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wise otter.Chemistry
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Chemical Equilibria

Topic 1 of 4

What equilibrium means

Both directions continue even when the visible properties stop changing.

A-Level 9476 (2026-2027)

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 understandingA sealed equilibrium mixture keeps the same colour for an hour. Does that prove the reactions have stopped?Think it through, then reveal the answer
No. Constant colour is consistent with constant concentrations while equal forward and reverse reactions continue. It does not mean the reactant and product concentrations are equal.