AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The effect of temperature changes on equilibrium (HT only): If the temperature of a system at equilibrium is increased:
A system at equilibrium has forward and reverse reactions occurring at equal rates, so concentrations stay constant.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A system at equilibrium has forward and reverse reactions occurring at equal rates, so concentrations stay constant. If the temperature is increased, the position of equilibrium shifts to oppose the change. The endothermic direction is favoured because it takes in the added thermal energy, reducing the temperature. For example, in N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (forward exothermic), raising the temperature makes the reverse, endothermic reaction more favourable, so the equilibrium yield of ammonia falls. Cooling does the opposite, favouring the exothermic direction. This statement is the starting point for predicting how heating changes the relative amount of products.
What earns the marks
- Equilibrium means forward and reverse reaction rates are equal, so the amounts of reactants and products remain constant.
- Increasing temperature adds thermal energy, and the system responds by favouring the direction that takes in energy.
- The endothermic direction is favoured when temperature is increased; the exothermic direction is favoured when temperature is decreased.
- The shift changes the relative amounts of substances present at the new equilibrium, not the rates being unequal at that new position.
- A catalyst speeds up both directions equally and does not change the position of equilibrium, unlike a temperature change.
Common mistakes and how to put them right
Saying the forward reaction always speeds up more when heated.
Put it right by stating that the endothermic direction is favoured, which may be the reverse reaction.
Confusing rate with position of equilibrium.
Put it right by explaining that a new equilibrium is reached where both rates are equal again but amounts differ.
Thinking a catalyst shifts equilibrium to raise yield.
Put it right by stating that a catalyst only reduces the time taken to reach equilibrium.
Exam tips
- State clearly that the endothermic direction is favoured when temperature increases, then apply it to the equation given.
- Use the equation's energy label or data to identify which direction is endothermic before predicting the shift.
- Link your prediction to relative amounts of products, for example 'the yield of product decreases', rather than only saying 'shifts left'.
Try the lesson
The start of the The effect of temperature changes on equilibrium (HT only): If the temperature of a system at equilibrium is increased: lesson, made from checked teaching notes for this specification point.
Have a go first
A reversible reaction reaches equilibrium. The temperature of the system is then increased. Which way does the equilibrium shift?
Listen
A system at equilibrium has forward and reverse reactions occurring at equal rates, so concentrations stay constant. If the temperature is increased, the position of equilibrium shifts to oppose the change. The endothermic direction is favoured because it takes in the added thermal energy, reducing the temperature. For example, in N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (forward exothermic), raising the temperature makes the reverse, endothermic reaction more favourable, so the equilibrium yield of ammonia falls. Cooling does the opposite, favouring the exothermic direction. This statement is the starting point for predicting how heating changes the relative amount of products. A strong answer earns marks for these points. Equilibrium means forward and reverse reaction rates are equal, so the amounts of reactants and products remain constant. Increasing temperature adds thermal energy, and the system responds by favouring the direction that takes in energy. The endothermic direction is favoured when temperature is increased; the exothermic direction is favoured when temperature is decreased. The shift changes the relative amounts of substances present at the new equilibrium, not the rates being unequal at that new position. A common slip: Saying the forward reaction always speeds up more when heated. Instead, stating that the endothermic direction is favoured, which may be the reverse reaction. Exam tip: State clearly that the endothermic direction is favoured when temperature increases, then apply it to the equation given.
Key words
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Equilibrium
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Endothermic direction
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Exothermic direction
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Position of equilibrium
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See every definition
- Equilibrium
- A state in which the forward and reverse reaction rates are equal, so the amounts of reactants and products remain constant.
- Endothermic direction
- The direction that takes in energy, and is favoured when the temperature of a system at equilibrium is increased.
- Exothermic direction
- The direction favoured when temperature is decreased, doing the opposite of the endothermic direction.
- Position of equilibrium
- Where the equilibrium sits, shifting to oppose a temperature change and changing the relative amounts of substances present at the new equilibrium.
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