6.2.6 The effect of temperature changes on equilibrium

Raising the temperature favours the endothermic direction

Definition

Equilibrium position

The relative amounts of reactants and products present when a reversible reaction reaches equilibrium in a closed system.

Increasing the temperature moves the equilibrium position in the endothermic direction.
This makes more of the products of the endothermic reaction.
Decreasing the temperature moves the equilibrium in the exothermic direction, making more of the products of the exothermic reaction.

Key Idea

A higher temperature shifts the equilibrium the endothermic way.
A lower temperature shifts the equilibrium the exothermic way.

Use the energy change of each direction to predict the shift

Definition

Dynamic equilibrium

The state reached in a closed system when the forward and reverse reactions occur at equal rates, so the concentrations of reactants and products remain constant.

First decide which direction is exothermic and which is endothermic.
A higher temperature adds energy, so the equilibrium moves the way that takes energy in.
A lower temperature removes energy, so the equilibrium moves the way that gives energy out.
The amount of each product changes to match the new equilibrium.

Example

Suppose the forward reaction $X⇌Y\text{X} \rightleftharpoons \text{Y}$ is exothermic.
Raising the temperature moves the equilibrium to the left, so the yield of Y falls.
Lowering the temperature moves it to the right, so the yield of Y rises.

Exam technique

Identify the exothermic and endothermic directions before you decide the shift.
Say a higher temperature favours the endothermic direction, and a lower temperature the exothermic direction.
Then state clearly whether the yield of the named product goes up or down.

Self review

Which direction does a higher temperature favour?
Which direction does a lower temperature favour?
If the forward reaction is exothermic, what does raising the temperature do to the yield of product?
What must you work out first before predicting a temperature shift?

The equilibrium position is the relative amounts of reactants and products present when a reversible reaction reaches equilibrium in a closed system. At dynamic equilibrium, the forward and reverse reactions continue at equal rates, so the concentrations remain constant.

Increasing the temperature moves the equilibrium position in the endothermic direction. Decreasing the temperature moves the equilibrium position in the exothermic direction.

This changes the relative amounts of substances present. More products of the favoured direction are formed, so the yield of a named product may increase or decrease.

What direction does a higher temperature favour at equilibrium?