NCERT Exemplar Class 11 Chemistry Chapter 7 Equilibrium

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NCERT Exemplar Class 11 Chemistry Chapter 7 Equilibrium

Multiple Choice Questions
Single Correct Answer Type

Q1. We know that the relationship between K_c and K_p is K_p = K_c(RT)^∆nWhat would be the value of An for the reaction NH₄C1(s) ⇆ NH₃(g) + HCl(g)?
(a) 1
(b) 0.5
(c) 1.5
(d) 2
Sol: (d) The relationship between K_p and K_c is
K_p = K_c (RT)^∆ⁿWhere ∆n = (number of moles of gaseous products) – (number of moles of gaseous reactants)
For the reaction,
NH₄C1(s) ⇆ NH₃(g) + HCl(g)
∆n = 2 – 0 = 2

Q2. For the reaction H₂(g) + I₂(g) ⇌2Hl(g), the standard free energy is ∆G° >0 The equilibrium constant (K) would be
(a) K = 0
(b)> 1
(c) K=1
(d)A:<l
Sol: (d) ∆G° = -RTln K. ∆G° > 0 means ∆G° is +ve. This can be so only if In K is -ve, i.e., K< 1.

Q3. Which of the following is not a general characteristic of equilibria involving physical processes?
(a) Equilibrium is possible only in a closed system at a given temperature.
(b) All measurable properties of the system remain constant.
(c) All the physical processes stop at equilibrium.
(d) The opposing processes occur at the same rate and there is dynamic but stable condition.
Sol: (c) All the physical processes like melting of ice and freezing of water, etc., do not stop at equilibrium.

Q4. PC1₅, PC1₃ and Cl₂ are at equilibrium at 500 K in a closed container and their concentrations are 0.8 x 10“³ molL^-1, 1.2 x 10^-3 mol L ¹ and 1.2 x 10^-3 mol L^-1 The value of K_c for the reaction PCl₅(g) ⇌PCl₃(g) + Cl₂(g) will be
(a) 8 x 10³ molL^-1
(b) 1.8 x 10^-3 molL^-1(c) 1.8 x 10“³ molL^-1
(d) 0.55 x 10⁴ molL^-1Sol: (b) PCl₅(g) ⇌ PCl₃(g) + Cl₂(g)

This equilibrium constant calculator will help you understand reversible chemical reactions.

Q5. Which of the following statements is incorrect?
(a) In equilibrium mixture of ice and water kept in perfectly insulated flask, mass of ice and water does not change with time.
(b) The intensity of red colour increases when oxalic acid is added to a solution containing iron (III) nitrate and potassium thiocyanate.
(c) On addition of catalyst the equilibrium constant value is not affected.
(d) Equilibrium constant for a reaction with negative AH value decreases as the temperature increases.

Sol: (b)Fe³⁺+SCN^– ⇌ FeSCN²⁺(Red)
When oxalic acid is added to a solution containing iron nitrate and potassium thiocyanate, oxalic acid reacts with Fe³⁺ ions to form a stable complex ion [Fe(C₂0₄)₃]^3-, thus, decreasing the concentration of free Fe³⁺ ions which in mm decreases the intensity of red colour.
Fe³⁺ + SCN^–⇌ [Fe(SCN)]²⁺ _(Red)

Q6. When hydrochloric acid is added to cobalt nitrate solution at room temperature, the following reaction takes place and the reaction mixture becomes blue. On cooling the mixture, it becomes pink. On the basis of this information mark the correct answer.
[Co(H₂0)₆]²⁺ (aq) + 4Cl^–(aq) [CoCl₄]²-(aq) + 6H₂0(l)
(Pink) (Blue)
(a)∆H > 0 for the reaction
(b) ∆H < 0 for the reaction
(c) ∆H = 0 for the reaction
(d) The sign of ∆H cannot be predicted on the basis of this information.
Sol:(a) Since the reaction shifts to backward direction on cooling, this means that the backward reaction is exothermic reaction. Therefore, the forward reaction is endothermic reaction and ∆H > 0.

Q7. The pH of neutral water at 25°C is 7.0. As the temperature increases, ionization of water increases, however, the concentration of H⁺ ions and OH^– ions are equal. What will be the pH of pure water at 60°C?
(a) Equal to 7.0
(b) Greater than 7.0
(c) Less than 7.0
(d) Equal to zero
Sol:(c) At 25°C, [H⁺] = [OH^–] = 10 ⁷ and K_w = [H⁺] [OH ^–] = 10^-14. On heating, K_w increases, i.e., [H⁺] [OH^–] > 10^-14 As [H⁺] = [OH^–], [H⁺]² > 10^-14 or [H⁺] > 10^-7 M or pH < 7.

Q8. The ionization constant of an acid, K_a is the measure of strength of an acid. The K_a values of acetic acid, hypochlorous acid and formic acid are 1.74 x 10^-5, 3.0 x 10^-8 and 1.8 x 10^-4 Which of the following orders of pH of 0.1 mol dm^-3 solutions of these acids is correct?
(a) acetic acid > hypochlorous acid > formic acid
(b) hypochlorous acid > acetic acid > formic acid
(c) formic acid > hypochlorous acid > acetic acid
(d) formic acid > acetic acid > hypochlorous acid

Q10. Acidity of BF3 can be explained on the basis of which of the following concepts?
(a) Arrhenius concept
(b) Bronsted Lowry concept
(c) Lewis concept
(d) Bronsted Lowry as well as Lewis concept
Sol: (c) According to Lewis concept, a positively charged or an electron deficient species acts as Lewis acid. BF₃ is an electron deficient compound with B having 6 electrons only.

Q11. Which of the following will produce a buffer solution when mixed in equal volumes?
(a) 1 mol dm^-3 NH₄OH and 0.1 mol dm^-3 HC1
(b) 0.05 mol dm^-3 NH₄OH and 0.1 mol dm^-3 HC1
(c) 1 mol dm^-3 NH₄OH and 0.05 mol dm^-3 HC1
(d) 1 mol dm^-3 CH₃COONa and 0.1 mol dm^-3 NaOH

Sol: (c) In (c), all HC1 will be neutralized and NH₄C1 will be formed. Also some NH₄OH will be left unneutralized. Thus, the final solution will contain NH₄OH and NH₄C1 and hence will form a buffer.

Q12. In which of the following solvents is silver chloride most soluble?
(a) 0.1 mol dm^-3 AgN0₃ solution
(b) 0.1 mol dm^-3 HC1 solution
(c) H₂0
(d) Aqueous ammonia
Sol: (d) Silver chloride forms a soluble complex with aqueous ammonia.
AgCl + 2NH₃→ [Ag(NH₃)₂]Cl

Q13. What will be the value of pH of 0.01 mol dm^-3 CH₃COOH (K._A -74 x 10^-5)?
(a) 3.4
(b) 3.6
(c) 3.9
(d) 3.0

Q14. K_a for CH₃COOH is 1.8 x 10^-5 and K_b for NH₄OH is 1.8 x 10^-5. The pH of ammonium acetate will be
(a) 7.005
(b) 4.75
(c) 7.0
(d) Between 6 and 7
Sol: (c) Ammonium acetate is a salt of weak acid and weak base.

Q15. Which of the following options will be correct for the stage of half completion of the reaction A ⇌B.
(a) ΔG° = 0
(b) ΔG° > 0
(c) ΔG° < 0
(d) ΔG° = -RT In 2
Sol: (a) A ⇌B
ΔG° = -RT In K
At the stage of half completion of reaction [A] = [B],
Therefore, K = 1. Thus, ΔG° = 0.

Q16. On increasing the pressure, in which direction will the gas phase reaction proceed to re-establish equilibrium, is predicted by applying the Le Chatelier’s principle. Consider the reaction,
N₂(g) + 3H₂(g) ⇌2NH₃(g)
Which of the following is correct, if the total pressure at which the equilibrium is established, is increased without changing the temperature?
(a) K will remain same
(b) K will decrease
(c) K will increase
(d) K will increase initially and decrease when pressure is very high
Sol:(a)N₂(g) + 3H₂(g) ⇌2NH₃(g)
According to Le Chatelier’s principle, at constant temperature, the equilibrium composition will change but K will remain same.

Q17. What will be the correct order of vapour pressure of water, acetone and ether at 30°C? Given that among these compounds, water has maximum boiling point and ether has minimum boiling point.
(a) Water < Ether < Acetone
(b) Water < Acetone < Ether
(c) Ether < Acetone < Water
(d) Acetone < Ether < Water
Sol: (b) Greater the boiling point, less is the vapour pressure. Hence, the correct order of vapour pressures will be:
water < acetone < ether.

Q19. In which of the following reactions, the equilibrium remains unaffected on addition of small amount of argon at constant volume?
(a) H₂(g) + I₂(g) ⇌2HI(g) ‘
(b) PCl₅(g) ⇌PCl₃(g) + Cl₂(g)
(c) N₂(g) + 3H₂(g) ⇌2NH₃(g)
(d) The equilibrium will remain unaffected in all the three cases.
Sol: (d) The equilibrium will remain unaffected in all three cases on addition of small amount of inert gas at constant volume.

Q20. For the reaction N₂0₄(g) ⇌2N0₂(g), the value of K is 50 at 400 K and 1700 at 500 K. Which of the following options is correct?
(a) The reaction is endothermic.
(b) The reaction is exothermic.
(c) If NO₂(g) and N₂0₄(g) are mixed 400 K at partial pressures 20 bar and 2 bar respectively, more N₂0₄(g) will be formed.
(d) The entropy of the system increases.
Sol: (a, c, d)
(a) As the value of K increases with increase of temperature and K = K_f / K_b, this means that k_f increases, i.e., forward reaction is favoured.
Hence, reaction is endothermic.

Q21. At a particular temperature and atmospheric pressure, the solid and liquid phases of a pure substance can exist in equilibrium. Which of the following term defines this temperature? .
(a) Normal melting point
(b) Equilibrium temperature
(c) Boiling point
(d) Freezing point
Sol: (a, d) These are normal melting point and freezing point since they are measured at atmospheric pressure.

Q22. The ionization of hydrochloric acid in water is given below:
HCl(aq) + H₂0(l) ⇌H₃0 ⁺ (aq) +Cl^–(aq)
Label two conjugate acid-base pairs in this ionization.
Sol: HCl (Acid) Cl^– (Conjugate base)
H₂0 (Base) H₃0⁺ (Conjugate acid

Q23. The aqueous solution of sugar does not conduct electricity. However, when sodium chloride is added to water, it conducts electricity. How will you explain this statement on the basis of ionization and how is it affected by concentration of sodium chloride?
Sol: (i) Sugar being a non-electrolyte does not ionize in water, whereas NaCl ionizes completely in water and produces Na⁺ and Cl^– ions which help in the conduction of electricity.
(ii) When concentration of NaCl is increased, more Na⁺ and Cl ions will be produced. Hence, conductance increases.

Q24. BF₃ does not have proton but still acts as an acid and reacts with NH₃. Why is it so? What type of bond is formed between the two?
Sol: BF₃ is an electron deficient compound. Hence, it acts as Lewis acid. NH₃has a lone pair of electrons. Hence, acts as Lewis base. A coordinate bond is formed between the two.
H₃N: →BF₃

Q25. Ionization constant of a weak base MOH, is given by the expression

Values of ionization constant of some weak bases at a particular temperature are given below:

Base	Dimethylamine	Urea	Pyridine	Ammonia
	5.4 x 10^-4	1.3 x 10^-14	1.77 x lO^-9	1.77 xlO^-5

Arrange the bases in decreasing order of the extent of their ionization at equilibrium. Which of the above base is the strongest?
Sol: Greater is the ionization constant (K_b) of a base, greater is the ionization of the base. Order of extent of ionization at equilibrium is dimethylamine > ammonia > pyridine > urea. Dimethylamine is the strongest base due to maximum value of K_b.

Q26. Conjugate acid of a weak base is always stronger. What will be the decreasing order of basic strength of the following conjugate bases?
OH^–, RO^–, ch₃coo^– , cl^–Sol: Conjugate acids of given bases are H₂0, ROH, CH₃COOH, HC1.
Their acidic strength is in the order
HCl > CH₃COOH > H₂0 >ROH Hence, basic strength is in the order RO^– > OH^– > CH₃COO^– > Cl^–

Q27. Arrange the following in increasing order of pH.
KN0₃(aq), CH₃COONa(aq), NH₄Cl(aq), C₆H₅COONH₄(aq)
Sol: KN0₃: salt of strong acid-strong base, solution is neutral, pH = 7. CHjCOONa: salt of weak acid-strong base, solution is basic, pH > 7.
NH₄Cl: salt of strong acid-weak base, solution is acidic, pH < 7.
C₆H₅COONH₄: both weak put NH₄OH is slightly stronger than C₆H₅COOH, pH close to 7 but slightly > 7.
Hence, in order of pH, NH₄C1 < C₆H₅COONH₄ > KN0₃ < CH₃COONa.

Q28. The value of K_c for the reaction
2HI(g) ⇌H₂(g) + I₂(g) is 1 x 10^-4. At a given time, the composition of reaction mixture is [HI] = 2 x 10^-5 mol, [H₂] = 1 x 10^-5 mol and [I₂] = 1 x 10^-5 mol. In which direction will the reaction proceed?

Q29. On the basis of the equation pH = -log [H⁺], the pH of 10^-8 mol dm^-3 solution of HC1 should be 8. However, it is observed to be less than 7.0. Explain the reason.
Sol: Concentration 10^-8 mol dm^-3 indicates that the solution is very dilute. So, we cannot neglect the contribution of H₃0⁺ ions produced from H₂0 in the solution. Total [H₃0⁺] = 10^-8 + 10^-7 M. From this we get the value of pH close to 7 but less than 7 because the solution is acidic.
From calculation, it is found that pH of 10^-8 mol dm^-3 solution of HC1 is equal to 6.96.

Q30. pH of a solution of a strong acid is 5.0. What will be the pH of the solution obtained after diluting the given solution a 100 times?
Sol: pH = 5 i.e., [H⁺] = 10^-5 mol L^-1On dilution by 100 times [H⁺] = 10^-7 mol L^-1For a very dilute solution,
Total [H⁺] = [H₃0⁺ ions from acid] + [H₂0⁺ ions from water]
= 10^–⁷ + 10^–⁷pH = -log[H⁺] = -log (2 x 10^-7) = 7 – log 2
= 7-0.3010 = 6.6990

Q31. A sparingly soluble salt gets precipitated only when the product of concentration of its ions in the solution (Q_sp) becomes greater than its solubility product. If the solubility of BaS0₄ in water is 8 x 10^-4 mol dm^-3, calculate its solubility in 0.01 mol dm^-3 of H₂S0₄.

Q32. pH of 0.08 mol dm^–³HOC1 solution is 2.85. Calculate its ionization constant.
Sol: pH of HOC1 = 2.85
-pH = log [H⁺] or -2.85 = log [H⁺]
=> [H⁺] = 1.413 x 10^-3

Q33. Calculate the pH of a solution formed by mixing equal volumes of two solutions A and B of a strong acid having pH = 6 and pH = 4 respectively.
Sol: pH of solution A = 6
[H⁺] = 10^-6mol L ¹pH of solution B = 4
[H⁺] = 10^-4 molL^-1On mixing one litre of each solution Total volume = 1 L + 1 L = 2 L
Total amount of H⁺ in 2 L solution formed by mixing solutions A and B = 10^-6 + 10^-4 mol

Q34. The solubility product of Al(OH)₃ is 2.7 x 10^-11. Calculate its solubility in g^–L and also find out pH of this solution. (Atomic mass of A1 = 27 u).

Q35. Calculate the volume of water required to dissolve 0.1 g lead (II) chloride to get a saturated solution. (K_sp of PbCl₂ = 3.2 x 10^-8, atomic mass of Pb = 207 u).

Q36. A reaction between ammonia and boron trifluoride is given below:
:NH₃ + BF₃ →H₃N : BF₃Identify the acid and base in this reaction. Which theory explains it? What is the hybridization of B and N in the reactants?
Sol:Although BF₃ does not have a proton but acts as Lewis acid as it is an electron deficient compound. It reacts with NH₃ by accepting the lone pair of electrons from NH₃ and completes its octet. The reaction can be represented by
BF₃ + :NH₃→BF₃ ← :NH₃Lewis electronic theory of acids and bases can explain it. Boron in BF₃ is sp²hybridised, whereas N in NH₃ is sp³ hybridised.

Q37. Following data is given for the reaction:
CaC0₃(s) → CaO(s) + C0₂(g)
∆_fH° [CaO(s)] = -635.1 kJ mol^–¹∆_fH° [CO_z(g)] = -393.5 kJ mol^–¹∆_fH° [CaC0₃(s)] = -1206.9 kJ mol^–¹Predict the effect of temperature on the equilibrium constant of the above reaction.
Sol: ∆_rH° = ∆_fH° [CaO] + ∆_fH° [C0₂] – ∆_fH° [CaC0₃]
= [-635.1] + [-393.5] – [-1206.9] = 178.3 kJ mol^-1Thus, the reaction is endothermic. Hence, according to Le Chatelier’s principle, on increasing the temperature, the equilibrium will proceed in the forward direction.
Matching Column Type Questions
Q38. Match the following equilibria with the corresponding condition

Column I		Column II
(i)	Liquid⇌Vapour	(a)	Saturated solution
(ii)	Solid ⇌Liquid	(b)	Boiling point
(iii)	Solid ⇌Vapour	(c)	Sublimation point
(iv)	Solute(s) ⇌Solute (solution)	(d)	Melting point ‘
		(e)	Unsaturated solution

Sol: (i) →(b), (ii) → (d), (iii) → (c), (iv) → (a)

(i) Liquid ⇌Vapour equilibrium exists at the boiling point.
(ii) Solid ⇌Liquid equilibrium exists at the melting point.
(iii) Solid ⇌ Vapour equilibrium exists at the sublimation point.
(iv) Solute(s) ⇌Solute (solution) equilibrium exists in saturated solution.

Q39. For the reaction: N₂(g) + 3H₂(g) ⇌2NH₃(g) equilibrium constant,

Some reactions are written below in Column I and their equilibrium constants in terms of K_c are written in Column II. Match the following reactions with the corresponding equilibrium constant.

Sol: (i) →(d), (ii) → (c), (iii) → (b)
ForN₂(g) + 3H₂(g)⇌2NH₃(g)

Q40. Match standard free energy of the reaction (Column I) with the corresponding equilibrium constant (Column II).

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