# A student sets up a sealed flask containing the following reversible reaction at constant temperature: H₂(g) + I₂(g) ⇌ 2HI(g). The system reaches dynamic equilibrium. The student then increases the pressure by reducing the volume of the flask. Explain what happens to the position of equilibrium and the concentration of HI when the pressure is increased.

> AQA A-Level Chemistry (7405) — 3.1.6 Chemical equilibria, Le Chatelier and Kc · Explain · 5 marks

> H₂(g) + I₂(g) ⇌ 2HI(g)

## Mark scheme (5 marks)

1. The position of equilibrium does not shift (stays the same / no change in position)
2. Because there are equal numbers of moles of gas on both sides of the equation (1 + 1 = 2)
3. Le Chatelier's principle states the system opposes / minimises the change applied
4. Because shifting in either direction would not reduce pressure (as moles of gas are equal either way), no shift occurs
5. The concentration of HI increases (because the same amount of HI is now in a smaller volume)

## Key terms

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

## Related

- [Revision notes for AQA A-Level Chemistry (7405)](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/a-student-sets-up-a-sealed-116eb7a1) · Published by Druglandscape Ltd.