The effect of temperature changes on equilibrium (HT only): Students should be able to interpret appropriate given data to predict the effect of a change in temperature on given reactions at equilibrium.

The idea

This Higher Tier skill requires you to use supplied data, such as equilibrium yields or concentrations at different temperatures, to decide how heating or cooling shifts a reversible reaction at equilibrium. First, identify whether the forward reaction is exothermic or endothermic from a given ΔH value or yield data. If the product yield rises as temperature falls, the forward reaction is exothermic. Then apply Le Chatelier's principle: increasing temperature favours the endothermic direction, while decreasing temperature favours the exothermic direction. Use the data to justify the shift.

For example: If product yield falls from 45% at 300 K to 20% at 500 K, the forward reaction is exothermic.

What earns the marks

• Identify from the given data whether the forward reaction is exothermic or endothermic, for example by comparing product yield or concentration at two temperatures.
• State the direction of shift caused by raising or lowering temperature, linking it to the endothermic or exothermic direction respectively.
• Use specific numerical values from the data to support the predicted shift rather than making a general statement.
• Explain the prediction in terms of Le Chatelier's principle, noting that the system opposes the temperature change by favouring the reaction that absorbs or releases heat.
• Apply the prediction to a named reaction or context, such as the Haber process, and comment on the yield of product or position of equilibrium.

Common mistakes and how to put them right

• Assuming that increasing temperature always increases product yield.

  Put it right by checking the data and identifying the exothermic direction, which is favoured by cooling.

• Confusing the direction of shift with the rate of reaction.

  Put it right by separating equilibrium position from reaction rate, since temperature affects both but in different ways.

• Ignoring the sign of ΔH or misreading it.

  Put it right by using the given data to infer whether the forward reaction is exothermic or endothermic before predicting the shift.

Exam tips

• Quote at least two numerical values from the table or graph to justify your prediction.
• Write the direction of shift clearly, for example 'equilibrium shifts to the left' or 'towards the reactants'.
• As this is Higher Tier only, expect to apply these principles to unfamiliar data sets.