The effect of temperature changes on equilibrium (HT only): If the temperature of a system at equilibrium is decreased:

The idea

Le Chatelier's principle predicts that decreasing the temperature of a system at dynamic equilibrium causes the system to shift its position to generate heat and oppose the change. Therefore, cooling forces the system to favour the exothermic reaction path. If the forward reaction is exothermic, decreasing the temperature shifts the equilibrium to the right, increasing the yield of products. Conversely, if the forward reaction is endothermic, cooling shifts the equilibrium to the left, decreasing the product yield. For example, in the Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the forward reaction is exothermic. Cooling the reaction mixture shifts the equilibrium to the right, raising the yield of ammonia. However, lower temperatures also reduce the rate of reaction.

What earns the marks

• State that decreasing the temperature causes the system to favour the exothermic reaction to release heat.
• Identify whether the forward or reverse reaction is exothermic based on the given enthalpy change.
• Conclude that the equilibrium shifts to the right if the forward reaction is exothermic, or to the left if it is endothermic.
• Deduce the final effect on the relative amount of products, such as an increased yield of ammonia in the Haber process.

Common mistakes and how to put them right

• Assuming cooling always increases product yield.

  Put it right: The effect depends entirely on whether the forward reaction is exothermic or endothermic.

• Confusing shift direction with reaction rate.

  Put it right: Cooling shifts the position towards the exothermic side to increase yield, but it simultaneously slows down the rate of reaction.

• Omitting the link to the exothermic direction: students often state the shift without explaining that the system is trying to release thermal energy.

Exam tips

• Check the sign of the enthalpy change (ΔH) to determine if the forward reaction is exothermic (negative) or endothermic (positive).
• Link the phrase 'shifts to the right/left' directly to the change in the amount of products to secure full marks.