A Level · Cambridge (CIE) · 9701

Dynamic equilibrium

Predict a change in equilibrium composition without confusing yield and reaction rate.

What you will learn

Describe dynamic equilibrium in a closed system.
Predict temperature effects for an exothermic forward reaction.

Equal rates, not stopped reactions

At dynamic equilibrium in a closed system, forward and reverse reactions continue at equal rates. The amounts of reactants and products stay constant over time, but they need not be equal to each other.

Disturbing an equilibrium changes the conditions. The system then approaches a new equilibrium composition. A catalyst speeds the approach by increasing both forward and reverse rates; it does not change the equilibrium composition at a fixed temperature.

Temperature changes the preferred direction

If the forward reaction is exothermic, heating favours the endothermic reverse direction and lowers the equilibrium product yield. Cooling favours the exothermic direction. A higher temperature may nevertheless make the reactions occur faster, so rate and final yield can move in different directions.

For an exothermic forward reaction: higher temperature → lower equilibrium product yield

Put the idea to work

Worked example

An exothermic synthesis is heated after reaching equilibrium. Explain the change in product yield.

Show the worked solution

The reverse reaction is endothermic.
Heating favours that direction relative to the forward reaction.
More product is converted back into reactants until a new equilibrium is reached.

Answer The equilibrium product yield decreases.

Common mistakes

Equal rates do not mean equal concentrations.
A catalyst changes the time to reach equilibrium, not the equilibrium yield.

Recall without your notes

What can you explain now?

Does adding a catalyst change the equilibrium constant at fixed temperature?

Compare with the explanation

It alters the rates of both directions but does not alter the equilibrium constant for that temperature.

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Keep learning Chemistry

Check the official syllabus 2028–2030

Course scope and assessment

Four hundred and forty-two original lessons map the published learning outcomes across AS Chemistry Topics 1–22 and A Level Topics 23–37, with guided, independent and transfer practice and explained answers. Every numbered subject-content topic and listed outcome tag now has a mapped lesson sequence. Topic 32 maps all 12 hydroxy-compound outcome tags in 11 lessons; Topic 33 maps all 15 carboxylic-acid and derivative outcome tags in 16 lessons; Topic 34 maps all 22 nitrogen-compound outcome tags in 20 lessons; Topic 35 maps all 12 polymerisation outcome tags in 16 lessons; Topic 36 maps all four organic-synthesis outcome tags in 14 lessons; Topic 37 maps all 22 chromatography and NMR outcome tags in 18 lessons. Cambridge's 2028–2030 Version 1 syllabus states that there are no significant changes which affect teaching from the preceding edition; the routes are aligned to that edition. This is 37 of the 37 numbered subject-content topics. Full cumulative exam assessment, Paper 3 laboratory competence, Paper 5 investigation practice and human subject-teacher review remain outstanding. Written practice is self-checked and does not certify exam readiness.