AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
In a reversible reaction at equilibrium: forward and reverse reactions…
Full specification wording
Chemical equilibria and Le Chatelier's principle: In a reversible reaction at equilibrium: forward and reverse reactions proceed at equal rates the concentrations of reactants and products remain constant
Equilibrium is dynamic, not static.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Equilibrium is dynamic, not static. Both reactions are still happening, but at exactly equal rates, so every molecule of product formed is matched by one converted back and the macroscopic composition is fixed. The two features examiners want are precisely these: the forward and reverse reactions proceed at equal rates, and the concentrations of reactants and products remain constant. The second point needs care. Constant does not mean equal: an equilibrium mixture can contain a great deal of product and very little reactant, and writing that the concentrations of reactants and products are the same is refused. A closed system is needed, since anything that escapes cannot come back, but the closed system is a condition rather than one of the two features.
What earns the marks
- Stating that the forward and reverse reactions proceed at equal rates.
- Stating that the concentrations of reactants and products remain constant.
- Identifying the equilibrium as dynamic, with both reactions still occurring.
- Recognising that a closed system is required so that no substance enters or leaves.
Common mistakes and how to put them right
Writing that the concentrations of reactants and products are equal or are the same, which examiners refuse.
Saying the reactions stop when equilibrium is reached.
Offering a closed system as one of the two required features, where it is ignored as an answer.
Stating equal amounts rather than constant concentrations.
Claiming equilibrium means the reaction is exactly half complete.
Exam tips
- Learn the two features word for word; this question is asked in almost identical form year after year.
- Write remain constant, never stay equal, when describing the concentrations.
- If asked for evidence that an equilibrium is dynamic, refer to a labelled isotope appearing on both sides over time.
Try the lesson
The start of the Chemical equilibria and Le Chatelier's principle: In a reversible reaction at equilibrium: forward and reverse reactions proceed at equal rates the concentrations of reactants and products remain constant lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement describes the rates of the forward and reverse reactions when a reversible reaction is at equilibrium?
Listen
Equilibrium is dynamic, not static. Both reactions are still happening, but at exactly equal rates, so every molecule of product formed is matched by one converted back and the macroscopic composition is fixed. The two features examiners want are precisely these: the forward and reverse reactions proceed at equal rates, and the concentrations of reactants and products remain constant. The second point needs care. Constant does not mean equal: an equilibrium mixture can contain a great deal of product and very little reactant, and writing that the concentrations of reactants and products are the same is refused. A closed system is needed, since anything that escapes cannot come back, but the closed system is a condition rather than one of the two features. A strong answer earns marks for these points. stating that the forward and reverse reactions proceed at equal rates. stating that the concentrations of reactants and products remain constant. identifying the equilibrium as dynamic, with both reactions still occurring. recognising that a closed system is required so that no substance enters or leaves. Exam tip: Learn the two features word for word; this question is asked in almost identical form year after year.
See it
Key words
Say what each word means before you turn the card.
dynamic equilibrium
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equal forward and reverse rates
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constant concentrations
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closed system
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See every definition
- dynamic equilibrium
- Both reactions are still occurring, with the forward and reverse reactions proceeding at equal rates.
- equal forward and reverse rates
- The forward and reverse reactions proceed at equal rates, so every molecule of product formed is matched by one converted back.
- constant concentrations
- The concentrations of reactants and products remain constant, and constant does not mean equal.
- closed system
- A closed system is required so that no substance enters or leaves, since anything that escapes cannot come back.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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