z=61 (Promethium, Pm) [Xe]⁵⁴4f⁵5d⁰ 6s²
Z = 91 (Protactinium, Pa) => [Rn]⁸⁶ 5f² 6d¹ 7s²
Z = 101 (Mendelevium, Md)=> [Rn]⁸⁶ 5f¹³ 6d⁰ 7s²
Z = 109 (Meitnerium, Mt) [Rn]⁸⁶ 5f¹⁴ 6d⁷ 7s²

8.35. Com pare the general characteristics of the first series of the transition metals with those of the second and third series metals in the respective vertical columns. Give special emphasis on the following points:
(i)electronic configurations
(ii)oxidation states
(iii)ionisation enthalpies and
(iv)atomic sizes
Sol: (i) Electronic configuration: The elements in the same vertical column generally have similar electronic configuration. First transition series shows only two exceptions, i.e., Cr = 3d⁵  4s¹  and Cu = 3d¹⁰ 4s¹. But second transition series shows more exceptions, i.e., Y = 4d¹ 5s², Nb = 4d¹ , 5s¹ , Mo=4d⁵  5s¹ , Ru=4d¹  5s¹ , Rh=4d⁸  5s¹ , Pd , =4d¹⁰ 5s°, Ag=4d¹⁰ 5s¹ . In third transition, there are two exceptions, i.e„ Pt = 5d⁹  6s¹  and Au = 5d¹⁰ 6s¹ .
Thus in the same vertical column, in a number of cases, the electronic configuration of the elements of three series are not similar.
(ii) Oxidation states: The elements in the same vertical column generally show similar oxidation states. The number of oxidation states shown by the elements in the middle of each series is maximum and minimum at the extreme ends.
(iii)Ionization enthalpies: The first ionization enthalpies in each series generally increases gradually as we more from left to right though some exceptions are observed in each series. The first ionization enthalpies of some elements in the second (4d) series are higher while some of them have lower value than the elements of 3d series in the same vertical column. However, the first ionization enthalpies of third (5d) series are higher than those of 3d and Ad series. This is because of weak shielding of nucleus by 4f-electrons in the 5d series.
(iv)Atomic sizes: In general, ions of the same charge or atoms in a given series show progressively decrease in radius with increasing atomic number though the decrease is quite small. But the size of the atoms of the Ad series is larger then the corresponding elements of the 3d series whereas size of elements of the 5d-series nearly the same as those of Ad series because of lanthanoid contraction.

8.36. Write down the number of 3d electrons in each of the following ions:Ti²⁺, V²⁺, Cr³⁺, Mn²⁺, Fe²⁺, Fe²⁺, Co²⁺, Ni²⁺ and Cu²⁺. Indicate how would you expect the five 3d orbitals to be occupied for these hydrated ions (octahedral).
Sol:

8.37. Comment on the statement that elements of first transition series possess many properties different from those of the heavier transition elements.
Sol: The heavier transition elements belong to fourth (Ad) and fifth (5d) and sixth (6d) transition series. Their properties are expected to be different from the elements belonging to the first (3d) series due to the following reasons :
(i) The atomic radii of the elements belonging to Ad and 5d series are more due to greater number of electron shells. However, the difference in Ad and 5d transition elements are comparatively less because of lanthanoid contraction.
(ii) Because of stronger inter atomic bonding, the m.p. and b.p. of the elements of Ad and 5d series are higher.
(iii) Ionisation enthalpies are expected to decrease as we move from one series to the other. However, the values for the elements of 5d series are higher as compared to the elements belonging to the other two series due to lanthanoid contraction.
Actually atomic size decreases on account of it and effective nuclear charge increases. As a result, there is an increase in ionisation energy in case of 3d elements.

8.38. What can be inferred from the magnetic moment values of the following complex species?

Sol:

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NCERT Solutions For Class 12 Chemistry Chapter 6 General Principles and Processes of Isolation of Elements

Topics and Subtopics in NCERT Solutions for Class 12 Chemistry Chapter 6 General Principles and Processes of Isolation of Elements:

Section Name	Topic Name
6	General Principles and Processes of Isolation of Elements
6.1	Occurrence of Metals
6.2	Concentration of Ores
6.3	Extraction of Crude Metal from Concentrated Ore
6.4	Thermodynamic Principles of Metallurgy
6.5	Electrochemical Principles of Metallurgy
6.6	Oxidation Reduction
6.7	Refining
6.8	Uses of Aluminium, Copper, Zinc and Iron

NCERT IN TEXT QUESTIONS 

6.1. Which of the ores mentioned  can be concentrated by magnetic separation method?
Ans: Ores Which are magnetic in nature can be separated from non-magnetic gangue particles by magnetic separation method. For ex: ores of iron such as haemetite (Fe₂O₃), magnetite (Fe₃O₄), siderite (FeCO₃) and iron pyrites (FeS₂ ) being magnetic can be separated from non-magnetic silica and other impurities by magnetic separation method.

6.2. What is the significance of leaching in the extraction of aluminium?
Ans: Leaching or chemical separation is quite effective to purify bauxite an ore of aluminium associated with the impurities of iron oxide. The ore is leached with concentrated solution of NaOH to form a soluble complex leaving behind the impurities.

6.3.

Ans: This is explained on the basis of K_eq, the equilibrium constant. In the given redox reaction, all reactants and products are solids at room temperature, so, there is no equilibrium between the reactants and products and hence the reactions does not occur at RT. At high temperature, Cr melts and values of TAS increases. As a result, the value of

6.4. Is it true that under certain conditions, Mg can reduce Al203 and Al can reduce MgO? What are those conditions?
Ans:

NCERT EXRECISES

6.1. Copper can be extracted by hydrometallurgy but not zinc. Explain.
Ans:

But with water, these metals (Al, Mg, Ca and K) forms their corresponding ions with the evolution of H₂ gas.
Thus, Al, Mg, Ca, K, etc., cannot be used to displace zinc from zinc solution, and only copper can be extracted by hydrometallurgy but not the zinc.

6.2.What is the role of depressant in froth-floatation process?
Ans: The role of depressant is to prevent one type of sulphide ore particles from forming the froth with air bubbles. NaCN is used as a depressant to separate lead sulphide (PbS) ore from zinc sulphide (ZnS) ore. NaCN forms a zinc complex, Na₂[Zn(CN)₄] on the surface of ZnS thereby preventing it from the formation of the froth.

In this condition, only lead sulphide forms froth and thus can be separated from zinc sulphide ore.

6.3. Why is the extraction of copper from pyrites more difficult than that from its oxide ore through reduction?
Ans:

6.4. Explain:
(i)Zone refining
(ii)Column chromatography.
Ans: (i) Zone refining: This method is used for production of semiconductors and other metals of very high purity, e.g., Ge, Si, B, Ca and In.
It is based on the principle that the impurities
are more soluble in the molten state (melt) than in the solid state of the metal.
The impure metal in the form of bar is heated at one end with a moving circular heater. As – the heater is slowly moved along the length of the rod, .the pure metal crystallises out of the melt whereas the impurities pass into the adjacent molten zone. Thi,s process is repeated several times till the impurities are completely driven to one end of the rod which is then cut off and discarded.
(ii) Chromatography: It is based on the principle that the different components of a mixture are adsorbed to different extents on an adsorbent.
In column chromatography, an adsorbent, such as alumina (Al₂O₃) or silica gel is packed in a column. This fonns the stationary phase. The mixture to be separated is dissolved in a suitable solvent (mobile phase) and applied to the top of the column. The adsorbed components are extracted (eluted) from the column with a suitable . solvent (eluent). The component which is more strongly adsorbed on the column takes longer time to travel through the column than a component which is weakly adsorbed. Thus, the various components of the mixture are seperated as they travel through absorbent (stationary phase).

6.5. Out of C and CO which is a better reducing agent at 673 K?
Ans: This can be explained thermodynamically, taking entropy and free energy changes into account.

As can be seen from ΔG° Vs T plot (Ellingham diagram), lines for the reactions, C ——–> C0₂ and C ——–> CO cross at 983 K. Below 983 K, the reaction (a) is energetically more favourable but above 673 K, reaction (b) is favourable and preferred. Thus, below 673 K both C and CO can act as a reducing agent but since CO can be more easily oxidised to C0₂ than C to C0₂ , therefore, below 673 K, CO is more effective reducing agent than carbon.

6.6. Name the common elements present in anode mud in the electro-refining of copper. Why are they so present?
Ans: Anode mud contains metals like Ag, Au, Pt etc. which are less reactive than Cu. Actually, they are not in a position
to lose electrons though they constitute the electrode which acts as anode. All these metals are left as residue under anode (known as anode mud) while the entire copper present participates in the oxidation half reaction.
Cu(s) → Cu²⁺(aq) + 2e^–

6.7. Write down the reactions taking place in different zones in the blast furnace during the extraction of iron.
Ans: In the blast furnace reduction of iron oxides take place at different temperature ranges as shown below.

6.8. Write chemical reactions taking place in the extraction of zinc from zinc blende.
Ans: The following processes are involved in the extraction of zinc from zinc blende:
(i) Concentration: Zinc blende ore is crushed and the concentration done by froth- floatation process.
(ii) Roasting: The concentrated ore is then roasted in presence of excess of air at about 1200 K as a result zinc oxide is formed.

(iii) Reduction : Zinc oxide obtained above is mixed with powdered coke and heated to 1673 K in a fire clay retort where it is reduced ‘ to zinc metal.

At 1673 K, zinc metal being volatile (boiling point 1180 K), distills over and is condensed.
(iv) Electrolytic refining: Impure zinc is made the anode while pure zinc strip is made the cathode. ZnSO₄ solution acidified with dil. H₂SO₄ is the electrolyte used. On passing electric current, pure zinc gets deposited on the cathode.

6.9. State the role of silica in the metallurgy of copper.
Ans: Silica (SiO₂) acts as an acidic flux in the metallurgy of copper and combines with FeO (the main impurity) to form FeSiO₃ which is a slag.

6.10. What is meant by the term “chromatography”?
Ans: Chromatography is a technique used for separation, purification, identification and characterization of the components of a mixture whether coloured or colourless. The term chromatography was originally derived from the Greek word ‘chroma’ meaning colour and ‘graphy for writing because the method was first used for the separation of coloured substances (plant pigments) into individual components.

6.11. What criterion is followed for the selection of the stationary phase in chromatography?
Ans: In chromatography, particularly in adsorption chromatography, the stationary phase is the adsorbent. It should fulfil certain criteria for better results.
(i) It should have high but selective adsorption power.
(ii) The particles should be spherical in shape and of uniform size.
(iii) The adsorbent should not react chemically with the solvents used for elution or with the components of the mixture under investigation.
(iv) The adsorbent should contain as small amount of the soluble components as possible.
(v) The adsorbent should be catalytically inactive and must have a neutral surface.
(vi) The adsorbent should be easily available.
(vi) The adsorbent should be perfectly white.

6.12. Describe a method for refining nickel.
Ans: When impure nickel is heated in presence of CO at 330-350 K, it forms volatile nickel tetracarbonyl leaving behind the impurities. The nickel tetracarbonyl thus obtained is then heated to higher temperature (450-470K), then it undergoes thermal decomposition to give pure nickel.

6.13. How can you separate alumina from silica in a bauxite ore associated with silica? Give equations, if any.
Ans: Pure alumina can be separated from silica in bauxite by Baeyer’s process. The bauxite ore associated with silica is heated with a concentrated solution of NaOH at 473-523 K and 35-36 bar pressure. Under these conditions, alumina dissolves as sodium meta-aluminate and silica as sodium silicate leaving behind the impurities.

The resulting solution is filtered to remove the undissolved impurities, sodium meta-aluminate can be precipitated as hydrated aluminium oxide by passing CO₂ vapours. The sodium silicate formed cannot be precipitated and can be filtered off.

6.14. Giving examples, differentiate between ‘roasting’ and ‘calcination’.
Ans: Calcination is a process of converting carbonates and hydroxide ores of metals to their respective oxides by heating them, strongly below their melting points either in absence or limited supply of air.

Roasting is a process of converting sulphide ores into its metallic oxides by heating strongly below its melting point in excess of air.

6.15. How is ‘cast-iron’ different from ‘pig iron’?
Ans: Cast iron differs from pig iron with respect to the carbon contents. Whereas carbon contents in pig iron are nearly four percent (4%), cast iron contains carbon to the extent of nearly three percent (3%).

6.16. Differentiate between “minerals” and “ores’.
Ans: Minerals: The natural substances in which the metals or their compounds occur in the earth is called minerais.
Ores: The minerals from which the metals can be coaveniently and economically extracted are called ores.
Note : All ores are minerals but all minerals are not ores.

6.17. Why copper matte is put in silica lined converter?
Ans: Copper matte consists of Cu₂S along with some unchanged FeS. When a blast of hot air is passed through molten matte placed in silica lined converter, FeS present in matte is oxidised to FeO which combines with silica (SiO₂) to form FeSiO₃slag.
₂S undergoes oxidation to form Cu₂0 which then reacts with more Cu₂S to form copper metal.

Thus, copper matte is heated in silica lined converter to remove FeS present in matte as FeSiO₃ slag.

6.18. What is the role of cryolite in the metallurgy of aluminium?
Ans: (a) It lowers the fusion (melting) point of the bath from 2323 K to about 1140 K.
(b) It makes alumina a good conductor of electricity.

6.19. How is leaching carried out in case of low grade copper ores?
Ans: Leaching in case of low grade copper ores is carried out with acids in presence of air. In this process, copper is oxidised to Cu²⁺ ions which pass into the solution.

6.20. Why is zinc not extracted from zinc oxide through reduction using CO?
Ans: The chemical reaction involving the reduction of ZnO by CO is :
ZnO(s) + CO(g) → Zn(s) + CO₂(g)
The process is thermodynamically not feasible because there is hardly any change in entropy as a result of the reaction. This is quite evident from the physical states of the reactants and products involved in the reaction

6.21. The value of Δ_fG° for formation of Cr₂O₃ is – 540 kJ mol^-1 and that of Al₂0₃ is – 827 kJ mol^-1 . Is the reduction of Cr₂O₃ possible with Al?
Ans: Chemical equation for the formation of Cr₂O₃ and Al₂0₃ are as follows :

6.22. Out of C and CO, which is a better reducing agent for ZnO?
Ans: The two reduction reactions are :

In the first case, there is increase in the magnitude of ΔS° while in the second case, it almost remains the same. In other words ΔG° will have more negative value in the first case when C(s) is used as the reducing agent than in the second case when CO(g) acts as the reducing agent. Therefore, C(s) is a better reducing agent.

6.23. The choice of a reducing agent in a particular case depends on thermodynamic factor. How far do you agree with this statement? Support your opinion with two examples.
Ans: We can study the choice of a reducing agent in a particular case using Ellingham diagram.
It is evident from the diagram that metals for which the standard free energy of formation oftheir oxides is more negative can reduce those metal oxides for which the standard free energy of formation of their respective oxides is less negative. It means that any metal will reduce the oxides of other metals which lie above it in the Ellingham diagram. This is because the standard free energy change (Δ_rG°) of the combined redox reaction will be negative by an amount equal to the difference in Δ_f G° of the two metal oxides. Thus both Al and Zn can reduce FeO to Fe but Fe cannot reduce Al₂0₃ to A1 and ZnO to Zn. In the same way, G can reduce ZnO to Zn but not CO.
Note : Only that reagent will be preferred as reducing agent which will lead to decrease in free energy value (ΔG°) at a certain specific temperature.

6.24. Name the processes from which chlorine is obtained as a by-product What will happen if an aqueous solution of NaCl is subjected to electrolysis?
Ans: Down process is used for the preparation of sodium metal, where chlorine is obtained as a by- product. This -process involves the electrolysis of a fused mixture of NaCl and CaCl₂ at 873 K.Sodium is discharged at the cathode while Cl₂ is obtained at the anode as a by-product.

If, an aqueous solution of NaCl is electrolysed, H₂ is evolved at the cathode while Cl₂ is obtained at the anode.

6.25. What is the role of graphite rod in the electrometallurgy of aluminium?
Ans: In the electrometallurgy of aluminium, oxygen gas is evolved at anode. O₂ reacts with graphite or carbon (graphite electrodes) to form carbon monoxide and carbon dioxide. In case if some other metal electrodes is used as anode, then oxygen will react with aluminium formed during the process to form aluminium oxide(Al₂O₃) which will pass into the reaction mixture resulting into wastage of Al. Since graphite is cheaper than aluminium, its wastage or can be tolerated.

6.26. Outline the principles of refining of metals by the following methods:
(i)Zone refining
(ii)Electrolytic refining
(iii)Vapour phase refining
Ans: (i) Zone refining: This method is used for production of semiconductors and other metals of very high purity, e.g., Ge, Si, B, Ca and In.
It is, based on the principle that the impurities are more soluble in the molten state (melt) than in the solid state of the metal.
The impure metal in the form of bar is heated at one end with a moving circular heater. As the heater is slowly moved along the length of the rod, the pure metal crystallises out of the melt whereas the impurities pass into the adjacent molten zone. This process is repeated several times till the impurities are completely driven to one end of the rod which is then cut off and discarded.
(ii)Electrolytic refining: Many metals, such as Cu, Ag, Au, Al, Pb, etc., are purified by this method. The impure metals is made the anode while a thin sheet of pure metal acts as a cathode. The electrolytic solution consists of a salt or a complex salt solution of the metal. On passing the current, the pure metal is deposited on the cathode while the impurities fall down as anode mud.
(iii)Vapour-phase refining: The crude metal is freed from impurities by first converting it into a suitable volatile compound by heating it with a specific reagent at a lower temperature and then decomposing the volatile compound at some higher temperature to give the pure metal.
(a)Mond’s process: When impure nickel is heated is a current of CO at 330-350 K, it forms volatile nickel tetracarbonyl complex leaving behind the impurities. The complex then heated to a higher temperature (450-470K) when it undergoes thermal decomposition giving pure nickel.

(b)Van Arkel method: This method is Used for preparing ultra-pure metals by removing all the oxygen and nitrogen present as impurities in metals like zirconium and titanium (which are used in space technology).Crude Zr is heated in an evacuated vessel with iodine at 870 K. Zirconium tetraiodide thus formed is separated. It is then decomposed by heating over a tungsten filament at 1800 – 2075 K to give pure Zr.

6.27. Predict conditions under which Al might be expected to reduce MgO.
Ans: The equations for the formation of the two oxides are

If we look at the plots for the formation of the two oxides of the Ellingham diagram, we find that they intersect at certain point. The corresponding value of ΔG° becomes zero for the reduction of MgO by Al metal.

This means that the reduction of MgO by Al metal can occur below this temperature. Aluminium (Al) metal can reduce MgO to Mg above this temperature because Δ°G for Al₂O₃ is less as compared to that of MgO.

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NCERT Solutions For Class 12 Chemistry Chapter 7 The p Block Elements

Topics and Subtopics in NCERT Solutions for Class 12 Chemistry Chapter 7 The p Block Elements:

Section Name	Topic Name	Section Name	Topic Name
7	The p-Block Elements	7.12	Simple Oxides
7.1	Group 15 Elements	7.13	Ozone
7.2	Dinitrogen	7.14	Sulphur – Allotropic Forms
7.3	Ammonia	7.15	Sulphur Dioxide
7.4	Oxides of Nitrogen	7.16	Oxoacids of Sulphur
7.5	Nitric Acid	7.17	Sulphuric Acid
7.6	Phosphorus – Allotropic Forms	7.18	Group 17 Elements
7.7	Phosphine	7.19	Chlorine
7.8	Phosphorus Halides	7.20	Hydrogen Chloride
7.9	Oxoacids of Phosphorus	7.21	Oxoacids of Halogens
7.10	Group 16 Elements	7.22	Interhalogen Compounds
7.11	Dioxygen	7.23	Group 18 Elements

NCERT Solutions CBSE Sample Papers ChemistryClass 12 Chemistry

NCERT IN TEXT QUESTIONS 

7.1. Why are pentahalides more covalent than trihalidcs?
Ans: The group 15 elements have 5 e^-1 s in their valence shell. It is difficult to lose 3e^-1s to form E³⁺ and even more difficult to lose 5e^-1 s to form E⁵⁺. Thus, they have very little tendency to form ionic compounds. Further, since the elements in +5 state have less tendency to lose e^-1s than in the +3 state, elements in +5 state have more tendency to share e^-1 s and hence pentahalides are more covalent than trihalides.

7.2. Why is BiH₃ the strongest reducing agent amongst all the hydrides of group 15 elements? (C.B.S.E. 2013)
Ans: Down the group, the atomic size of the element (E) increases and the bond length of the corresponding E—H bond also increases. This adversely affects the bond dissociation enthalpy. This means that amongst the trihydrides of the members of nitrogen family, the bond dissociation enthalpy of Bi—H bond is the least. Therefore, BiH₃ is the strongest reducing agent among the hydrides of group 15 elements.

7.3. Why is N₂ less reactive at room temperature?
Ans: Due to presence of triple bond between two N-atoms (N = N), the bond dissociation energy of N₂ is very high. As a result, N₂ becomes less reactive at room temperature.

7.4. Mention the conditions required to maximise the yield of ammonia.
Ans: Ammonia is prepared by Haber’s process as given below:

7.5. How does ammonia react with a solution of Cu²⁺?
Ans:

7.6. What is the covalence of nitrogen in N₂O₅ ?
Ans: In N₂O₅ , each N-atom has four shared pairs of e-1 s as shown:

7.7. Why is bond angle in \({ PH }_{ 4 }^{ + }\) ion higher than in PH₃ ? (Pb. Board 2009)
Ans: In both PH₃ and \({ PH }_{ 4 }^{ + }\) ion, the phosphorus atom is sp³ hybridised. However, in PH₃ the central atom has apyramidal structure due to the presence of lone electron pair on the phosphorus atom.

Because of lone pair : shared pair repulsion which is more than that of shared pair : shared pair repulsion, the bond angle in PH₃ is nearly 93-6°. In \({ PH }_{ 4 }^{ + }\) ion, there is no lone electron pair on the phosphorus atom. It has a tetrahedral structure with bond angle of 109°-28′. Thus, the bond angle in \({ PH }_{ 4 }^{ + }\) ion is higher than in PH₃.

7.8. What happens when white phosphorus is heated with concentrated NaOH solution in an inert atmosphere of CO₂?
Ans:

7.9. What happens when PCl₅ is heated?
Ans:

7.10. Write a balanced equation for the hydrolytic reaction of PC is in heavy water.
Ans:

7.11. What is the basicity of H₃PO₄?
Ans:

7.12. What happens when H₃PO₄ is heated?
Ans: On heating, H₃PO₃ disproportionates to form PH₃ and H₃PO₄ with O.S. of-3and + 5.

7.13. List the important sources of sulphur.
Ans: Sulphur mainly occurs in the combined states in earth’s crust in the form of sulphates and sulphides.
Sulphates : gypsum (CaSO₄.2H₂O); epsom (MgSO₄.7H₂O); baryte (BaSO₄), etc.
Sulphides : Galena (PbS); zinc blende (ZnS); copper pyrites (CuFeS2); iron pyrites (FeS₂), etc. Traces of sulphur occur’as H₂S and in organic materials such as eggs, proteins, garlic, onion, mustard, hair and wool.

7.14. Write the order of thermal stability of the – hydrides of Group 16 elements.
Ans: The thermal stability of hydrides of group 16 elements decreases down the group. This is because down the group, size of the element (M) increases, M-H bond length increases and thus, stability of M-H bond decreases so that it can be broken down easily. Hence, we have order of thermal stability as H₂O > H₂S > H₂Se > H₂Te > H₂PQ

7.15. Why is H₂O a liquid and H₂S a gas?
Ans: Due to high electronegativity of O than S, H₂O undergoes extensive intermolecular H-bonding. As a result, H₂O exists as an associated molecule in which each O is tetrahedrally surrounded by four H₂O molecules. Therefore, H₂O is a liquid at room temperature.
On the other hand,H₂S does not undergo H- bonding. It exists as discrete molecules which are held together by weak van der waals forces of attraction. A small amount of energy is required to break these forces of attraction. Therefore, H₂S is a gas at room temperature.

7.16. Which of the following does not react with oxygen directly? Zn, Ti, Pt, Fe
Ans: Platinum (Pt) is a noble metal and does not react with oxygen directly.

7.17. Complete the following reactions:
(i)C₂H₂ + O₂ -> (ii) 4Al + 3 O₂ ->
Ans:

7.18. Why does O₃ act as a powerful oxidising agent?
Ans: On heating, O₃ readily decomposes to give O₂ and nascent oxygen.

Since nascent oxygen is very reactive, therefore, O₃ acts as a powerful oxidising agent.

7.19. How is O₃ estimated quantitatively?
Ans: When O₃ is treated with excess of KI solution buffered with borate buffer (pH = 9.2), I2 is liberated quantitatively.

The I₂ thus liberated is titrated against a standard solution of sodium thiosulphate using starch as an indicator.

7.20. What happens when sulp’hur dioxide is passed through an aqueous solution of Fe(III) salt?
Ans: SO₂ acts as a reducing agent and reduces aqueous solution of Fe (III)salt to Fe (II) salt.

7.21. Comment on the nature of two S-O bonds formed in S02 molecule. Are the two S-O bonds in this molecule equal ?
Ans: SO₂ exists as an angular molecule with OSO bond angle of 119.5°. It a resonance hybrid of two canonical-forms:

7.22. How is the presence of SO₂ detected?
Ans: SO₂ is a pungent smelling gas. It can be detected by two test:

 

7.23. Mention three areas in which H₂SO₄ plays an important role.
Ans: (i) Sulphuric acid is used for the manufacture of a number of chemicals like hydrochloric acid, phosphoric acid, nitric acid along with a large number of organic compounds.
(ii) A mixture of concentrated nitric acid and concentrated sulphuric acid is used in the manufacture of explosives like picric acid, T.N.T, dynamite etc.
(iii) Dilute solution of acid is employed in petroleum refining in order to remove the unwanted impurities of sulphur.

Question 24.
Write the conditions to maximise the yield of H₂SO₄ by Contact process.
Solution:
The key step in the manufacture of sulphuric acid is oxidation of SO₂ to SO₃ in presence of V₂O₅ catalyst.

The reaction is exothermic and reversible. Hence, low temperature and high pressure are the favourable conditions for maximum yield of SO₃. In practice a pressure of 2 bar and temperature of 720 K is maintained.

Question 25.
Why is K_a2 « K_a1 for H₂SO₄ in water?
Solution:
H₂SO₄ is a very strong acid in water largely because of its first ionisation to H₃O⁺ and HSO₄^– The ionisation of HSO₄^– to H₃O⁺ and SO₄^2- is very very small. That is why, K_a2« K_a1.

Question 26.
Considering the parameters such as bond dissociation enthalpy, electron gain enthalpy and hydration enthalpy, compare the oxidising powers of F₂ and Cl₂.
Solution:
The oxidising powers of both the members of halogen family are expressed in terms of their electron accepting tendency and can be compared as their standard reduction potential values.
F₂ + 2e^– → 2F^–; E° = 2-87 V, Cl₂ + 2e^– → 2Cl^– ; E° = 1-36 V
Since the E° of fluorine is more than that of chlorine, it is a stronger oxidising agent.
Explanation : Three factors contribute towards the oxidation potentials of both the halogens. These are :
(i) Bond dissociation enthalpy: Bond dissociation enthalpy of F2 (158 kJ mol^-1) is less compared to that of Cl₂ (242·6 kJ mol^-1).
(ii) Electron gain enthalpy: The negative electron gain enthalpy of F (- 332·6 kJ mol^-1) is slightly less than of Cl (-348·5 kJ mol^-1).
(iii) Hydration enthalpy: The hydration enthalpy of F- ion (515 kJ mol^-1) is much higher than that of Cl- ion (381 kJ mol^-1) due to its smaller size.
From the available data, we may conclude that lesser bond dissociation enthalpy and higher hydration enthalpy compensate lower negative electron gain enthalpy of fluorine as compared to chlorine. Consequently, F₂ is a more powerful oxidising agent than Cl₂.

Question 27.
Give two examples to show the anomalous behaviour of fluorine.
Solution:

1. Ionisation enthalpy, electro-negativity and electrode potential are higher for fluorine than the expected trends of other halogen.
2. Fluorine does not show any positive oxidation state except in HOF.

Question 28.
Sea is the greatest source of some halogens. Comment.
Solution:
Sea water contains chlorides, bromides and iodides of sodium, potassium, magnesium and calcium but sodium chloride being the maximum makes sea water saline. Various sea weeds contain upto 0.5% iodine.

Question 29.
Give the reason for bleaching action of Cl₂.
Solution:
Chlorine bleaches by oxidation Cl₂ + H₂O → HCl + HOCl → HCl + [O]
The nascent oxygen reacts with dye to make it colourless.

Question 30.
Name two poisonous gases which can be prepared from chlorine gas.
Solution:
COCl₂ (phosgene), CCl₃NO₂ (tear gas)

Question 31.
Why is ICI more reactive than l2?
Solution:
In general, interhalogen compounds are more reactive than halogens due to weaker X-X’ bonding than X-X bond. Thus, ICI is more reactive than I₂.

Question 32.
Why is helium used in diving apparatus?
Answer:
Helium along with oxygen is used in the diving apparatus by the sea divers. Since it is very little soluble in blood, it reduces decompression and causes less discomfort to the diver in breathing. A mixture of helium and oxygen does not cause pain due to very low solubility of helium in blood as compared to nitrogen.

Question 33.
Balance the following equation :
XeF₆ + H₂O → XeO₂F₂ + 4HF
Solution:

Question 34.
Why has it been difficult to study the chemistry of radon?
Solution:
Radon is radioactive with very short half-life which makes the study of chemistry of radon difficult.

NCERT EXERCISES

7.1. Discuss the general characteristics of Group 15 elements with reference to their electronic configuration, oxidation state, atomic size, ionisation enthalpy and electronegativity.
Sol: In group 15 of the Periodic Table, the elements, nitrogen (₇N), phosphorus (₁₅P), arsenic (₃₃As), antimony (₅₁Sb) and bismuth (₈₃Bi) are present. The elements of this group can exhibit various oxidation states ranging between -3 to + 5. Negative oxidation state will be exhibited when they combine with less electronegative element andpositive oxidation state will be exhibited with more electronegative element. Positive oxidation state becomes more favourable as we more down the group due to increasing metallic character & electropositivity. Although due to inert pair effect the stability of +5 state will also decrease. The only stable compound of Bi (V) is BiF₅.
The atomic (covalent) and ionic radii (in a particular oxidation state) of the elements of nitrogen family (group 15) are smaller than the corresponding elements of carbon family (group 14). On moving down the group, the covalent and ionic radii (in a particular oxidation state) increase with increase in atomic number. There is a considerable increase in covalent radius from N to P. However, from As to Bi, only a small increase is observed.
As the size increases on moving down the group, the ionisation enthalpy increases. The ionisation enthalpy of nitrogen group elements is more than the corresponding elements of oxygen group. This is because of more stable half-filled outermost p- subshell of nitrogen group elements. Electronegativity decreases down the group with increase in atomic size.

7.2. Why is the reactivity of nitrogen different from that of phosphorus?
Sol: Molecular nitrogen exists as a diatomic molecule (N₂) in which the two nitrogen atoms are linked to each other by triple bond (N≡N). It is a gas at room temperature. Multiple bonding is not possible in case of phosphorus due to its large size. It exists as P₄ molecule (solid) in which P atoms are linked to one another by single covalent bonds. Because of greater bond dissociation enthalpy (946 kJ mol^-1) of N≡N bond, molecular nitrogen is very less reactive as compared to molecular phosphorus.

7.3. Discuss the trends in chemical reactivity of group 15 elements.
Sol: Hydrides: All elements of group 15 form gaseous hydrides of the type MH₃.
In all the hydrides the central atom is sp³ hybridized and their shape is pyramidal due to presence of lone pair of electrons.
(a)The basic strength of the hydrides decreases as we move down the group.
Thus, NH₃ is the strongest base.
NH₃ > PH₃ > AsH₃ > SbH₃
(b)The thermal stability of the hydrides decreases as the atomic size increases, i.e., the M – H bond strength decreases which means reducing character increases.
(c)In the liquid state, the molecules of NH₃are associated due to hydrogen bonding. The molecules of other hydrides are not associated.
(d)NH₃ is soluble in water whereas other hydrides are insoluble.
(e)All the hydrides, except NH₃, are strong reducing agents and react with metal ions (Ag⁺, Cu²⁺, etc.) to form phosphides, arsenides or antimonides.
Halides: The elements of group 15 form two series of halides MX₃ and MX₅.
(a)All the elements of the group form trihalides. The ionic character of trihalides increases as we move down the group. Except NCl₃ all the trihalides are hydrolysed by water. This is due to the absence of d-orbitals in nitrogen.
(b)PF₃ is not hydrolysed because fluorine being more electronegative than oxygen forms more stable bonds with phosphorus than P – O bonds.
(c)N cannot form NX₅ because of non-availability of rforbitals. Bi cannot form BiX₃ because of reluctance of 6s electrons of Bi to participate in bond formation.
(d)The hybridisation of M in MX₃ is sp³ and shape is pyramidal. M in MX₅ is sp³ as hybridised and shape is trigonal pyramidal. The axial bonds in MX₅ are weaker and longer, So MX₅ are less stable and decompose on heating eg:

Oxides:
(a)Nitrogen forms a number of oxides. The rest of the members (P, As, Sb and Bi) of the group form two types of oxides : E₂0₃ and E₂O₅.
(b)The reluctance of P, As, Sb and Bi to enter into pπ -pπ multiple bonding leads to cage structures of their oxides and they exist as dimers, E₄O₆ and E₅O₁₀.
(c)The basic nature of die oxides increases with increase in atomic number of the element. Thus, the oxides of nitrogen (except N₂0 and NO), P (III) and As (III) are acidic, Sb (III) oxide is amphoteric and Bi (III) oxide is basic.

7.4. Why does NH₃ form hydrogen bond but PH₃ does not?
Sol: Nitrogen has an electronegativity value 3.0, which is much higher than that of H (2.1). As a result, N – H bond is quite polar and hence NH₃ undergoes intermolecular H – bonding.

Phosphorus have an electronegativity value 2-1. Thus, P – H bond is not polar and hence PH₃ does not undergo H – bonding.

7.5. How is nitrogen prepared in the laboratory? Write the chemical equations of the reactions . involved.
Sol: In laboratory, nitrogen is prepared by heating an equimolar aqueous solution of ammonium chloride and sodium nitrite. As a result of double decomposition reaction, ammonium nitrite is formed. Ammonium nitrite is unstable and decompose to form nitrogen gas.

7.6. How is ammonia manufactured industrially?
Sol: Commercially, by Haber’s process.

iron oxide, K₂O, Al₂0₃ The optimum conditions for the production of NH₃ are pressure of 200 atm and temperature of 100K.

7.7. Illustrate how copper metal can give different products on reaction with HN0₃.
Sol: On heating with dil HN0₃, copper gives copper nitrate and nitric oxide.

7.8. Give the resonating structures of N0₂ and N₂O₅.
Sol: Resonating structures of N0₂ are:

7.9. The HNH angle value is higher than HPH, H AsH and HSbH angles. Why?
(Hint: Can be explained on the basis of sp³ hybridisation in NH₃ and only s-p bonding , between hydrogen and other elements of the group).
Sol: In all these cases, the central atom is sp³ hybridized. Three of the four sp³ orbitals form three σ-bonds, while the fourth contains the lone pair of electrons. On moving down from N to Sb, the electronegativity of the central atom goes on decreasing. As a result of this, bond  pairs of electrons lie away and away from the central atom. This is because of the force of repulsion between the adjacent bond pairs goes on decreasing and the bond angles keep on decreasing from NH₃ to SbH₃. Thus, bond angles are in the order:

7.10. Why does R₃P=0 exist but R₃N=0 does not (R is an alkyl group) ?
Sol: Nitrogen does not have vacant d-orbitals on its valence shell. Therefore, it cannot extend its dπ-pπ bonding is not possible. As a result, the molecules of R₃N = 0 does not exist. However, phosphorus and rest of the members of the group 15 have vacant d-orbitals in the valence shell which can be involved in dπ-pπ bonding. Under the circumstances, R₃P=0 molecule can exist.

7.11. Explain why NH₃ is basic while BiH₃ is only feebly basic.
Sol: In both NH₃ and BiH₃, N and Bi have a lone paif of electrons on the central atom and hence should behave as Lewis bases. But NH₃ is much more basic than BiH₃. Since the atomic size of N is much smaller than that of Bi, therefore, electron density on N-atom is much higher than that on Bi-atom. Thus, the tendency of N in NH₃ to donate its lone pair of electrons is much more in comparison to tendency of Bi in BiH₃. Hence, NH₃ is more basic than BiH₃.

7.12. Nitrogen exists as diatomic molecule and phosphorus as P₄. Why?
Sol: Nitrogen exists as a diatomic molecule having a triple bond between the two N-atoms, This is due its small size that it forms pπ-pπ multiple bonds with itself and with carbon /oxygen as well. On the other hand, phosphorus due to its larger size does not form multiple pπ-pπ bonds with itself. It prefers to form P – P single bonds and hence it exists as tetrahedral P₄ molecule.

7.13. Write main differences between the properties of white phosphorus and red phosphorus.
Sol:

Structure of white and red phosphorus are given below:

7.14. Why does nitrogen show catenation properties less than phosphorus ? (C.B.S.E. Foreign 2009)
Sol: The valence shell electronic configuration of N is 2s²2p³. In order to complete the octet, the two nitrogen atoms share three electron pairs in the valence p-sub-shell and get linked by triple bond (N=N). Thus molecular nitrogen exists as discrete diatomic species and there is no scope of any self linking or catenation involving a number of nitrogen atoms. However, in case of phosphorus, multiple bonding is not feasible due to comparatively large atomic size of the element. Molecular phosphorus exists as tetra-atomic molecule (P4) in white phosphorus. These tetrahedrons are further linked by covalent bonds to form red variety which is in polymeric form. Thus, catenation in nitrogen is less than in phosphorus.

7.15. Give the disproportionation reaction of H₃ P0₃.
Sol: On heating, H₃ P0₄ undergoes self – oxidation-reduction, i.e: disproportionation to form PH₃.

7.16. Can PCl₅ act as an oxidising as well as a reducing agent Justify.
Sol: The oxidation state of P in PCl₅ is+5. Since P has five electrons in its valence shell, therefore, it cannot donate electron and cannot increase its oxidation state beyond + 5, Thus, PCl₅ cannot act as a reducing agent. It can act as oxidizing agent by itself undergoing reduction.

7.17. Justify the placement of O, S, Se, Te and Po in the same group’of the periodic table in terms of electronic configuration, oxidation state and hydride formation.
Sol: (1)Electronic configuration:
O (At. no. = 8) = [He] 2s² 2p⁴
S (At. no. = 16) = [Ne] 3s² 3p⁴
Se (At. no. = 34) = [Ar] 3d¹⁰ 4s² 4p⁴
Te (At. no. = 52) = [Kr] 4d¹⁰ 5s² 5p⁴ ,
Po (At. no. = 84) = [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁴ ,
Thus, all these elements have the same ns² np⁴ (n = 2 to 6) valence shell electronic configuration, hence are justified to be placed in group 16 of the Periodic Table.
(2)Oxidation state : Two more electrons are needed to acquire the nearest noble gas configuration. Thus, the minimum oxidation state of these elements should be – 2. O and to some extent S show – 2 oxidation state. Other element being more electropositive than O and S, do not show negative oxidation state. As these contain six electrons, thus, maximum oxidation state shown by them is+ 6. Other oxidation state shown by them are + 2 and + 4. O do not show+4 and + 6 oxidation state, due to the absence of d-orbitals. Thus, on the basis of maximum and minimum oxidation states, these elements are justified to be placed in the same group 16 of the periodic table.
(3)Hydride formation: All these elements share two of their valence electrons with 1 s- orbital of hydrogen to form hydrides of the general formula EH₂, i.e., H₂0, H₂S, H₂Se, H₂Te and H₂Po. Thus, on the basis of hydride formation, these elements are justified to be placed in the same group 16 of the Periodic Table.

7.18. Why is dioxygen a gas but sulphur a solid?
Sol: Due to the small size and high electronegativity, oxygen forms pπ- pπ multiple bonds. As a result, oxygen exists as diatomic (O₂) molecules. These molecules are held together by weak van der Waal’s forces of attraction which can be overcome by collisions of the molecules at room temperature. Therefore, O₂ is a gas at room temperature. Due to its bigger size and lower electronegativity, sulphur does not form pn-pn multiple bonds. It prefers to form S – S single bonds. S – S single bond is stronger then O-O single bond. Thus, sulphur has higher tendency for catenation than oxygen. Due to higher tendency for catenation and lower tendency for pπ – pπ multiple bonds sulphur exits as octa-atomic (S_g) molecule. Due to bigger size, the force of attraction holding the S_g molecules together are much stronger which cannot be overcome by collisions of molecules at room temperature. Therefore, sulphur is solid at room temperature.

7.19. Knowing the electron gain enthalpy values of O—>O^– and O—>O^2- as -141 and 702 kJ mol^-1 respectively, how can you account for the formation of a large number of oxides having O^2- species and not O^–?
Sol: Let us consider the reaction of oxygen with monopositve metal, we can have two compounds. MO(O in -1 state) and M₂O (O in -2 state). The energy required for formation of O^-2 is compensated by increased coulombic attraction between M⁺ and O^-2. Coulombic force of attraction, F_A is proportional to product of charges on ions i.e.

where q₁ and q₂ are charges on ions and r is distance between ions. Same logic can be applied if metal is dispositive.

7.20. Which aerosols deplete ozone?
Sol: Aerosols like chlorofluorocarbons (CFC’s), i.e., freon (CCl₂F₂), depletes the ozone layer by supplying Cl* free radicals which convert O₃ to O₂

7.21. Describe the manufacture of H₂SO₄ by contact process?
Sol: Preparation of sulphuric acid:By Contact Process: Burning of sulphur or sulphide ores in presence of oxygen to produce SO₂. Catalytic oxidation of SO₂ with O₂to give SO₃ in the presence of V₂O₅.

Then SO₃ made to react with sulphuric acid of suitable normality to obtain a thick oily liquid called oleum.

Then oleum is diluted to obtain sulphuric acid of desired concentration.

The sulphuric acid obtained by contact process is 96-98% pure.

7.22. How is SO₂ an air pollutant?
Sol: (1) SO₂ dissolves in moisture present in air to form H₂SO₄ which damages building materials especially marble (acid – rain).- CaCO₃ + H₂SO₃ ——->CaSO₃ + H₂0 + CO₂
(2)It corrodes metals like Fe and steel. It also brings about fading and deterioration of fabrics, leather, paper, etc., and affecting the colour of paints.
(3)Even in low concentration (= 0.03 ppm), it has damaging effect on the plants. If exposed for a long time, i.e., a few days or weeks, it slows down the formation of chlorophyll i. e., loss of green colour. This is called chlorosis.
(4)It is strongly irritating to the respiratory track. It cause throat and eye irritation, resulting into cough, tears and redness in eyes. It also cause breathlessness and effects larynx i. e. „ voice box.

7.23. Why are halogens strong oxidising agents?
Sol: Members of the halogen family act as strong oxidising agents on account of their electron accepting tendency both in the molecular as well as atomic form.

This is attributed to their high electronegativity, negative electron gain enthalpy values and alsp low bond dissociation enthalpies sinve they contain single covalent bonds(X — X) in their molecules. Fluorine is most reactive among the halogens and the reactivity down the group.

7.24. Explain why fluorine forms only one oxoacid, HOF.
Sol: Cl, Br and I form four series of oxo acids of general formula HOX, HOXO, HOXO₂, and H0XO₃. In these oxo-adds, the oxidation states of halogens are + 1, + 3, + 5, and + 7 respectively. However, due to high electronegativity, small size and absence of d-orbitals, F does not form oxo-acids with + 3, + 5 and + 7, oxidation states. It just forms one oxo-acid (HOF).

7.25. Explain why inspite of nearly the same electronegativity, nitrogen forms hydrogen bonding while chlorine does not.
Sol: Both .nitrogen (N) and chlorine (Cl) have electronegativity of 3.0. However, only nitrogen is involved in the hydrogen bonds (e.g., NH3) and not chlorine. This is due to smaller atomic size of nitrogen (atomic radius =70 pm) as compared to chlorine (atomic radius = 99) pm), therefore, N can cause greater polarisation of N-H bond than Cl in case of Cl—H bond.Consequently, N atom is involved in hydrogen bonding and not chlorine.

7.26. Write two uses of ClO₂
Sol: (1) ClO₂ is an excellent bleaching agent. It is 30 times stronger bleaching agent then the Cl₂. It is used as a bleaching agerit for paper pulp in paper industry and in textile industry. (2) ClO₂ is also a powerful oxidising agent and chlorinating agent. It acts as a germicide for disinfecting water. It is used for purifying drinking water.

7.27. Why are halogens coloured?
Sol: The halogens are coloured because their molecules absorb light in the visible region. As a result of which their electrons get excited to higher energy levels while the remaining light is transmitted. The color of halogens is the color of this transmitted light.

7.28. Write the reactions of F₂ and Cl₂ with water.
Sol:

7.29. How can-you prepare Cl₂ from HCl and HCl from CI₂? Write reactions only.
Sol:

7.30. What inspired N. Bartlett for carrying out reaction between Xe and PtF₆?
Sol: N. Bartlett observed that PtF₆ reacts with O₂to give an compound O₂+ [PtF₆]^–.
PtF₆ (g) + O₂ (g) ——–>O₂+[PtF₆]^–
Since the first ionization enthalpy of Xe (1170 kJ mol^-1 )is fairly close to that of 02 molecule (1175 kJ mol^-1 ), he thought that PtF₆ should also oxidise Xe to Xe⁺. This inspired Bartlett to carryout the reaction between Xe and PtF₆. When PtF₆ and Xe were made to react, a rapid reaction took place and a red solid, Xe+[PtF₆]^– was obtained.

7.31. What are the oxidation states of phosphorus in the following: –
(i) H₃PO₃ (ii)PCl₃
(iii) Ca₃P₂(iv)Na₃PO₄
(v) POF₃
Sol:

7.32. Write balanced equations for the following:
(i) NaCl is heated witlrsulphuric acid in the presence of MnO₂
(ii) Chlorine gas is passed into a solution of Nal in water.
Sol:

7.33. How are xenon fluorides XeF₂, XeF₄ and XeF₆ obtained?
Sol: XeF₂, XeF₄ and XeF₆ are obtained by direct reaction between Xe and F₂ as follows:

7.34. With which neutral molecule is ClO^– isoelectronic? Is this molecule Lewis acid or base ? (Pb. Board 2009)
Sol: ClO^– has (17 + 8 + 1) = 26 electrons. It is iso-electronic with two neutral molecules.
Oxygen difluoride (OF₂) : 8 + 18 = 26 electrons
Chlorine fluoride (ClF) : 17 + 9 = 26 electrons
Out of these, ClF can act as Lewis base. The atom chlorine has three lone electron pairs which it donates to form compounds like ClF₃, ClF₅ and ClF₇.

7.35. How are XeO₃ and XeOF₄prepared?
Sol:

7.36. Arrange the following in the order of property indicated for each set: –
(i) F₂ , Cl₂ , Br₂ , I₂ – increasing bond dissociation enthalpy.
(ii) HF, HCI, HBr, HI – increasing acid . strength.
(iii) NH₃, PH₃, AsH₃, SbH₃, BiH₃ – increasing Sol. base strength.
Sol: (i) Bond dissociation enthalpy decreases as the bond distance increases from F₂ to I₂ due to increase in the size of the atom, on moving from F to I.
F – F bond dissociation enthalpy is smaller then the Cl – Cl and even smaller than Br – Br. This is because F atom is very small and have large electron-electron repulsion among the lone pairs of electrons in F₂ molecule where they are much closer to each other than in case of Cl₂. The increasing order of bond dissociation enthalphy is I, < F₂ < Br₂ < Cl₂
(ii) Acid strength of HF, HCI, HBr and HI depends upon their bond dissociation enthalpies. Since the bond dissociation enthalpy of H – X bond decreases from H – F to H-l as the size of atom increases from F to I.
Thus, the acid strength order is HF < HCI < HBr < HI
The weak acidic strength of HF is also due to H-bonding due to which release of H⁺  becomes difficult.
(iii) NH₃, PH₃, ASH₃, SbH₃ and BiH₃ behaves as Lewis bases due to the presence of lone pair of electrons on the central atom. As we move from N to Bi, size of atom increases. Electron density on central atom decreases and hence the basic strength decreases from NH₃ to BiH₃. Thus basic strength order is BiH₃<SbH₃<AsH₃<PH₃<NH₃

7.37. Which one of the following does not exist ?
(i)XeOF₄ (ii)NeF₂
(iii)XeF₄ (iv)XeF₆
Sol: NeF₂ does not exist. This is because the sum of first and second ionization enthalpies of Ne are much higher than those of Xe. Consequently, F₂ can oxidise Xe to Xe²⁺ but cannot oxidise Ne to Ne²⁺.

7.38. Give the formula and describe the structure of a noble gas species which is isostructural with: (i) ICI₄^– (ii) IBr₂^– (iii) Br0₃^–
Ans: (i) ICI₄^–: In ICI₄^–, central atom I has seven valence electrons and one due to negative charge. Four out of these 8 electrons are utilized in forming four single bonds with four Cl atoms. Four remaining electrons constitutes the two lone pairs. It is arranged in square planar structure. ICI₄^– has 36 valence electrons. A noble gas species having 36 valence electrons is XeF₄ (8 + 4 x 7 = 36). XeF₄ is also square planar.

(ii) IBr₂^–: In IBr₂^–, central atom I has eight electrons. Two of these are utilized in forming two single bonds with two Br atom. Six remaining electrons constitutes three lone pairs. It is arranged in linear structure.

IBr₂^– has 22 valence electrons. A noble gas species having 22 valence electrons is XeF₂ (8+2 x 7=22).
XeF₂ is also linear.
(iii) In Br0₃^– ion the central Br atom has 8 valence electrons (7 +1). Out of these, it shares 4 with two atoms of O forming Br = O bonds. Out of the remaining four .electrons, 2 are donated to the third O atom which accounts for its negative charge. The remaining 2 electrons constitute one lone pair. In order to minimise the force of repulsion, the structure of Br0₃^– ion must be pyramidal. Br0₃^– ion has (7 + 3 x 6 + 1) = 26 valence electrons and is isoelectronic as well as iso-structural with noble gas species Xe0₃ which has also 26(8 + 3 x 6) electrons.

7.39. Why do noble gases have comparatively large atomic size?
Sol: The members of the noble gas family have comparatively large atomic size as compared to rest of the members present in the same period. Actually, for these elements, van der Waals’ radii are considered while for rest of the elements either covalent radii or metallic radii are taken into account. Since van der Waals’ radii arise simply due to van der Waals’ forces of attraction, these are expected to have comparatively large magnitude.

7.40. List the uses of neoirand argon gases.
Sol: Uses of Neon
Neon is used in discharge tubes and fluorescent bulbs for advertisement display purposes. Glow’of different colours ‘neon signs’ can be produced by mixing neon with other gases. Neon bulbs and used in botanical gardens and in green’ houses.
Uses of Argon
Argon is used mainly to provide an inert atmosphere in high temperature metallurgical processes such as arc welding of metals and alloys. In the laboratory, it is used for handling substance which are air sensitive.
It is used in filling incandescent and fluorescent lamps where its presence retards the sublimation of the filament and thus increases the life of the lamp.It is also used in “neon signs” for obtaining lights of different colours.

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