Chemical Equilibrium - Past JEE Questions

State True or False

1. If equilibrium constant for the reaction A2 + B2 ↔ 2AB, is K, then for the backward reaction AB ↔ ½ A2 + ½ B2, the equilibrium constant is 1/K.

(JEE 1984)

Answer: False

Reason: For A2 + B2 ↔ 2AB,
K = [AB] ²/[A2][B2]

For AB ↔ ½ A2 + ½ B2 equilibrium constant is

= [A2] ^1/2[B2] ^1/2/[AB]

This is not equal to 1/K.

2. Solubility of sodium hydroxide increases with increase in temperature.
(JEE 1985).

Answer: False

The dissolution of NaOH in water is an exothermic process. According to Le Chatelier principle, solubility decreases with increase in temperature. Increase in temperature favours backward reaction.



3. The rate of exothermic reaction increases with increasing temperature.
(JEE 1990).

Answer: True
Reason: In general, rate of a reaction increases with increase in temperature which is due to increase in the rate constant of the reaction. As per the Arrhenius equation relating k and T, as temperature increases k increases. (Note: At equilibrium if the temperature is increased, for an exothermic reaction the equilibrium shifts leftward).


Fill in blanks


4. For a given reversible reaction, at a fixed temperature equilibrium constants Kp and Kc are related by ____________________ .
(JEE 1994)

Answer : Kp = Kc (RT) ^{Δ v}g



5. A ten fold increase in pressure on the reaction

N2(g) + 3H2(g) ↔ 2NH3(g) at equilibrium results in _____________ in Kp.

(JEE 1996)

Answer: no change

Reason: Kp of an equilibrium reaction is independent of pressure of the system.

6. For a gaseous reaction 2B → A, the equilibrium constant Kp is _____________ to/than Kc.
(JEE 1997)

Answer: smaller




MCQs with multiple answers

7. When NaNO3 is heated in a closed vessel, oxygen is liberated and NaNO2 is left behind. At equilibrium

a. addition of NaNO2 favours reverse reaction.
b. addition of NaNO3 favours forward reaction.
c. increasing temperature favours forward reaction.
d. increasing temperature favours reverse reaction.

(1986)

Answer: ( c), (d),

Reason:

2NaNO3(s) ↔ 2NaNO2(s) + O2 (g)

K_p = Pressure of O2

Changing the quantities of NaNO3 and NaNO3 does not affect equilibrium position as K_p or Pressure of O2 remains constant.

Increasing temperature favours the forward reaction as this is endothermic direction.
Increasing pressure shifts the reaction in the backward direction as Pressure of O2 has to be kept constant.

8. For the reaction

PCl5 (g) ↔ PCl3 (g) + Cl2 (g)

The forward reaction at constant temperature is favoured by

a. introducing an inert gas at constant volume.
b. introducing chlorine gas at constant volume.
c. introducing an inert gas at constant pressure.
d. introducing PCl5 at constant volume.
e. increasing volume of the container
(1991)

Answer: c, d, and e.

Reason: Introducing an inert gas at constant pressure would increase the volume of the system. So the equilibrium shifts towards the side with larger number of gaseous molecules, i.e. the reaction proceeds in the forward direction.

Increasing volume produces the same result.

Introducing PCl5 at constant volume increases the concentration of the reactant and thus equilibrium is shifted in the forward direction.