5.6.2.4The effect of changing conditions 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 changing conditions on equilibrium (HT only). 14 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. The relative amounts of reactants and products at equilibrium depend on the conditions. Le Chatelier's principle lets you predict which way the position of equilibrium shifts when a condition is changed. Questions give you an equation and some information, and ask you to predict and explain.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
5
State Le Chatelier's principleIf a condition of a system at equilibrium is changed, the system responds to counteract the change.
6
Explain what shifting right or left meansShifting right gives more products at equilibrium; shifting left gives more reactants.
6
Know a catalyst does not move equilibriumA catalyst speeds up both reactions equally, so equilibrium is reached faster but its position does not change.
7
Predict the effect of a change in conditionsUse the information given (equation, energy change, numbers of molecules) to say which way the position shifts.
8
Explain a prediction fully using Le ChatelierSay what the change is, how the system counteracts it, which way the position shifts and what happens to the product.
Notes
Le Chatelier's principle
The relative amounts of all the reactants and products at equilibrium depend on the conditions: concentration, temperature and, for gases, pressure.
Le Chatelier's principle: if a system is at equilibrium and a change is made to any of the conditions, the system responds to counteract the change.
The system does not undo the change completely; it reduces its effect.
Position of equilibrium
The position of equilibrium describes the relative amounts of reactants and products.
If it shifts to the right, the forward reaction is favoured, so there are more products at equilibrium.
If it shifts to the left, the reverse reaction is favoured, so there are more reactants at equilibrium.
After the change, the forward and reverse rates become equal again and a new equilibrium is reached.
Quick guide to the changes
Increase the concentration of a substance: the position shifts away from that substance, to use some of it up.
Increase the temperature: the position shifts in the endothermic direction, which takes in energy.
Increase the pressure (gases only): the position shifts towards the side with fewer molecules, which lowers the pressure.
Decreasing any of these has the opposite effect.
A catalyst does not change the position of equilibrium. It speeds up the forward and reverse reactions equally, so equilibrium is reached more quickly.
Cheatsheet
Le Chatelier: the system responds to counteract the change
Shift right = more products; shift left = more reactants
Add a substance → shift away from it; remove a substance → shift towards it
Heat up → endothermic direction favoured; cool down → exothermic direction favoured
Raise the pressure → side with fewer gas molecules; lower it → side with more
Catalyst: no change to the position; equilibrium is reached faster
How to answer each type of question
State Le Chatelier's principle
2 marks5
Start with the condition: a system at equilibrium has one of its conditions changed.
Finish with the response: the system responds to counteract (oppose) the change.
Example. State Le Chatelier's principle.
Show the model answer
If a change is made to the conditions of a system at equilibrium (1), the system responds to counteract the change (1).
Predict and explain the effect of a change
2 to 3 marks7
Identify the change: concentration, temperature or pressure.
Say which way the position of equilibrium shifts to counteract it.
Say what happens to the amount of product, or to the colour if colours are given.
Example. Brown nitrogen dioxide gas forms colourless dinitrogen tetroxide gas in a reversible reaction. 2NO2(g) ⇌ N2O4(g) The forward reaction is exothermic. A sealed syringe containing the equilibrium mixture is put into iced water. Predict how the colour of the mixture changes. Explain your answer.
Show the model answer
The mixture becomes paler (1). Lowering the temperature shifts the position of equilibrium in the exothermic direction, to the right (1), so there is less brown NO2 and more colourless N2O4 (1).
Explain the effect of a catalyst on an equilibrium
2 marks6
Say the catalyst increases the rates of the forward and reverse reactions equally.
Conclude: the position of equilibrium does not change, but equilibrium is reached faster.
Example. A manufacturer adds a catalyst to a reversible reaction. Explain why the catalyst does not change the amount of product in the equilibrium mixture.
Show the model answer
The catalyst increases the rates of the forward and reverse reactions equally (1), so the position of equilibrium does not change; equilibrium is just reached more quickly (1).
Shortcuts and memory tricks
Le Chatelier's principle in three words: 'do the opposite'. Whatever you change, the system tries to oppose it.
Right = products: 'shifts to the right' always means more product.
Three levers move the position: concentration, temperature and pressure. A catalyst only changes the speed.
Where marks are lost
Writing 'the equilibrium shifts to counteract the change' without saying which direction and what happens to the amount of product.
Saying a catalyst increases the yield or shifts the position of equilibrium.
Saying the system cancels the change completely. It only reduces the effect.
Mixing up the position of equilibrium (how much product) with how quickly equilibrium is reached (the rate).
Exam technique
Structure your answer: change → direction of shift → effect on the amount of product (or colour). Each part is often a mark.
Use the phrase 'the position of equilibrium shifts to the right (towards the products)' or 'to the left (towards the reactants)'.
Read the information given carefully: the question tells you whether the forward reaction is exothermic and gives the equation you need.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Complete the sentence about Le Chatelier's principle. If a system is at equilibrium and a change is made to any of the conditions, the system responds to ________ the change.
counteract
Complete the sentence. According to Le Chatelier's principle, if a system at equilibrium is subjected to a change, the position of equilibrium moves to ________ the effect of the change.
counteract
What is meant by 'the position of equilibrium shifts to the right'?
More C is formed: the amount of C increases and the amounts of A and B decrease.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
(a) Complete the sentence about Le Chatelier's principle. If a system is at equilibrium and a change is made to any of the conditions, the system responds to ________ the change.[1]
(b) Which change does not affect the position of equilibrium? Tick (✓) one box.[1]
Adding a catalyst
Changing the temperature
Changing the concentration of a reactant
Changing the pressure of a reaction involving gases
(c) What is meant by 'the position of equilibrium moves to the right'?[1]
(d) A catalyst does not change the position of equilibrium, but catalysts are still used in industrial processes that involve reversible reactions. Explain why.[2]
Show the answer and mark scheme
(a)Answer: counteract
counteract / oppose / reduce the effect of
(b)Answer: Adding a catalyst
(c)Answer: The relative amount of products at equilibrium increases.
the relative amount of products (at equilibrium) increases
(d)Answer: The catalyst speeds up the forward and reverse reactions equally, so equilibrium is reached faster.
the catalyst increases the rates of the forward and reverse reactions (equally)
so equilibrium is reached faster / more product is made in a given time
Question 2Medium2 marks
(a) A reversible reaction between gases has more molecules on the left-hand side of the equation than on the right. Which change shifts the position of equilibrium to the right? Tick (✓) one box.[1]
Decreasing the pressure
Increasing the pressure
Adding a catalyst
Removing some of the reactant gases
(b) Complete the sentence. According to Le Chatelier's principle, if a system at equilibrium is subjected to a change, the position of equilibrium moves to ________ the effect of the change.[1]
Show the answer and mark scheme
(a)Answer: Increasing the pressure
(b)Answer: counteract
counteract / oppose / minimise
Question 3Hard6 marks
Methanol is manufactured from carbon monoxide and hydrogen. CO(g) + 2H2(g) ⇌ CH3OH(g) The forward reaction is exothermic. Typical conditions are:
a temperature of 250 °C
a pressure of 50 to 100 atmospheres
a copper-based catalyst.
Explain why these conditions are used. Refer to the rate of reaction, the yield of methanol and cost in your answer.[6]
Show the answer and mark scheme
Answer: See indicative content.
the forward reaction is exothermic, so a lower temperature gives a higher yield of methanol
but at a low temperature the rate is too slow, so 250 °C is a compromise between rate and yield
there are 3 molecules of gas on the left and 1 on the right, so a high pressure gives a higher yield
a high pressure also increases the rate (more frequent collisions)
very high pressures are expensive (strong equipment, energy for compressors) and more dangerous, so 50 to 100 atmospheres is a compromise
the catalyst increases the rate (equilibrium is reached faster) without changing the yield, so a lower temperature can be used
unreacted carbon monoxide and hydrogen can be recycled
Marked with levels of response: the full level descriptors are in the app.