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.

Acids and bases - Past JEE questions

(Bronsted and Lewis concepts);

1. State whether the following statement is true or false

Aluminium chloride (AlCl3) is a Lewis acid because it can donate electrons. (1982)

2. Fill in the blanks

The conjugate base of HSO4ˉ in acqueous solution is ___________. (1982)


3. The conjugate acid of NH2ˉ is:
a. NH3
b. NH2OH
c. NH4⁺
d. N2H4

(1985)


Answers some time later

Chemical Equilibrium - Multiple Choice Questions

One Answer only

1. The ion that cannot be precipitated by both HCl and H2S is

a. Pb²⁺
b. Ag⁺
c. Cu⁺
d. Sn²⁺

(JEE 1982)

Ans: (d)

2. A certain buffer solution contains equal concentration of X^- and HX. The Kb for X^- is 10^-10. The pH of the buffer is:

a. 4
b. 7
c. 10
d. 14
(JEE 1984)

ans: a

3. A certain weak acid has a dissociation constant of 1.0*10^-4. The equilibrium constant for its reaction with a strong base is:

a. 1.0*10^-4
b. 1.0*10^-10
c. 1.0*10¹⁰
d. 1.0*10¹⁴
(JEE 1984)

Answer: ( c)

4. An example of a reversible reaction is:
(JEE 1985)

5. The conjugate acid of NH₂^- is:

a. NH3
b. NH4OH
c. NH₄⁺
d. N2H4
(JEE 1985)

Answer: (a)

6. the compound that is not a Lewis acid is:

a. BF3
b. AlCl3
c. BeCl2
d. SnCl4
(JEE 1985)

Answer: (c )

BeCl2 cannot accept a lone pair of electrons.


7. the compound whose 0.1 M solution is basic is:

a. ammonium acetate]
b. ammonium chloride
c. Ammonium sulphate
d. sodium acetate
(JEE 1986)

Answer (d)

Chemical Equilibrium - True or False Type Questions

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

3. From the solution containing copper (+2) and zinc (+2) ions , copper can be selectively precipitated using sodium sulphide. (JEE 1987)

False

Na2S will produce an alkaline solution, both copper (+2) and zinc (+2) ions would be precipitated.

4. 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).

Chemical Equilibrium (Ionic Equilibrium) - Fill Blanks

1. The conjugate base of HSO₄^- in acqueous solution is_____________.

(JEE 19820)

Answer: SO₄^2-

2. The hydrolysis of ethyl acetate in ______________________ medium is a _________ order reaction.

(JEE 1986)

Answer : acidic, first order

3. ____________ acid gives _____________ ion.
(options given: hydrobromic acid, hypobromous. Perbromic, bromide, bormite, perbromate)

(JEE 1988)

Answer: Hydrobromic acid, bromide (*Refer TMH Chemistry IIT 2007-Table 7.1)

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


4a. Solubility of iodine in water is greatly increased by the addition of iodide ions because of the formation of _______________________. (JEE 1994)

Answer: I3^-,

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

7. In the reaction I^- + I2 → I3^-, the Lewis acid is _______________ .
(JEE 1997)
Answer: I2