Equilibrium iMPORTANT QUESTION

Very Short Answer Type Questions

Question 1. What is meant by equilibrium?
Answer: Equilibrium is a state at which rate of forwarding reaction is equal to the rate of backward reaction.

Question 2. State the law of mass action?
Answer: It states that the rate at which a substance reacts is directly proportional to its molar concentration.

Question 3. What is meant by reaction quotient?
Answer: It is defined as the ratio of the product of molar concentration of products to the product of molar concentration of reactants at any stage of the reaction.

Question 4. Define ionic equilibrium.
Answer: The equilibrium between ions and unionised molecules is called ionic equilibrium.

Question 5. What is meant by ionic product of water (k_w)?
Answer: It is the product of concentration of [ H₃O⁺] and [OH^–] at a specific temperature.
Kw = [H₃O⁺] [OH^–]
= 1.0 x 10-14 at 298 K

Question 6. Define solubility product.
Answer:  It is product of molar concentration of ion raised to the power of number of ions produced per compound in saturated solution.

Question 7. How does common ion affect the solubility of electrolyte?
Answer: Solubility of electrolyte decreases due to common ion effect.

Question 8. Write conjugate add and conjugate base of H₂O?
Answer: Conjugate acid is H₃O⁺ and conjugate base is OH^–.

Question 9. Give two characteristics of a buffer solution.
Answer:

• Its pH does not change on the addition of small amount of acid or base.
• Its pH does not change on dilution or standing.

Question 10. How does a catalyst affect the equilibrium constant?
Answer: The equilibrium constant is not affected by a catalyst.

Question 11. State Ostwald’s dilution law.
Answer: Ostwald’s dilution law states that the degree of dissociation of weak electrolyte is inversely proportional to square root of its concentration.

Where, Kfl and Kfc are acid dissociation and base dissociation constants.

Question 12. What is basic buffer?
Answer: Basic buffer is the buffer whose pH is more than 7. It is a mixture containing weak base and its salt with a strong acid, e.g., NH₄Cl + NH₄OH

Question 13. S0₃^2-is Bronsted base or acid and why?
Answer: S0₃^2-is Bronsted base because it can accept H⁺.

Question 14.

What is the relationship between K_p and K_c?
Answer: K_p = K_c
because ∆n = 0.

Question 15. Define common ion effect. 
Answer: The supervision in concentration of one of the ions by adding other ion as common ion is called common ion effect.

Short Answer Type Questions

Question 1. The following concentration were obtained for the formation of NH₃ from N₂ and H₂ at equilibrium at 500 K.[N₂(g)] = 1.5 x 10^-2 M [H₂ (g)] = 3.0 x 10^-2 M [NH₃] = 1.2 x 10^-2 M. Calculate equilibrium constant.
Answer:

Question 2. Write the equilibrium constant (K_c) expression for the following reactions.

Answer:

Question 3. Given the equilibrium N₂O₄ (g) ——->2NO₂ (g)   K=0.15 atm at 298 K

• (a) What is K_p using pressure in torr?
• (b) What is K_c using units of moles per litre.

Answer:

Question 4. In the reaction A + B———> C + D, what will happen to the equilibrium if concentration of A is increased?

• (b) The equilibrium constant for a reaction is 2 x 10^-23  at 25°C and 2 x 10^-2 at 50°C. Is the reaction endothermic or exothermic?
• (c) Mention at least three ways by which the concentration of S0₃ can be increased in the following reaction in a state of equilibrium.

Answer: 

(a) The reaction will shift in the forward direction.
(b) Endothermic
(c) (i) lowering the temperature (ii) increasing pressure.
(iii) increasing concentration of oxygen.

Question 5. (i) Define Le Chatelier’s principle.
(ii) Following reactions occur in a Blast furnace.
Fe₂0₃(s) + 3CO(g) ———–>2Fe(s) + 3CO₂(g)
use Le chatelier’s principle to predict the direction of reaction when equilibrium mixture is disturbed by
(a) adding Fe₂0₃ (b) removing CO₂ .
(c) removing CO.
Answer: 

(a) When a system under equilibrium is subjected to a change in temperature, pressure or concentration, then the equilibrium shifts in such a direction so as to undo the effect of the change.
(ii) (a) On adding Fe₂0₃(s), the equilibrium will remain unaffected.
(b) By removing CO₂ (g), the equilibrium will be shifted in the forward direction.
(c) By removing CO(g), the equilibrium will be shifted in the backward direction.

Long Answer Type Questions

Question 1.

(i) Point out the differences between ionic product and solubility product.
(ii) The solubility of AgCl in water at 298 K is 1.06 x 10^-5 mole per litre. Calculate its solubility product at this temperature.
Answer: (i)

Question 2. At certain temperature and under a pressure of 4 atm, PCl₅ is 10% dissociated. Calculate the pressure at which PCl₅ will be 20% dissociated at temperature remaining constant.
Answer:  Calculation of K_p

HOTS Questions

Question 1. For the equilibrium 2 NOCl(g)——-> 2NO(g) + Cl₂(g) the value of the equilibrium constant Kc is 3.75 x 10^-6 at 1069 K. Calculate the K_p for the reaction at this temperature?
Answer: We know that K_p = K_c(RT)^∆n
For the above reaction, ∆n = (2 + 1) – 2 = 1 K_p = 3.75 x 10’6 (0.0831 x 1069)
K_p = 0.033.

Question 2. The values of Ksp of two sparingly soluble salts Ni(OH)₂ and AgCN are 2.0 x 10^-15  and 6 x 10^-17  respectively. Which salt is more soluble? Explain.
Answer:

Question 3. The value of Kc for the reaction 2A——>B + C is 2 x 10^-3. At a given time, the composition of reaction mixture is [A] = [B] = [C] = 3 x 10^-4 M. In which direction the reaction will proceed?
Answer: 

For the reaction the reaction quotient Q_c is given by Q_c = [B] [C]/[A]2 as [A] = [B] = [C] = 3 x 10-4 M Q_c = (3 x 10^-4) (3 x10^-3)/(3 x 10) = 1
as Q_c > K_c, so, the reaction will proceed in the reverse direction.

Question 4. PCl₅, PCl₃ and Cl₂ are at equilibrium at 500 K and having concentration 1.59M PCl₅ 1.59M Cl₂ and 1.41M PCl₅. Calculate K_c for the reaction PCl₅———>PC₁₃+ Cl₂
Answer: The equilibrium constant Kc for the above reaction can be written as:

Question 5. Dihydrogen gas is obtained from natural gas by partial oxidation with steam as per following endothermic reaction:

CH₄(g) + H₂O(g) ——> CO(g) + 3 H₂(g)

• (a) Write an expression for K_pfor the above reaction.
• (b) How will the values of K_p and composition of equilibrium mixture be affected by
  • (i) increasing the pressure
  • (ii) increasing the temperature
  • (iii) using a catalyst?

Answer:

(b) (i) value of K_p will not change, equilibrium will shift in backward direction.

(ii) value of K_p will increase and reaction will proceed in forward direction.

(iii) no effect.