Chemistry Grade 11
Unit 5 Chemical Equilibrium

1: Which of the following is a characteristic of a reversible reaction? 
A) It proceeds to completion in one direction only 
B) The products cannot reform the reactants 
C) It can proceed in both forward and reverse directions simultaneously 
D) It always has a very large equilibrium constant 
Answer: C 
Explanation: Reversible reactions can proceed in both forward and reverse directions under the same conditions, eventually reaching a state of dynamic equilibrium.

2: At chemical equilibrium, which statement is TRUE? 
A) The concentrations of reactants and products are equal 
B) The forward and reverse reaction rates are equal 
C) All reactants have been converted to products 
D) The reaction has stopped completely 
Answer: B 
Explanation: At dynamic equilibrium, the rate of the forward reaction equals the rate of the reverse reaction, so concentrations remain constant but reactions continue at the molecular level.

3: Which condition is NOT required for a system to attain chemical equilibrium? 
A) The system must be closed 
B) Temperature must remain constant 
C) The reaction must be irreversible 
D) The reaction must be reversible 
Answer: C 
Explanation: Chemical equilibrium can only be attained in reversible reactions; irreversible reactions proceed to completion and cannot reach equilibrium.

4: For the reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the correct equilibrium constant expression (Kc) is: 
A) Kc = [NH₃] / ([N₂][H₂]³) 
B) Kc = [NH₃]² / ([N₂][H₂]³) 
C) Kc = ([N₂][H₂]³) / [NH₃]² 
D) Kc = [N₂][H₂]³ / [NH₃] 
Answer: B 
Explanation: Kc = [products]ᶜᵒᵉᶠᶠⁱᶜⁱᵉⁿᵗˢ / [reactants]ᶜᵒᵉᶠᶠⁱᶜⁱᵉⁿᵗˢ, so Kc = [NH₃]² / ([N₂][H₂]³).

5: If Kc >> 1 for a reaction, this indicates that at equilibrium: 
A) Reactants are favored 
B) Products are favored 
C) Reactants and products are present in equal amounts 
D) The reaction does not proceed 
Answer: B 
Explanation: A large Kc value (much greater than 1) means the equilibrium mixture contains mostly products; the reaction favors the forward direction.

6: For the equilibrium: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), Kc = 4.0 at a certain temperature. What is Kc for the reverse reaction: 2SO₃(g) ⇌ 2SO₂(g) + O₂(g)? 
A) 4.0 
B) 2.0 
C) 0.25 
D) 16.0 
Answer: C 
Explanation: The equilibrium constant for the reverse reaction is the reciprocal of the forward reaction: Kc(reverse) = 1/Kc(forward) = 1/4.0 = 0.25.

7: Which factor does NOT affect the value of the equilibrium constant Kc? 
A) Temperature 
B) Initial concentrations of reactants 
C) Presence of a catalyst 
D) Both B and C 
Answer: D 
Explanation: Kc depends only on temperature; it is unaffected by initial concentrations, pressure, volume, or catalysts (which only speed up attainment of equilibrium).

8: For the reaction: H₂(g) + I₂(g) ⇌ 2HI(g), Kc = 50 at 448°C. If [H₂] = 0.1 M, [I₂] = 0.2 M, and [HI] = 0.5 M at a given moment, what is the reaction quotient Qc and which direction will the reaction proceed? 
A) Qc = 12.5; reaction proceeds forward 
B) Qc = 12.5; reaction proceeds reverse 
C) Qc = 25; reaction proceeds forward 
D) Qc = 25; reaction proceeds reverse 
Answer: A 
Explanation: Qc = [HI]²/([H₂][I₂]) = (0.5)²/(0.1×0.2) = 0.25/0.02 = 12.5. Since Qc < Kc (12.5 < 50), the reaction proceeds forward to reach equilibrium.

9: According to Le Chatelier's principle, if the pressure is increased on the equilibrium system: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the equilibrium will shift: 
A) To the left, favoring reactants 
B) To the right, favoring products 
C) No shift occurs 
D) Depends on temperature 

Answer: B 
Explanation: Increasing pressure favors the side with fewer moles of gas. Reactants: 4 moles gas; products: 2 moles gas. So equilibrium shifts right to reduce pressure.

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