AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › The rate and extent of chemical change › Reversible reactions and dynamic equilibrium
Practise Equilibrium. 5 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
In a closed system, a reversible reaction reaches equilibrium when the forward and reverse reactions happen at exactly the same rate. You need to explain what 'dynamic' means, interpret graphs and data showing when equilibrium is reached, and avoid the classic trap of saying the amounts are equal.
Key facts
Closed system: reactants and products cannot escape
Equilibrium: rate of forward reaction = rate of reverse reaction
Dynamic: both reactions keep going, at the same rate
At equilibrium, concentrations are constant (not necessarily equal)
Before equilibrium: forward rate falls, reverse rate rises
Horizontal lines on a concentration–time graph = equilibrium
Notes
Reaching equilibrium
Equilibrium can only be reached in a closed system: apparatus that prevents the reactants and products from escaping, such as a sealed container.
At the start there are only reactants, so only the forward reaction happens, at its fastest rate.
As the reactants are used up, the forward reaction slows down. As the products build up, the reverse reaction speeds up.
Equilibrium is reached when the forward and reverse reactions happen at exactly the same rate.
What equilibrium means
The equilibrium is dynamic: both reactions are still happening, but at the same rate.
So the amounts (and concentrations) of the reactants and products stay constant.
Constant does not mean equal. At equilibrium there may be much more product than reactant, or much less.
If a product escapes (an open system), it cannot react to re-form the reactants, so equilibrium is not reached.
Graphs and data
diagram
Rate against time: the forward rate falls and the reverse rate rises (from zero) until they meet. After that, both lines are horizontal at the same value.
Rates: the forward rate falls, the reverse rate rises, and they meet at equilibrium.
Concentration against time: reactant concentrations fall and product concentrations rise, then all the lines become horizontal. Equilibrium is reached when they first level off.
Horizontal lines on a concentration graph do not mean the reaction has stopped. Both reactions are still going on.
Concentrations: the lines go flat at equilibrium, but they are not equal.
How to answer each type of question
Recall: what is meant by dynamic equilibrium
2 marksGrade 5
Say both the forward and reverse reactions are still happening.
Say they happen at the same rate.
Example. Hydrogen and iodine react in a sealed flask to form hydrogen iodide. H2(g) + I2(g) ⇌ 2HI(g) What is meant by saying that this reaction is at dynamic equilibrium?
Show the model answerHide the model answer
The forward and reverse reactions are both still happening (1) at the same rate (1).
Don’t lose marks
Saying the concentrations of reactants and products are equal at equilibrium.
Saying the reaction has stopped at equilibrium.
Forgetting that equilibrium can only be reached in a closed system.
Writing that the reactions happen 'at the same time' instead of 'at the same rate'.
More tips
Memory tricks
Picture walking down an up escalator at the same speed it moves: you stay in one place, but both movements carry on.
Equilibrium = equal RATES, not equal AMOUNTS.
Closed system = nothing escapes.
Exam technique
Two marks for 'dynamic equilibrium' usually need both ideas: both reactions continue AND they happen at the same rate.
Use 'rate' when writing about the reactions, and 'amount' or 'concentration' when writing about the substances.
On graphs and tables, give the time when the values first stop changing as the time equilibrium was reached.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 4
Define a closed systemApparatus that prevents the reactants and products from escaping.
Grade 5
State when equilibrium is reachedWhen the forward and reverse reactions happen at exactly the same rate.
Grade 5
Know that amounts stay constant at equilibriumThe concentrations of reactants and products stop changing, but they are not necessarily equal.
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. 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