Describe what is meant by a reversible reaction and explain how a reversible reaction carried out in a closed system eventually reaches dynamic equilibrium.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Some chemical reactions can proceed in both the forward and backward directions. When such a reaction is carried out in a closed system, a special condition known as dynamic equilibrium can be established.
Model answer (4 marks)
A reversible reaction is one in which the products can react together to reform the reactants.
In a closed system the reaction starts with the forward process only (or the forward rate is initially higher), so the concentration of products rises.
As the product concentration increases, the rate of the backward reaction rises while the forward rate falls.
Dynamic equilibrium is reached when the forward and backward rates are equal; the concentrations of reactants and products then remain constant.
In a closed system the reaction starts with the forward process only (or the forward rate is initially higher), so the concentration of products rises.
As the product concentration increases, the rate of the backward reaction rises while the forward rate falls.
Dynamic equilibrium is reached when the forward and backward rates are equal; the concentrations of reactants and products then remain constant.
Examiner tips
- Use the exact definition of a reversible reaction; mention products reforming reactants.
- Explain the sequence: start with forward, product build‑up, rates change, equality of rates.
- State that concentrations become constant at equilibrium.
- Use the term "dynamic equilibrium" and link it to equal rates.
Common mistakes
- Confusing reversible with irreversible reactions; not stating that products can reform reactants.
- Failing to mention that the system is closed and that the forward rate initially dominates.
- Losing the point that concentrations remain constant once equilibrium is reached.
Mark scheme (4 marks)
- A reversible reaction is one in which the products can react together to reform the reactants.
- At the start, only the forward reaction occurs (or the forward reaction is faster than the backward reaction), so the concentration of products increases.
- As products build up, the rate of the backward reaction increases (and the rate of the forward reaction decreases).
- Dynamic equilibrium is reached when the rate of the forward reaction equals the rate of the backward reaction, so the concentrations of reactants and products remain constant.
Key terms in this question
reversible reaction · dynamic equilibrium · closed system
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
More Reversible reactions and equilibria questions
- The Haber process is a reversible reaction used to produce ammonia. The reaction…
- When copper sulfate solution is mixed with excess anhydrous white copper sulfate…
- A closed flask contains the following reversible reaction: dinitrogen tetroxide …
- Explain what is meant by dynamic equilibrium in a reversible reaction, and descr…