Explain what is meant by a reversible reaction reaching equilibrium, and describe what happens to the rate of the forward and reverse reactions at equilibrium.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A reversible reaction is one in which the products can react to reform the original reactants, so the reaction can proceed in both directions.
Equilibrium is achieved only in a closed system where neither reactants nor products can escape.
At equilibrium the rate of the forward reaction equals the rate of the reverse reaction.
Consequently the concentrations of reactants and products remain constant and do not change.
Equilibrium is achieved only in a closed system where neither reactants nor products can escape.
At equilibrium the rate of the forward reaction equals the rate of the reverse reaction.
Consequently the concentrations of reactants and products remain constant and do not change.
Examiner tips
- Use the word "reversible" to show you understand the bidirectional nature of the reaction.
- Mention that equilibrium occurs in a closed system to avoid loss of species.
- State that forward and reverse rates are equal at equilibrium.
- Explain that concentrations stay constant once equilibrium is reached.
Common mistakes
- Confusing equilibrium with a stopped reaction – the reaction still proceeds but at equal rates.
- Omitting the requirement of a closed system.
- Failing to mention that concentrations remain constant at equilibrium.
Mark scheme (4 marks)
- A reversible reaction is one where the products can react to re-form the original reactants / the reaction can go in both directions.
- Equilibrium is only reached when the reaction occurs in a closed system / apparatus that prevents reactants or products from escaping.
- At equilibrium the forward reaction and the reverse reaction occur at exactly the same rate.
- The concentrations of reactants and products remain constant / do not change at equilibrium.
Key terms in this question
reversible reaction · equilibrium · reverse reaction
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Reversible reactions and dynamic equilibrium questions
- A student says: 'Once a reversible reaction reaches equilibrium, the reaction ha…
- A reversible reaction takes place in a sealed container. Describe what happens t…
- A reversible reaction takes place in a closed container. After some time, the sy…
- A reversible reaction is carried out in a closed container. The reaction reaches…