# 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

> 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.

## 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

- [equilibrium constant](https://www.gradenine.co.uk/glossary/equilibrium-constant)
- [Kc](https://www.gradenine.co.uk/glossary/kc)
- [equilibrium concentration](https://www.gradenine.co.uk/glossary/equilibrium-concentration)
- [position of equilibrium](https://www.gradenine.co.uk/glossary/position-of-equilibrium)

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/hydrogen-and-iodine-react-together-in-f0342abe) · Published by Druglandscape Ltd.