5.6.2.6The effect of temperature changes on equilibrium (HT only)
AQA GCSE Combined Science (8464), Higher tier · Chemistry › The rate and extent of chemical change › Reversible reactions and dynamic equilibrium
Practise The effect of temperature changes on equilibrium (HT only). 13 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Higher tier only. Changing the temperature of a system at equilibrium changes the relative amount of products, depending on whether the forward reaction is exothermic or endothermic. You need to predict the effect, and use data to decide whether a reaction is exothermic or endothermic.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
5
Predict heating an exothermic reactionRaising the temperature decreases the relative amount of products at equilibrium.
6
Predict heating an endothermic reactionRaising the temperature increases the relative amount of products at equilibrium.
7
Explain temperature effects using Le ChatelierIncreasing the temperature favours the endothermic direction, which takes in energy.
7
Deduce the energy change from yield dataIf the yield falls as the temperature rises, the forward reaction is exothermic.
8
Explain colour changes caused by temperatureLink the direction of the shift to the colour of the substance that increases.
8
Weigh yield against rate when choosing temperatureA lower temperature gives more product for an exothermic reaction, but the rate is slower.
Notes
The rules
Increase the temperature: the relative amount of products at equilibrium increases for an endothermic reaction and decreases for an exothermic reaction.
Decrease the temperature: the relative amount of products at equilibrium decreases for an endothermic reaction and increases for an exothermic reaction.
'An exothermic reaction' here means the forward reaction is exothermic, so the reverse reaction is endothermic.
Why it happens
Increasing the temperature: the system counteracts the change by favouring the endothermic direction, which takes in energy.
Decreasing the temperature: the system favours the exothermic direction, which releases energy.
So work out which direction is endothermic: the position shifts that way when the mixture is heated.
Using data and observations
If the percentage of product at equilibrium goes down as the temperature goes up, the forward reaction is exothermic.
If the percentage of product goes up as the temperature goes up, the forward reaction is endothermic.
Example: 2NO2 (brown) ⇌ N2O4 (colourless), where the forward reaction is exothermic. Heating the sealed mixture shifts the position left, so it turns darker brown.
Temperature also affects rate. For an exothermic reaction, a lower temperature gives a greater amount of product at equilibrium, but the reaction is slower, so equilibrium takes longer to reach. grade 8+
Cheatsheet
Heat up → endothermic direction favoured
Cool down → exothermic direction favoured
Forward reaction exothermic: higher temperature → less product
Forward reaction endothermic: higher temperature → more product
Yield falls as temperature rises → forward reaction is exothermic
A higher temperature increases the rates of both the forward and reverse reactions
How to answer each type of question
Predict and explain the effect of changing the temperature
2 marks6
Find out whether the forward reaction is exothermic or endothermic.
Heating: the position shifts in the endothermic direction. Cooling: it shifts in the exothermic direction.
State what happens to the amount of product.
Example. Sulfur dioxide reacts with oxygen to form sulfur trioxide. 2SO2(g) + O2(g) ⇌ 2SO3(g) The forward reaction is exothermic. Predict the effect of increasing the temperature on the amount of sulfur trioxide at equilibrium. Explain your answer.
Show the model answer
The amount of sulfur trioxide decreases (1). Increasing the temperature favours the endothermic direction, which is the reverse reaction, so the position of equilibrium shifts to the left (1).
Use data to decide whether a reaction is exothermic or endothermic
3 marks7
Describe the trend: what happens to the amount of product as the temperature rises?
Less product at a higher temperature means heating shifts the position left.
So the reverse reaction is endothermic and the forward reaction is exothermic (and the opposite for the other trend).
Example. Methanol is made from carbon monoxide and hydrogen. CO(g) + 2H2(g) ⇌ CH3OH(g) The percentage of methanol in the equilibrium mixture at a fixed pressure was: 200 °C: 55%; 250 °C: 38%; 300 °C: 21%; 350 °C: 10% Is the forward reaction exothermic or endothermic? Explain how the data show this.
Show the model answer
Exothermic (1). As the temperature increases, the percentage of methanol at equilibrium decreases (1). So increasing the temperature shifts the position to the left, which means the reverse reaction is endothermic and the forward reaction is exothermic (1).
Explain a colour change on heating or cooling
3 marks8
Use the colour change to say which substance has increased.
So say which way the position has shifted.
Heating favours the endothermic direction, so name the direction that must be endothermic.
Example. A sealed tube contains colourless gas A in equilibrium with brown gas B. A(g) ⇌ 2B(g) When the tube is put into hot water, the mixture turns darker brown. Is the forward reaction exothermic or endothermic? Explain your answer.
Show the model answer
Endothermic (1). The mixture turns darker, so more brown gas B has formed and the position of equilibrium has shifted to the right (1). Increasing the temperature favours the endothermic direction, so the forward reaction is endothermic (1).
Explain the choice of temperature (yield and rate)
3 marks8
Give the effect on yield: for an exothermic forward reaction, a lower temperature gives more product at equilibrium.
Give the effect on rate: a lower temperature makes the reaction slower (less frequent collisions, fewer with the activation energy).
Conclude: the temperature chosen gives a reasonable yield in a reasonable time.
Example. The forward reaction in the methanol process is exothermic. A manufacturer uses 250 °C rather than 150 °C, even though the percentage of methanol at equilibrium is higher at 150 °C. Suggest why the manufacturer uses 250 °C.
Show the model answer
At 150 °C the rate of reaction would be too slow (1), because the particles collide less frequently and fewer collisions have energy equal to or greater than the activation energy (1). At 250 °C a reasonable yield is made in a much shorter time, so more methanol is produced each hour (1).
Shortcuts and memory tricks
Heat favours endo: adding heat pushes the reaction in the direction that soaks up heat.
Treat energy like a substance in the equation: for an exothermic forward reaction, energy is on the product side, so adding heat pushes the position left.
Data check: product percentage and temperature move in opposite directions → forward reaction exothermic; in the same direction → endothermic.
Where marks are lost
Saying that increasing the temperature shifts every equilibrium to the right. It depends on which direction is endothermic.
Mixing up the forward and reverse directions when the question says 'the reaction is exothermic'.
Saying a higher temperature slows down the reverse reaction. Heating speeds up both reactions; the position shifts because the endothermic direction is favoured.
Naming the direction of the shift but forgetting to say what happens to the amount of product.
Exam technique
Always name the direction that is favoured (endothermic or exothermic, forward or reverse) in your explanation.
When the question gives a table of yields, quote numbers from it to support your answer.
In yield-versus-rate questions, give both sides: the effect on the amount of product and the effect on the rate.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
The energy change for the forward reaction is +57 kJ/mol. What is the energy change for the reverse reaction?
−57 kJ/mol
What happens to the amounts of nitrogen and oxygen at equilibrium when the temperature is increased?
They decrease.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy3 marks
Nitrogen and oxygen react in a reversible reaction to form nitrogen monoxide: N2(g) + O2(g) ⇌ 2NO(g) The forward reaction is endothermic.
(a) The temperature is increased. What happens to the amount of nitrogen monoxide at equilibrium? Tick (✓) one box.[1]
It decreases.
It increases.
It stays the same.
(b) Give the reason for your answer.[1]
(c) What happens to the amounts of nitrogen and oxygen at equilibrium when the temperature is increased?[1]
Show the answer and mark scheme
(a)Answer: It increases.
(b)Answer: The forward reaction is endothermic, so increasing the temperature favours the forward reaction.
the forward reaction is endothermic, so increasing the temperature shifts the position of equilibrium to the right / favours the (endothermic) forward reaction
(c)Answer: They decrease.
they decrease
Question 2Medium6 marks
Nitrogen dioxide, NO2, is a brown gas. Dinitrogen tetroxide, N2O4, is a colourless gas. They are in equilibrium: N2O4(g) ⇌ 2NO2(g) A sealed tube containing the equilibrium mixture was put into hot water and the mixture became darker brown. The tube was then put into iced water and the mixture became paler.
(a) Is the forward reaction exothermic or endothermic? Explain your answer using the observations.[3]
(b) Explain why the mixture became paler in iced water.[2]
(c) The energy change for the forward reaction is +57 kJ/mol. What is the energy change for the reverse reaction?[1]
Show the answer and mark scheme
(a)Answer: Endothermic: in hot water the mixture became darker, so more NO2 formed; a higher temperature shifts the equilibrium in the endothermic direction.
endothermic
in hot water the mixture became darker brown, so more NO2 formed (the equilibrium shifted to the right)
increasing the temperature shifts the equilibrium in the endothermic direction
(b)Answer: Lowering the temperature shifts the equilibrium in the exothermic direction (to the left), so NO2 turns into colourless N2O4.
decreasing the temperature shifts the equilibrium in the exothermic direction (to the left)
so NO2 is converted into (colourless) N2O4
(c)Answer: −57 kJ/mol
−57 kJ/mol
Question 3Hard3 marks
The forward reaction in the industrial manufacture of an ester from an alcohol and a carboxylic acid is very slightly exothermic.
(a) Explain the effect of increasing the temperature on the equilibrium yield of ester.[2]
(b) Despite this, a moderately high temperature is used industrially. Suggest why.[1]
Show the answer and mark scheme
(a)Answer: The yield decreases slightly: increasing the temperature favours the endothermic (reverse) direction, shifting the equilibrium slightly to the left.
the equilibrium yield decreases (slightly)
increasing the temperature favours the endothermic (reverse) direction, shifting the equilibrium slightly to the left
(b)Answer: A higher temperature gives a much faster rate of reaction, so equilibrium (or a usefully high conversion) is reached in a practical amount of time.
a higher temperature gives a much faster rate of reaction, so a useful amount of ester forms in a reasonable time