Gaokao Exam Gaokao Chemistry: Chemical Equilibrium and Reactions 1 — Questions and Answers
Question 1: For the equilibrium N₂(g) + 3H₂(g) ⇌ 2NH₃(g), increasing pressure at constant temperature will:
- Shift the equilibrium toward products (NH₃) (Correct answer)
- Shift the equilibrium toward reactants
- Have no effect on the equilibrium position
- Decrease the equilibrium constant
Correct answer: Shift the equilibrium toward products (NH₃)
By Le Chatelier's principle, increasing pressure favours the side with fewer moles of gas; the product side has 2 mol vs 4 mol reactants, so equilibrium shifts to products.
Question 2: The equilibrium constant expression for 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) is:
- K = [SO₃]²/([SO₂]²[O₂]) (Correct answer)
- K = [SO₂]²[O₂]/[SO₃]²
- K = [SO₃]/([SO₂][O₂])
- K = [SO₃]²[SO₂]²[O₂]
Correct answer: K = [SO₃]²/([SO₂]²[O₂])
The equilibrium constant expression uses products over reactants, each raised to the power of the stoichiometric coefficient.
Question 3: At 25°C, for the reaction H₂(g) + I₂(g) ⇌ 2HI(g), K = 50. If [H₂] = [I₂] = 0.1 M and [HI] = 0.5 M initially, the reaction quotient Q is:
- 25 (Correct answer)
- 50
- 100
- 5
Correct answer: 25
Q = [HI]²/([H₂][I₂]) = 0.25/(0.1 × 0.1) = 25. Since Q < K, the reaction proceeds forward.
Question 4: For an endothermic reaction at equilibrium, increasing temperature will:
- Increase K and shift equilibrium toward products (Correct answer)
- Decrease K and shift equilibrium toward products
- Increase K and shift equilibrium toward reactants
- Leave K unchanged
Correct answer: Increase K and shift equilibrium toward products
For an endothermic reaction, heat is a 'reactant'; adding heat (increasing T) shifts equilibrium to the product side and increases K.
Question 5: Adding a catalyst to a reaction at equilibrium:
- Speeds up both forward and reverse reactions equally, leaving equilibrium position unchanged (Correct answer)
- Shifts equilibrium toward products only
- Shifts equilibrium toward reactants only
- Changes the equilibrium constant
Correct answer: Speeds up both forward and reverse reactions equally, leaving equilibrium position unchanged
A catalyst lowers activation energy equally for forward and reverse reactions, reaching equilibrium faster but not changing K or the equilibrium position.
Question 6: For the reaction A(g) + B(g) ⇌ C(g) + D(g) with K = 4, if [A] = [B] = 1 M and [C] = [D] = x at equilibrium, then x =
- 2 M (Correct answer)
- 4 M
- 0.5 M
- 1 M
Correct answer: 2 M
K = x²/(1)(1) = 4 → x = 2. Wait — if initial concentrations are exactly [A]=[B]=1 at equilibrium, then x² = 4, x = 2.
Question 7: The degree of dissociation of a weak acid increases when:
- The solution is diluted with water (Correct answer)
- A common ion is added to the solution
- The solution is concentrated
- The temperature is decreased
Correct answer: The solution is diluted with water
Dilution decreases the ion concentration, shifting equilibrium toward more dissociation and increasing the degree of dissociation.
Gaokao Exam Gaokao Chemistry: Chemical Equilibrium and Reactions 1
For the equilibrium N₂(g) + 3H₂(g) ⇌ 2NH₃(g), increasing pressure at constant temperature will: