The effect of temperature changes on equilibrium (HT only) — AQA GCSE Chemistry
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The effect of temperature changes on equilibrium (HT only) explained
The effect of temperature changes on equilibrium (HT only) covers 3 specification points, including The relative amount of products at equilibrium… and The relative amount of products at equilibrium….
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Open a point below for its explanation and revision guidance.
If the temperature of a system at equilibrium is increased: the relative amount of products at equilibrium increases for an endothermic reaction the relative amount of products at equilibrium decreases for an exothermic reaction.
Changing temperature affects the position of equilibrium because it changes the rates of the forward and reverse reactions differently, depending on whether the forward reaction is endothermic or exothermic. If the temperature is increased, the system absorbs the extra heat by favouring the endothermic direction. For an endothermic forward reaction, the forward reaction is favoured, so the relative amount of products at equilibrium increases. For an exothermic forward reaction, the reverse reaction is endothermic and is favoured instead, so the relative amount of products at equilibrium decreases. For example, in the endothermic forward reaction N₂O₄(g) ⇌ 2NO₂(g), increasing temperature shifts equilibrium to the right, producing more NO₂.
If the temperature of a system at equilibrium is decreased: the relative amount of products at equilibrium decreases for an endothermic reaction the relative amount of products at equilibrium increases for an exothermic reaction.
Decreasing the temperature of a system at equilibrium favours the exothermic direction, because the system releases heat to oppose the cooling. If the forward reaction is endothermic, the reverse reaction is exothermic and is favoured, so the relative amount of products at equilibrium decreases. If the forward reaction is exothermic, the forward reaction is favoured, so the relative amount of products at equilibrium increases. For example, in the exothermic forward reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), lowering the temperature shifts equilibrium to the right, producing more ammonia. This principle is used in the Haber process to increase ammonia yield, although lower temperatures also slow the rate of reaction.
Students should be able to interpret appropriate given data to predict the effect of a change in temperature on given reactions at equilibrium.
Le Chatelier's principle allows students to predict how a system at dynamic equilibrium responds to a temperature change. If a reaction is exothermic in the forward direction, raising the temperature shifts the equilibrium position in the endothermic (reverse) direction, causing the product yield to fall. Given data, such as a table of percentage yield at different temperatures, students must compare values to deduce whether the forward reaction is exothermic or endothermic. For example, if yield falls from 60% at 300 K to 30% at 500 K, the forward reaction is exothermic. Students are not required to consider the equilibrium constant (Kc) at GCSE level, focusing solely on qualitative shifts in equilibrium position.
Your focus
- Describe how increasing temperature affects the position of equilibrium for endothermic and exothermic reactions.
- Explain the effect in terms of the system opposing the temperature increase.
- Predict changes in the relative amount of products when temperature is increased.
Show all 9 objectives
- Describe how decreasing temperature affects the position of equilibrium for endothermic and exothermic reactions.
- Explain the effect in terms of the system opposing the temperature decrease.
- Predict changes in the relative amount of products when temperature is decreased.
- Predict the direction of equilibrium shift when temperature is increased or decreased.
- Interpret given yield or concentration data to classify a forward reaction as exothermic or endothermic.
- Explain the effect of temperature changes on equilibrium yield using Le Chatelier's principle.
The effect of temperature changes on equilibrium (HT only) exam tips
Marking Points
- Increasing temperature favours the endothermic direction.
- For an endothermic forward reaction, the forward reaction is favoured and the relative amount of products increases.
- For an exothermic forward reaction, the reverse reaction is favoured and the relative amount of products decreases.
- The system absorbs the added heat by shifting in the endothermic direction.
- The new equilibrium has a different composition from the original equilibrium.
- The explanation should refer to the relative amounts of products, not just the direction of the shift.
- Decreasing temperature favours the exothermic direction.
- For an endothermic forward reaction, the reverse reaction is favoured and the relative amount of products decreases.
- For an exothermic forward reaction, the forward reaction is favoured and the relative amount of products increases.
- The system releases heat to oppose the decrease in temperature.
- The new equilibrium has a different composition from the original equilibrium.
- The explanation should refer to the relative amounts of products at equilibrium.
- State that a temperature change alters the equilibrium position, shifting it to counteract the change.
- Predict the qualitative effect on the yield of a named product, linking the shift to the direction of the forward reaction.
- Deduce whether a forward reaction is exothermic or endothermic by analysing yield data at different temperatures.
- Explain that an increase in temperature shifts the equilibrium in the endothermic direction to absorb the excess heat.
Examiner Tips
- 💡State whether the forward reaction is endothermic or exothermic before predicting the effect.
- 💡Use the phrase 'the equilibrium shifts in the endothermic direction' to show clear reasoning.
- 💡Link the prediction to the relative amount of products at the new equilibrium.
- 💡Identify whether the forward reaction is endothermic or exothermic before predicting the effect.
- 💡Use the phrase 'the equilibrium shifts in the exothermic direction' to make your reasoning clear.
- 💡Mention the effect on the relative amount of products, not just the direction of the shift.
- 💡Name the direction of shift explicitly, for example 'shifts to the left' or 'shifts towards the reactants'.
- 💡Link the prediction to the sign of the enthalpy change or given temperature data provided in the question.
Common Mistakes
- Assuming that increasing temperature always increases the amount of product; this is only true for an endothermic forward reaction.
- Confusing endothermic and exothermic directions; the endothermic direction is always favoured when temperature increases.
- Thinking that temperature changes affect only the rate and not the position of equilibrium; temperature changes affect both.
- Assuming that decreasing temperature always decreases the amount of product; this is only true for an endothermic forward reaction.
- Confusing the favoured direction; decreasing temperature favours the exothermic direction, not the endothermic direction.
- Forgetting that a lower temperature also slows down the rate at which equilibrium is reached, even though the yield may be higher.
- Discussing the equilibrium constant (Kc); the correction is to focus solely on the equilibrium position and product yield.
- Assuming that raising temperature always increases product yield; the correction is that it increases yield only when the forward reaction is endothermic.
- Confusing the rate of reaction with equilibrium position; the correction is to distinguish between reaching equilibrium faster (rate) and the final yield (position).