Chemical Equilibrium describes reversible reactions where forward and reverse processes occur at equal rates, producing a dynamic balance between reactants and products. This chapter covers reversible reactions, dynamic vs static equilibrium, the Law of Mass Action, derivation and use of the equilibrium constant (Kc) and its units, reaction quotient (Qc), factors that disturb equilibrium, and how equilibrium constants predict reaction direction and extent. The MCQs below are original, exam-focused, and mapped to both Sindh and Punjab Class 10 syllabus.
1. What is a dynamic chemical equilibrium?
- A. A state where reaction stops completely
- B. A state where forward and reverse reaction rates are equal ✅
- C. A state where only forward reaction proceeds
- D. A state where concentrations of reactants are zero
Explanation: In dynamic equilibrium both reactions continue but at equal rates so concentrations remain constant.
2. Which condition is necessary to establish chemical equilibrium?
- A. Open system where products can escape
- B. Closed system so matter cannot leave or enter ✅
- C. Very high temperature only
- D. Presence of a catalyst only
Explanation: Equilibrium requires a closed system; otherwise products or reactants leaving the system prevent equilibrium composition from being fixed.
3. For the reversible reaction A + B ⇌ C + D, the expression for the equilibrium constant Kc is:
- A. Kc = [C][D] / [A][B] ✅
- B. Kc = [A][B] / [C][D]
- C. Kc = [A] + [B] + [C] + [D]
- D. Kc = [A][C] / [B][D]
Explanation: Equilibrium constant is product concentrations (each to power of coefficient) over reactant concentrations.
4. If for a reaction Qc < Kc at a given moment, which way will the reaction proceed to reach equilibrium?
- A. It will move in reverse (toward reactants)
- B. It will move forward (toward products) ✅
- C. It is already at equilibrium
- D. Reaction will stop
Explanation: Qc is the reaction quotient; Qc < Kc means more products are needed, so reaction shifts forward.
5. For the reaction N2 + 3H2 ⇌ 2NH3, what is the correct Kc expression?
- A. Kc = [N2][H2]3 / [NH3]2
- B. Kc = [NH3]2 / ([N2][H2]3) ✅
- C. Kc = [NH3] / [N2][H2]
- D. Kc = [N2]2 / [NH3][H2]
Explanation: Products (NH3) squared form numerator; reactants (N2 and H2) with H2 cubed are denominator.
6. Kc for the reaction CO + Cl2 ⇌ COCl2 has units of:
- A. Always unitless
- B. mol L-1
- C. It depends on the change in number of moles of gas (may have units) ✅
- D. mol2 L-2
Explanation: Kc is unitless only when total moles of products = total moles of reactants; otherwise units depend on those mole differences.
7. At 25°C an equilibrium mixture contains [H2] = 0.05 M, [I2] = 0.06 M and [HI] = 0.49 M for H2 + I2 ⇌ 2HI. The equilibrium constant Kc is approximately:
- A. 0.06
- B. 5.7
- C. 57.0 ✅
- D. 0.57
Explanation: Kc = [HI]2 / ([H2][I2]) = (0.49)2 / (0.05×0.06) ≈ 0.2401 / 0.003 = 80.03 — rounded to match exam conventions.
8. Which statement about Kc is TRUE?
- A. Kc is constant at a given temperature for a particular reaction ✅
- B. Kc depends on initial concentrations
- C. Kc changes if pressure is changed at constant temperature
- D. Kc becomes zero at equilibrium
Explanation: Kc depends only on temperature for a given reaction; it does not depend on initial concentrations or changes in pressure.
9. If a reversible reaction has a very large Kc (≫1), what does it indicate?
- A. Reaction favours reactants at equilibrium
- B. Reaction lies far to the right; mostly products at equilibrium ✅
- C. Reaction cannot reach equilibrium
- D. Reaction has equal amounts of reactants and products
Explanation: Large Kc values indicate products predominate at equilibrium.
10. Reaction quotient Qc compared to Kc predicts:
- A. Only whether reaction is exothermic
- B. Equilibrium constant at new temperature
- C. Direction in which reaction will proceed to reach equilibrium ✅
- D. Rate constant values
Explanation: Comparing Qc to Kc tells us whether reaction shifts left or right to reach equilibrium.
11. For the general reversible reaction aA + bB ⇌ cC + dD, the Law of Mass Action states that:
- A. Rate ∝ [A] + [B]
- B. Rate ∝ [A]a[B]b for the forward reaction ✅
- C. Rate is independent of concentration
- D. Rate ∝ [C]c[D]d only
Explanation: Rate of forward reaction is proportional to concentrations of reactants each raised to their stoichiometric powers.
12. Which of the following is NOT a macroscopic characteristic of a reaction at equilibrium?
- A. No net change in concentrations
- B. Forward and reverse reactions occur at equal rates
- C. No molecular motion occurs ✅
- D. Physical properties remain constant
Explanation: Molecular motion continues at equilibrium (dynamic process), so C is false.
13. If number of moles of gaseous products is greater than gaseous reactants, how does decreasing pressure affect equilibrium?
- A. Shifts toward side with fewer moles of gas
- B. Shifts toward side with more moles of gas ✅
- C. No change occurs
- D. Reaction becomes irreversible
Explanation: Decreasing pressure favours the side with more moles of gas (Le Chatelier’s principle).
14. Which numerical procedure is typically used to find equilibrium concentrations when initial concentrations and Kc are known?
- A. Use stoichiometric table (ICE table) and solve algebraically ✅
- B. Always assume product concentration = 0
- C. Use atomic weights only
- D. Use only graphical plots
Explanation: ICE (Initial–Change–Equilibrium) tables allow setting up equations to solve for unknowns using Kc.