AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The effect of temperature changes on equilibrium (HT only): the relative amount of products at equilibrium decreases for an endothermic reaction
Le Chatelier's principle predicts how a system at dynamic equilibrium responds to changes.
Study notes
From the checked teaching notes for this specification point.
The idea
Le Chatelier's principle predicts how a system at dynamic equilibrium responds to changes. If the forward reaction is endothermic, it absorbs heat energy. When the temperature of such a system is decreased, the system responds by opposing the change, shifting the position of equilibrium to release heat. This favours the exothermic reverse reaction. The reverse rate temporarily exceeds the forward rate until equilibrium is re-established. This shift to the left reduces the equilibrium yield, meaning less product is present.
For example: In the endothermic forward reaction N₂O₄(g) ⇌ 2NO₂(g), cooling the mixture shifts the equilibrium to the left, producing a paler mixture as the proportion of brown NO₂ falls.
What earns the marks
- State that decreasing the temperature of an equilibrium mixture causes the system to shift to oppose the change by releasing heat.
- Identify that for a forward endothermic reaction, the reverse reaction must be exothermic.
- Explain that a temperature decrease therefore favours the reverse exothermic reaction.
- Conclude that the position of equilibrium shifts to the left, reducing the equilibrium yield of products.
- Apply this principle to a specific equation by naming the product whose equilibrium yield will fall upon cooling.
Common mistakes and how to put them right
Stating that cooling an endothermic reaction increases the product yield.
Put it right by applying Le Chatelier's principle, which shows cooling favours the exothermic reverse reaction, decreasing the product.
Confusing the rate of reaction with the position of equilibrium.
Put it right by noting that while cooling decreases both forward and reverse rates, it decreases the endothermic forward rate more, shifting equilibrium to the left.
Assuming the forward reaction is always exothermic.
Put it right: Carefully checking the provided enthalpy change sign; a positive value indicates an endothermic forward reaction.
Exam tips
- Always check the sign of the enthalpy change to determine if the forward reaction is endothermic before predicting the effect of a temperature change.
- When explaining a shift, explicitly state that the system opposes the temperature decrease by favouring the exothermic direction, leading to a lower yield of products.
Try the lesson
The start of the The effect of temperature changes on equilibrium (HT only): the relative amount of products at equilibrium decreases for an endothermic reaction lesson, made from checked teaching notes for this specification point.
Have a go first
A reversible reaction has an endothermic forward reaction and is at equilibrium. The temperature of the mixture is decreased. What happens to the relative amount of products at the new equilibrium?
Listen
Le Chatelier's principle predicts how a system at dynamic equilibrium responds to changes. If the forward reaction is endothermic, it absorbs heat energy. When the temperature of such a system is decreased, the system responds by opposing the change, shifting the position of equilibrium to release heat. This favours the exothermic reverse reaction. The reverse rate temporarily exceeds the forward rate until equilibrium is re-established. This shift to the left reduces the equilibrium yield, meaning less product is present. For example, in the endothermic forward reaction N₂O₄(g) ⇌ 2NO₂(g), cooling the mixture shifts the equilibrium to the left, producing a paler mixture as the proportion of brown NO₂ falls. A strong answer earns marks for these points. State that decreasing the temperature of an equilibrium mixture causes the system to shift to oppose the change by releasing heat. Identify that for a forward endothermic reaction, the reverse reaction must be exothermic. Explain that a temperature decrease therefore favours the reverse exothermic reaction. Conclude that the position of equilibrium shifts to the left, reducing the equilibrium yield of products. A common slip: Stating that cooling an endothermic reaction increases the product yield. Instead, applying Le Chatelier's principle, which shows cooling favours the exothermic reverse reaction, decreasing the product. Exam tip: Always check the sign of the enthalpy change to determine if the forward reaction is endothermic before predicting the effect of a temperature change.
See it
Put it in order
Put these steps in the order they happen.
- 1. Temperature is decreased
- 2. System opposes the change by releasing heat
- 3. Reverse exothermic reaction is favoured
- 4. Reverse rate temporarily exceeds forward rate
- 5. Position of equilibrium shifts to the left
- 6. Equilibrium yield of products decreases
- 7. Forward reaction is endothermic
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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