The effect of temperature changes on equilibrium (HT only)
AQA·Chemistry (Combined Science)·5.6.2.6·Trilogy — Higher
Temperature effects on equilibrium
At equilibrium, the forward and reverse reactions continue at the same rate. Changing the temperature changes their rates by different amounts, so one direction is temporarily faster than the other. The composition of the mixture changes until the rates are equal again at a new equilibrium.
To predict the change in composition, first identify whether the forward reaction takes in heat or releases it. The reverse reaction has the opposite heat change. An increase in temperature favours the direction that takes in heat; a decrease favours the direction that releases heat.
When the forward reaction is endothermic
An endothermic forward reaction takes in heat, so the reverse reaction releases heat.
- Increasing temperature favours the forward reaction. The new equilibrium mixture contains a greater relative amount of products.
- Decreasing temperature favours the reverse reaction. The new equilibrium mixture contains a smaller relative amount of products.
When the forward reaction is exothermic
An exothermic forward reaction releases heat, so the reverse reaction takes in heat.
- Increasing temperature favours the reverse reaction. The new equilibrium mixture contains a smaller relative amount of products.
- Decreasing temperature favours the forward reaction. The new equilibrium mixture contains a greater relative amount of products.