Effect of Temperature on Equilibrium (Higher Tier)

Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, so the concentrations of reactants and products remain constant.

Le Chatelier's Principle and Temperature

Le Chatelier's Principle states that if a system at equilibrium is subjected to a change in conditions, the system will respond to partially oppose that change and restore a new equilibrium.

When temperature changes, the equilibrium position shifts to counteract the change:

• If temperature increases, the equilibrium shifts in the direction that absorbs heat (endothermic direction).
• If temperature decreases, the equilibrium shifts in the direction that releases heat (exothermic direction).

This happens because the system tries to reduce the effect of the temperature change by favouring the reaction that either takes in or gives out heat.

Endothermic reactions absorb heat from the surroundings (temperature increases favour these).

Exothermic reactions release heat to the surroundings (temperature decreases favour these).

For example, consider the reversible reaction:

This reaction is exothermic in the forward direction (releases heat). Increasing temperature shifts equilibrium left (towards reactants), decreasing temperature shifts it right (towards products).

Effect of Increasing Temperature

When temperature is increased, the system shifts equilibrium towards the endothermic direction to absorb the extra heat and reduce the temperature rise.

• This means the yield of products formed by the endothermic reaction increases.
• The yield of products formed by the exothermic reaction decreases.

In the Haber process for ammonia production:

Increasing temperature shifts equilibrium towards nitrogen and hydrogen (reactants), reducing ammonia yield.

However, higher temperatures increase the rate of reaction because particles have more energy, so there is a trade-off between rate and yield.

For instance, if a reaction is:

Increasing temperature shifts equilibrium to the left (towards A), increasing reactant concentration and decreasing product B.

Effect of Decreasing Temperature

When temperature decreases, the system shifts equilibrium towards the exothermic direction to release heat and oppose the temperature drop.

• This increases the yield of products formed by the exothermic reaction.
• It decreases the yield of products formed by the endothermic reaction.

Using the Haber process again, lowering temperature shifts equilibrium towards ammonia (product), increasing yield.

However, lower temperatures slow the reaction rate, so industrial processes must find a compromise.

For example, if the reaction is:

Decreasing temperature shifts equilibrium to the right (towards D), increasing product concentration.

Practical Implications

In industry, temperature is carefully controlled to optimise the yield of desired products while maintaining a reasonable reaction rate.

• Higher temperatures increase reaction rates but may reduce product yield if the forward reaction is exothermic.
• Lower temperatures increase yield but slow down the reaction, making the process less efficient.

In the Haber process, the temperature is typically around 450°C, which is a compromise between a reasonable rate and acceptable ammonia yield.

This balance is crucial because running the process at low temperature would give high yield but very slow production, while high temperature would give fast production but low yield.