# The reaction between hydrogen and iodine vapour is reversible. In a closed container, the following reaction reaches dynamic equilibrium: H₂(g) + I₂(g) ⇌ 2HI(g). The forward reaction is exothermic. Explain what happens to the position of equilibrium and to the concentration of HI when the temperature of the system is increased.

> Pearson Edexcel International GCSE Chemistry (4CH1) — 3.3 Reversible reactions and equilibria · Explain · 4 marks

## Mark scheme (4 marks)

1. Increasing temperature shifts the equilibrium position in the direction of the endothermic reaction
2. The backward/reverse reaction is endothermic (because the forward reaction is exothermic)
3. Therefore the equilibrium position shifts to the left (towards the reactants)
4. The concentration of HI decreases

## Key terms

- [dynamic equilibrium](https://www.gradenine.co.uk/glossary/dynamic-equilibrium)

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Source: [GradeNine](https://www.gradenine.co.uk/q/the-reaction-between-hydrogen-and-iodine-0dfc1d68) · Published by Druglandscape Ltd.