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Equilibrium — NEET & JEE MCQs with answers
Equilibrium (Unit 6) covers both physical and chemical equilibria, where opposing processes occur at equal rates in a closed system, resulting in no net change in measurable properties. The unit extends from phase equilibria (solid–liquid, liquid–vapour, solid–vapour, dissolution) to chemical equilibria characterised by the equilibrium constant $Kc$ and $Kp$, Le Chatelier's principle, and the acid–base equilibria of aqueous solutions. Practical applications include buffer solutions, solubility products of sparingly soluble salts, and the industrial synthesis of ammonia.
Topics in this chapter
- 6.1 Equilibria Involving Physical Processes
- 6.2 Equilibrium in Chemical Processes - Dynamic Equilibrium
- 6.3 Law of Chemical Equilibrium and Equilibrium Constant
- 6.4 Homogeneous Equilibria
- 6.5 Heterogeneous Equilibria
- 6.6 Applications of Equilibrium Constants
- 6.7 Relationship between Equilibrium Constant K, Reaction Quotient Q and Gibbs Energy G
- 6.8 Factors Affecting Equilibria — Le Chatelier's Principle
- 6.9 Ionic Equilibrium in Solution
- 6.10 Acids, Bases and Salts
- 6.11 Ionization of Acids and Bases
- 6.12 Buffer Solutions
- 6.13 Solubility Equilibria of Sparingly Soluble Salts
Free practice questions
The value of $K_c$ for a reaction does NOT depend on:
- A. The stoichiometry of the balanced equation
- B. The temperature
- C. The change in Gibbs energy
- D. The rate of the reaction
Show answer
Answer: D — NCERT opens §6.7 by stating that the value of $K_c$ for a reaction does not depend on the rate of the reaction.
NCERT Class 11 Chemistry Ch 6
In the contact process equilibrium 2SO2(g) + O2(g) ⇌ 2SO3(g), the very large equilibrium constant (Kc = 1.7 × 10^26) indicates that the reaction:
- A. Favours the reactants strongly
- B. Cannot reach equilibrium
- C. Tends to go to completion (strongly favours products)
- D. Hardly proceeds at all
Show answer
Answer: C — The text notes the value of K is suggestive of the reaction going to completion (essentially complete conversion to products), although the rate is slow.
NCERT Class 11 Chemistry · Ch 6 · 6.8.5 Effect of a Catalyst
For a salt of a weak acid and a weak base, the pH of its solution is given by:
- A. pH = 7 + ½(pKb - pKa)
- B. pH = 7 - ½(pKa + pKb)
- C. pH = 7 + ½(pKa - pKb)
- D. pH = ½(pKa + pKb)
Show answer
Answer: C — NCERT eq. 6.38: pH = 7 + ½(pKa - pKb) for a salt of a weak acid and a weak base; the degree of hydrolysis is independent of concentration.
NCERT Class 11 Chemistry · Ch 6 · 6.11 Ionization of Acids and Bases
The solubility product of a salt of the type AB2 is 1.08 x 10^-10. Its molar solubility S (mol/L) is:
- A. 1.04 x 10^-5
- B. 6 x 10^-4
- C. 2.7 x 10^-11
- D. 3 x 10^-4
Show answer
Answer: D — Ksp = 4S^3, so S = (Ksp/4)^(1/3) = (1.08 x 10^-10/4)^(1/3) = 3 x 10^-4 mol/L.
NCERT Class 11 Chemistry · Ch 6 · 6.13 Solubility Equilibria of Sparingly Soluble Salts
A basic buffer has [Salt]/[Base] = 0.1 with pKb = 4.74. The pH of the buffer is (pH + pOH = 14):
- A. 8.26
- B. 9.26
- C. 10.26
- D. 3.74
Show answer
Answer: C — pOH = 4.74 + log 0.1 = 4.74 − 1 = 3.74; pH = 14 − 3.74 = 10.26.
NCERT Class 11 Chemistry · Ch 6 · 6.12 Buffer Solutions
The equilibrium constant expression is written from:
- A. the balanced chemical equation
- B. the rate law determined experimentally
- C. the order of the reaction
- D. the mechanism of the reaction
Show answer
Answer: A — The exponents in $K_c$ are the stoichiometric coefficients of the balanced equation, not experimentally determined orders.
NCERT Class 11 Chemistry Ch 6
For the reaction $PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)$, the relation between $K_p$ and $K_c$ is:
- A. $K_p = K_c(RT)^{-1}$
- B. $K_p = K_c(RT)$
- C. $K_p = K_c$
- D. $K_p = K_c(RT)^{2}$
Show answer
Answer: B — $\Delta n = (1+1) - 1 = +1$, so $K_p = K_c(RT)$.
NCERT Class 11 Chemistry · Ch 6 · 6.4 Relationship Between Kp and Kc
When a liquid evaporates in a closed container and equilibrium is reached, which of the following is true?
- A. The rate of evaporation exceeds the rate of condensation
- B. The rate of evaporation equals the rate of condensation
- C. Evaporation stops completely
- D. Condensation stops completely
Show answer
Answer: B — At equilibrium in a closed container the rate of evaporation equals the rate of condensation, giving a constant vapour pressure. The number of molecules leaving the liquid equals the number returning from the vapour.
NCERT Class 11 Chemistry · Ch 6 · 6.1 Equilibria Involving Physical Processes
The conjugate acid of the Brönsted base NH2- is:
- A. NH3
- B. N3-
- C. NH2OH
- D. NH4+
Show answer
Answer: A — Adding one proton to the amide ion NH2- gives NH3, its conjugate acid.
NCERT Class 11 Chemistry · Ch 6 · 6.10 Acids, Bases and Salts
For the equilibrium $\mathrm{CO_2(g) + C(s) \rightleftharpoons 2CO(g)}$, which statement is correct?
- A. It is heterogeneous, and carbon does not appear in the $K_p$ expression
- B. It is heterogeneous, and carbon appears in the $K_p$ expression
- C. It is homogeneous, and carbon appears in the $K_p$ expression
- D. It is homogeneous, and carbon does not appear in the $K_p$ expression
Show answer
Answer: A — Graphite is a pure solid, forming a phase separate from the gases; hence the equilibrium is heterogeneous and the solid carbon is omitted from $K_p$.
NCERT Class 11 Chemistry Ch 6
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