# 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

## 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

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

## Related

- [Revision notes for Eduqas A-Level Chemistry](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/the-reaction-between-hydrogen-and-iodine-00cbad39) · Published by Druglandscape Ltd.