H2 Chemistry Equilibrium: Equal Rates Do Not Mean Equal Amounts

Start with what an equilibrium shift means

An SGExams student asked why shifting a chemical equilibrium increases some substances and decreases others. This is a conceptual question worth answering before memorising a list of directions. The discussion is from 2022; current examination requirements should come from the official syllabus, not an older forum reply.

The 2026 H2 Chemistry 9476 syllabus includes dynamic equilibrium, effects of disturbances and equilibrium constants. This guide concentrates on the meaning of those ideas. Use the syllabus for your own examination year and your college’s examples when extending the explanation to calculations.

Separate equal rates from equal quantities

In a closed system at dynamic equilibrium under constant conditions, the forward and reverse reactions continue at equal rates. There is no net change in the amounts of the reacting species. This does not require equal reactant and product concentrations; one side can be present in much greater quantity.

Imagine people entering and leaving a room at the same rate. The number inside can remain steady without being equal to the number outside. This limited analogy illustrates equal rates, not molecular details. Chemical equilibrium must still be described using the actual reversible reaction and its conditions.

Use the balanced equation to track a shift

Consider the illustrative reversible reaction A ⇌ B in a closed container at constant temperature and volume. If A is added to an equilibrium mixture, its concentration immediately increases. The system then undergoes net forward reaction as it moves towards a new equilibrium: A is consumed and B is formed.

Keep two comparisons separate. During the adjustment, A decreases from its value immediately after addition. That statement alone does not prove its final concentration is below its original, pre-addition value. Questions about graphs often depend on distinguishing the instant of disturbance from the later adjustment.

Track stoichiometry without assuming equal concentrations

For a reaction such as N₂ + 3H₂ ⇌ 2NH₃, a net forward reaction consumes nitrogen and hydrogen and forms ammonia in the proportions shown by the balanced equation. Those change ratios do not mean the equilibrium amounts must be in a 1:3:2 ratio.

When describing a shift, state what was changed, which direction is favoured and which species are consumed or formed. Include relevant conditions. Avoid the vague claim that every change makes “more products”; removing a product and increasing temperature are different disturbances and require different reasoning.

Keep the equilibrium constant separate from speed

For a given reaction, changing concentration at constant temperature changes the mixture and can cause adjustment, but it does not change the equilibrium constant. A catalyst helps the system reach equilibrium faster without changing that constant or the equilibrium composition under the same conditions. Temperature changes can change the constant.

For a temperature question, identify whether the forward reaction is exothermic or endothermic before predicting a direction. For a gas-pressure question, inspect the balanced gaseous equation and how pressure is changed. Do not apply a memorised rule about the number of gas molecules without checking the actual disturbance.

Test the explanation before calculating

Choose an appropriate equilibrium question from our H2 Chemistry papers. Check the syllabus, write the balanced reaction and label the initial equilibrium, disturbance and new equilibrium. Sketch qualitative concentration changes before attempting numerical work.

If your prediction conflicts with the answer, locate the assumption that failed: equal rates versus amounts, the comparison time, stoichiometry or a temperature-dependent constant. Ask your tutor to review that step. A clear explanation of one disturbance gives a stronger foundation for equilibrium calculations than a long list of unexplained arrows.

Sources and further reading

Checked on 3 November 2026. The linked discussion is real; the examples and practice routines are our original guidance. Older forum posts are not evidence of current examination rules.

• [JC] help for H2 chem — 28 September 2022.
• SEAB H2 Chemistry 9476 syllabus, 2026