Hydrogen and iodine react together in a closed container to form hydrogen iodide. The equation for the reaction is: H₂(g) + I₂(g) ⇌ 2HI(g). At a certain temperature, the equilibrium concentrations are: [H₂] = 0.10 mol/dm³, [I₂] = 0.10 mol/dm³, and [HI] = 0.80 mol/dm³. Write the expression for the equilibrium constant, Kc, for this reaction, calculate its value, and explain what a large value of Kc tells us about the position of equilibrium.

WJEC A-Level Chemistry (Wales) — 3.8 Equilibrium constants · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Hydrogen and iodine react together in a closed container to form hydrogen iodide: H₂(g) + I₂(g) ⇌ 2HI(g). At equilibrium, the concentrations are: [H₂] = 0.10 mol/dm³, [I₂] = 0.10 mol/dm³, [HI] = 0.80 mol/dm³.

Model answer (5 marks)

Kc = [HI]²/([H₂][I₂])
Kc = (0.80)²/(0.10×0.10) = 0.64/0.01 = 64
A large Kc (64) indicates the equilibrium lies far to the right, i.e. the concentration of HI is much greater than that of H₂ and I₂ at equilibrium.

Examiner tips

  • Write the correct expression with exponents and correct order of terms.
  • Show the full substitution and calculation, keeping units consistent.
  • Explain that a large Kc means the product concentration dominates.
  • Use the phrase ‘equilibrium lies to the right’ to match the mark scheme.

Common mistakes

  • Using the wrong expression (e.g. Kc = [H₂][I₂]/[HI]²).
  • Calculating 0.64/0.1 instead of 0.01, giving 6.4 instead of 64.
  • Failing to explain that a large Kc favours the products.”

Mark scheme (5 marks)

  1. Correct Kc expression: Kc = [HI]² / ([H₂][I₂])
  2. Correct substitution of values into the expression: (0.80)² / (0.10 × 0.10)
  3. Correct value of Kc = 64
  4. A large value of Kc means the equilibrium lies to the right / favours the products
  5. A large Kc means the concentration of products is much greater than the concentration of reactants at equilibrium

Key terms in this question

equilibrium constant · Kc · equilibrium concentration · position of equilibrium

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