GCSE Combined Science revision · Chemistry: reactions and energy

Higher: equilibrium and pressure

Higher only

The idea

For reactions involving gases, increasing the pressure shifts the equilibrium towards the side with fewer molecules of gas, as shown by the balanced equation. Decreasing the pressure shifts it towards the side with more gas molecules. If both sides have the same number of gas molecules, pressure has no effect on the position.

Count the gas molecules on each side.
Higher pressure favours the side with fewer.
Lower pressure favours the side with more.

Watch out Count only gas molecules, using the numbers in front of each formula.

See it worked

Worked example

N₂ + 3H₂ ⇌ 2NH₃. Explain the effect of increasing pressure on the yield of ammonia.

Left side: 4 gas molecules; right side: 2.
Higher pressure favours the side with fewer molecules, so the yield of ammonia rises.

Answer The yield of ammonia increases

Your turn

Question 1Calculator allowed1 mark

For 2SO₂ + O₂ ⇌ 2SO₃ (all gases), increasing the pressure:

Question 2Calculator allowed1 mark

For H₂ + I₂ ⇌ 2HI (all gases), increasing the pressure:

Question 3Calculator allowed1 mark

How many gas molecules are on the left side of N₂ + 3H₂ ⇌ 2NH₃?

In past papers

10 official questions in 10 papers since 2018. The latest:

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