Reversible Reactions and Dynamic Equilibrium
Master Reversible Reactions and Dynamic Equilibrium for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. In a closed system a reversible reaction reaches equilibrium, where the forward and backward reactions happen at the same rate.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
In a reversible reaction the products can react to re-form the original reactants. The reaction is shown with a double arrow, ⇌, rather than a single one.
When a reversible reaction occurs in a closed system, equilibrium is reached when the forward and reverse reactions occur at exactly the same rate. This is called dynamic equilibrium.
Dynamic is the key word. At equilibrium both reactions are still happening — they have not stopped — but because they proceed at the same rate, the concentrations of reactants and products remain constant. A closed system is required so that nothing can enter or leave.
Revision notes
Reversible reactions
The products of the reaction can react to produce the original reactants.
The symbol ⇌ is used instead of a single arrow. An example is ammonium chloride ⇌ ammonia + hydrogen chloride.
Reaching equilibrium
Equilibrium is reached when the forward and reverse reactions occur at exactly the same rate.
This requires a closed system, so that no reactants or products can escape or be added. At equilibrium the concentrations of all substances remain constant.
Why dynamic matters
At equilibrium the reactions have not stopped — both continue.
Because they proceed at equal rates, there is no net change in concentration. Saying the reaction has stopped is a common and costly error.
Key points
- A reversible reaction can go both ways.
- The symbol ⇌ shows a reversible reaction.
- Equilibrium needs a closed system.
- At equilibrium the forward and reverse rates are equal.
- Concentrations remain constant at equilibrium.
- Both reactions continue at equilibrium.
Worked examples
Example 1
Explain what is meant by dynamic equilibrium. [3 marks]
Model answer
Dynamic equilibrium is reached when the forward and reverse reactions are occurring at exactly the same rate (1 mark). Both reactions continue to happen (1 mark), but because the rates are equal the concentrations of reactants and products remain constant (1 mark).
Example 2
Explain why equilibrium can only be reached in a closed system. [2 marks]
Model answer
In a closed system no reactants or products can escape or be added (1 mark), so the reverse reaction can occur and the concentrations can settle at constant values (1 mark).
Example 3
State what the symbol ⇌ indicates. [1 mark]
Model answer
That the reaction is reversible, so the products can react to re-form the reactants (1 mark).
Common mistakes
Saying the reactions stop at equilibrium.
Both continue; they simply proceed at equal rates.
Saying the concentrations of reactants and products are equal.
They are constant, not necessarily equal to each other.
Forgetting the closed system requirement.
Without it, products escape and equilibrium cannot be established.
Using a single arrow for a reversible reaction.
The double arrow ⇌ must be used.
Exam tips
- Use the word dynamic and explain that both reactions continue.
- Say concentrations are constant, not equal.
- Always mention the closed system requirement.
- Draw the double arrow correctly.
Key terms
- Reversible reaction
- A reaction whose products can re-form the reactants.
- Dynamic equilibrium
- When forward and reverse reactions occur at equal rates.
- Closed system
- A system where nothing can enter or leave.
- Equilibrium
- A state where concentrations remain constant.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.