The reaction between hydrogen and iodine reaches a dynamic equilibrium in a closed container. H₂(g) + I₂(g) ⇌ 2HI(g) Write the expression for the equilibrium constant, Kc, for this reaction. Then explain what a large value of Kc tells us about the position of equilibrium and the relative amounts of products and reactants present at equilibrium.

Eduqas A-Level Chemistry — 3.8 Equilibrium constants · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Kc = [HI]² / ([H₂][I₂])
A large Kc indicates that the equilibrium lies to the right, i.e. the concentration of HI is much greater than that of H₂ and I₂ at equilibrium. Kc is a constant that depends only on temperature, not on the initial concentrations or pressure.

Examiner tips

  • Write the Kc expression correctly with products in the numerator and reactants in the denominator.
  • Explain that a large Kc means the equilibrium lies to the right and that product concentrations dominate.
  • Mention that Kc is temperature‑dependent only.
  • Use the exact wording from the mark scheme.

Mark scheme (5 marks)

  1. Correct Kc expression: Kc = [HI]² / ([H₂][I₂])
  2. Products appear in the numerator and reactants in the denominator of the Kc expression
  3. A large Kc means the equilibrium position lies to the right (towards the products)
  4. A large Kc means the concentration of products is much greater than the concentration of reactants at equilibrium
  5. Kc only changes with temperature (not with changes in concentration or pressure)

Key terms in this question

equilibrium constant · Kc · dynamic equilibrium · position of equilibrium

Related

More Equilibrium constants questions

▶ Try answering this question with AI marking (free) →