The Effect of Pressure Changes on Equilibrium

Higher tier only

We can predict the effect of changes in pressure on systems in equilibrium
Changes in pressure only affects gases
In gaseous reactions:
- An increase in pressure will favour the reaction that produces the least number of molecules
- A decrease in pressure will favour the reaction that produces the greatest number of molecules
If there are the same number of moles of gases on either side of the equation, then there is NO effect on the position of equilibrium when the pressure is changed
- Increasing the pressure will increase the rate of the forward reaction and backward reaction equally which is why the position of equilibrium is unchanged

Nitrogen dioxide molecules can dimerise and form dinitrogen tetroxide in the following equilibrium reaction:

2NO₂ (g) ⇌ N₂O₄(g)

dark brown colourless

What will the colour change be if the pressure is increased? Explain your answer.

Answer:

The number of gas molecules produced by the forward reaction = 1
The number of gas molecules produced by the reverse reaction = 2
An increase in the pressure will favour the reaction that produces the least number of molecules
- This is the forward reaction
So, the equilibrium shifts to the right
- This means that the mixture will appear increasingly colourless as the concentration of N₂O₄ increases

Changes in pressure affect those systems that contain a gas only.

Use the balanced equation to determine which side has the most molecules of gas.

The Effect of Pressure Changes on Equilibrium (AQA GCSE Chemistry: Combined Science)