Topic 7: Equilibria

Question 1

Definitions I:
(a) Reversible reaction
(b) Dynamic equilibrium
(c) Le Chatelier’s principle

Answer 1

Definitions I:
(a) Reaction that can proceed in both forward and reverse directions.
(b) State where forward and reverse reaction rates are equal, concentrations constant.
(c) Equilibrium shifts to minimise changes in conditions like pressure or temperature.

Question 2

Definitions II:
(a) Equilibrium constant, Kc
(b) Mole fraction
(c) Partial pressure

Answer 2

Definitions II:
(a) Ratio of product concentrations to reactants, each raised to stoichiometric powers.
(b) Ratio of moles of a gas to total moles in a mixture.
(c) Pressure exerted by one gas in a mixture, proportional to mole fraction.

Question 3

Definitions III:
(a) Strong acid
(b) Weak base
(c) Neutralisation

Answer 3

Definitions III:
(a) Acid fully dissociated in water, e.g., HCl → H⁺ + Cl⁻.
(b) Base partially dissociated in water, e.g., NH₃ ⇌ NH₄⁺ + OH⁻.
(c) Reaction of H⁺(aq) with OH⁻(aq) to form H₂O(l).

Question 4

In the Haber process (N₂ + 3H₂ ⇌ 2NH₃), how do pressure and temperature affect equilibrium? [3 Marks]

Answer 4

High pressure shifts right, more NH₃ (4 → 2 moles gas) (1 mark)
High temperature shifts left, reaction exothermic (1 mark)
Balance yield vs. rate (1 mark)

Question 5

For 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), Kc = 280 at 1000 K, calculate [SO₃] if [SO₂] = 0.1 mol/dm³, [O₂] = 0.2 mol/dm³. [2 Marks]

Answer 5

Kc = [SO₃]² / ([SO₂]²[O₂]), 280 = [SO₃]² / (0.1² × 0.2) (1 mark)
[SO₃] = √(0.56) = 0.75 mol/dm³ (1 mark)

Question 6

Sketch the pH titration curve for HCl vs. NaOH and CH₃COOH vs. NH₃. [3 Marks]

Answer 6

HCl vs. NaOH: steep rise at pH 7 (1 mark)
CH₃COOH vs. NH₃: gradual, midpoint ~9 (1 mark)
Labelled axes, equivalence points (1 mark)

Question 6

In a sealed flask, H₂ + I₂ ⇌ 2HI has Kp = 50 at 700 K. What affects Kp? [2 Marks]

Answer 6

Only temperature affects Kp (1 mark)
Pressure, concentration, catalyst change position, not constant (1 mark)

Question 7

Name the formulas of:
(a) Two common acids [1 Mark]
(b) Two common alkalis [1 Mark]

Answer 7

(a) HCl, H₂SO₄ (1 mark)
(b) NaOH, NH₃ (1 mark)

Question 8

For the Contact process (2SO₂ + O₂ ⇌ 2SO₃), why use 450°C and a catalyst? [2 Marks]

Answer 8

450°C balances exothermic yield and rate (1 mark)
V₂O₅ catalyst lowers activation energy, increases SO₃ (1 mark)

Question 8

Why does adding Mg to HCl produce H₂ faster than to CH₃COOH? [2 Marks]

Answer 8

HCl fully dissociates, higher [H⁺], faster reaction (1 mark)
CH₃COOH weak, lower [H⁺], slower (1 mark)

Question 9

If 0.5 mol N₂ and 1.5 mol H₂ form 0.8 mol NH₃ at equilibrium, calculate Kc (V = 1 dm³). [2 Marks]

Answer 9

[N₂] = 0.1, [H₂] = 0.3, [NH₃] = 0.8 mol/dm³ (1 mark)
Kc = (0.8²) / (0.1 × 0.3³) = 237 (1 mark)

Question 10

Select an indicator for titrating HNO₃ with KOH (strong acid-strong base). [2 Marks]

Answer 10

Phenolphthalein, changes at pH 8-10 (1 mark)
Matches sharp pH jump at equivalence (1 mark)