AQA GCSE Chemistry (8462), Higher tier · The rate and extent of chemical change › Reversible reactions and dynamic equilibrium
Practise Equilibrium. 12 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. Equilibrium is reached when the forward and reverse reactions happen at the same ________.
rate
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) Complete the sentence. Equilibrium is reached when the forward and reverse reactions happen at the same ________.[1]
(b) Equilibrium can only be reached in a closed system. What is meant by a closed system?[1]
(c) Which statement about a reaction at equilibrium is correct? Tick (✓) one box.[1]
The forward and reverse reactions have stopped.
The concentrations of the reactants and products are equal.
The concentrations of the reactants and products do not change.
Only the forward reaction is happening.
(d) Explain why this type of equilibrium is described as dynamic.[1]
Show the answer and mark scheme
(a)Answer: rate
rate
(b)Answer: Apparatus that stops reactants and products escaping.
apparatus that prevents reactants and products escaping
(c)Answer: The concentrations of the reactants and products do not change.
(d)Answer: The forward and reverse reactions are both still happening.
the forward and reverse reactions are both still happening
Question 2Medium3 marks
A student is monitoring a reversible reaction in which the reactant is colourless and the product is orange.
(a) Describe how the student could tell, just by observing the colour of the mixture, that the reaction has reached equilibrium.[1]
(b) Suggest why using a colorimeter, rather than judging the colour by eye, might give more reliable evidence for when equilibrium is reached.[2]
Show the answer and mark scheme
(a)Answer: The colour of the mixture stops changing (becomes constant).
the colour of the mixture stops changing / stays constant
(b)Answer: A colorimeter gives a precise numerical reading, so very small ongoing changes in colour (which the eye might miss) can still be detected, giving a more reliable indication of when the colour has truly stopped changing.
a colorimeter gives a precise numerical reading of colour/absorbance, rather than a subjective judgement by eye
so it can detect very small ongoing changes that the eye might not notice, giving stronger evidence that a constant value (equilibrium) has been reached
Question 3Hard3 marks
A student says: 'Once a reversible reaction reaches equilibrium, no more product can ever be made from it.'
Evaluate this statement.[3]
Show the answer and mark scheme
Answer: The statement is wrong. At equilibrium the forward reaction is still happening, so product is still being made, but it is used up by the reverse reaction at the same rate, so the amount stays constant. Also, the amounts only stay constant while the conditions stay the same: if a condition is changed (e.g. product removed, or the temperature or pressure changed), the position of equilibrium can shift and more product can be made.
the forward reaction is still happening at equilibrium, so product is still being made (the equilibrium is dynamic)
but product is used up by the reverse reaction at the same rate, so the amount of product stays constant
the amounts only stay constant while the conditions stay the same
if a condition is changed (e.g. product removed, temperature or pressure changed), the position of equilibrium can shift so that more product is made