AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Reversible reactions: In some chemical reactions, the products of the reaction can react to produce the original reactants. Such reactions are called reversible reactions and are represented:
Most reactions you meet are shown as one-way: reactants change into products and stop.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Most reactions you meet are shown as one-way: reactants change into products and stop. In a reversible reaction, the products can react together to reform the original reactants, so both changes happen in the same mixture. We show this with the special symbol ⇌ instead of a single arrow. For example, heating hydrated copper(II) sulfate drives off water to give white anhydrous copper(II) sulfate; adding water to the white solid regenerates the blue hydrated salt. The forward change and the reverse change are separate reactions, and the symbol ⇌ reminds you that both are possible. Reversibility does not mean the reaction never finishes; it means the system can move in either direction depending on conditions such as temperature, pressure or concentration.
What earns the marks
- A reversible reaction is one in which the products can react to form the original reactants.
- The reaction is represented using the symbol ⇌ between reactants and products, not a single arrow.
- The forward reaction converts reactants into products; the reverse reaction converts products back into reactants.
- An example is the hydration and dehydration of copper(II) sulfate: blue hydrated ⇌ white anhydrous + water.
- Recognising ⇌ in an equation shows the reaction is reversible rather than complete in one direction.
Common mistakes and how to put them right
Writing a single arrow → for a reversible reaction.
Put it right: Use ⇌ to show both directions are possible.
Thinking reversible means the reaction never reaches an end point.
Put it right: Reversible reactions can still settle at a balance point where forward and reverse rates are equal.
Assuming the reverse reaction always happens at the same rate as the forward reaction.
Put it right: The rates depend on the conditions, so one direction may be favoured.
Exam tips
- When you see ⇌ in an equation, state clearly that the reaction is reversible and name the forward and reverse changes.
- Use a named example such as hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water to explain the idea.
- If asked to explain, link the direction of the reaction to a change in conditions such as heating or adding water.
Try the lesson
The start of the Reversible reactions: In some chemical reactions, the products of the reaction can react to produce the original reactants. Such reactions are called reversible reactions and are represented: lesson, made from checked teaching notes for this specification point.
Have a go first
Which symbol is used in a chemical equation to show that a reaction is reversible?
Listen
Most reactions you meet are shown as one-way: reactants change into products and stop. In a reversible reaction, the products can react together to reform the original reactants, so both changes happen in the same mixture. We show this with the special symbol ⇌ instead of a single arrow. For example, heating hydrated copper(II) sulfate drives off water to give white anhydrous copper(II) sulfate; adding water to the white solid regenerates the blue hydrated salt. The forward change and the reverse change are separate reactions, and the symbol ⇌ reminds you that both are possible. Reversibility does not mean the reaction never finishes; it means the system can move in either direction depending on conditions such as temperature, pressure or concentration. A strong answer earns marks for these points. A reversible reaction is one in which the products can react to form the original reactants. The reaction is represented using the symbol ⇌ between reactants and products, not a single arrow. The forward reaction converts reactants into products; the reverse reaction converts products back into reactants. An example is the hydration and dehydration of copper(II) sulfate: blue hydrated ⇌ white anhydrous + water. A common slip: Writing a single arrow → for a reversible reaction. Instead, use ⇌ to show both directions are possible. Exam tip: When you see ⇌ in an equation, state clearly that the reaction is reversible and name the forward and reverse changes.
See it
Key words
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Reversible reaction
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⇌
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Forward reaction
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Reverse reaction
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See every definition
- Reversible reaction
- A reaction in which the products can react to produce the original reactants.
- ⇌
- The symbol used between reactants and products to show that both directions are possible, not a single arrow.
- Forward reaction
- The change that converts reactants into products.
- Reverse reaction
- The change that converts products back into reactants.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
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