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wise otter.Chemistry
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9476 / 2027
Chemical Equilibria

Lesson 1 of 4

What equilibrium means

Both directions continue even when the visible properties stop changing.

A-Level 9476 (2026-2027)

  • I can explain reversibility and dynamic equilibrium through rates.Syllabus

    Syllabus 9476, 9(a). Explain reversibility and dynamic equilibrium through rates.

Equal rates give constant composition, not equal amounts

A closed system can keep reacting in both directions with no net change.

A reversible reaction can proceed in both directions under appropriate conditions. In a closed system initially containing reactants, the forward reaction forms products. As products accumulate, the reverse reaction becomes possible and its rate increases. Dynamic equilibrium is reached when the forward and reverse rates are equal, so macroscopic properties such as concentration and colour remain constant.

Follow A <=> B from a reactant-rich starting mixture
  1. Initially

    Forward rate is large relative to reverse rate; A decreases and B increases.

  2. As composition changes

    The forward and reverse rates approach one another. Their dependence on concentration comes from their mechanisms.

  3. At dynamic equilibrium

    Forward rate = reverse rate, both usually nonzero. Amounts remain constant but need not be equal.

A closed system prevents material loss; it need not be thermally insulated. Constant temperature is needed when discussing one fixed equilibrium constant. The same equilibrium composition can be approached from either side if the overall amounts, temperature and volume or pressure constraints are the same.

Check your understanding

A sealed equilibrium mixture keeps the same colour for an hour. Does that prove the reactions have stopped?

  1. Yes. A constant colour shows that no molecules are reacting any more.
  2. No. A constant colour shows the forward reaction has finished, but the reverse still runs.
  3. No. The concentrations are constant because the forward and reverse rates are equal.
Show the answer

No. The concentrations are constant because the forward and reverse rates are equal.

At dynamic equilibrium both reactions continue at the same rate, so nothing visible changes. Constant colour does not mean equal concentrations either.