Changing the position of equilibrium
Le Chatelier's principle (Higher tier only)
The describes how much of the mixture is reactants and how much is products. If the equilibrium lies to the right, there are more products; to the left, more reactants.
if a system at equilibrium is subjected to a change in conditions, the system responds to counteract the change.
This lets you predict what happens when the concentration, temperature or pressure is changed. The example used here is the :
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
The forward reaction is .
Key terms
Changing concentration (Higher tier only)
- If the concentration of a , more products form until equilibrium is reached again. The position shifts to the right.
- If a , more reactants react to replace it. The position also shifts to the right.
- If the concentration of a product is increased, the position shifts to the left.
In the Haber process, ammonia is removed by cooling the gases so the ammonia turns to liquid. Adding more nitrogen or hydrogen also gives more ammonia.
Key terms
Changing temperature (Higher tier only)
- Raising the temperature favours the direction, which takes in energy and counteracts the rise.
- Lowering the temperature favours the direction.
In the Haber process, the forward reaction is exothermic. So raising the temperature shifts the position to the left and gives . Lowering it gives more ammonia, but makes the reaction very slow.
Key terms
Changing pressure (Higher tier only)
Pressure only affects reactions involving gases. Count the on each side of the equation:
- Increasing the pressure favours the side with fewer gas molecules.
- Decreasing the pressure favours the side with more gas molecules.
In the Haber process there are 4 molecules of gas on the left (1 N₂ + 3 H₂) and 2 on the right (2 NH₃). So increasing the pressure shifts the position to the right and gives .
If both sides have the same number of gas molecules, pressure has no effect on the position.
A doesn't change the position of equilibrium. It only makes equilibrium reached sooner. (The conditions actually chosen are in The Haber process, C10.)