AQA GCSE Chemistry (8462), Higher tier · The rate and extent of chemical change › Reversible reactions and dynamic equilibrium
Practise Changing conditions and equilibrium. 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.
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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.