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H2 Chemistry

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Chemical Equilibria

Explain a changing equilibrium through competing rates, then calculate composition and justify industrial choices.

A-Level 9476 (2026-2027)

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4 topics

  1. What equilibrium means

    Both directions continue even when the visible properties stop changing.

  2. Concentration, pressure and temperature

    Predict the response and distinguish a shifted position from a changed constant.

  3. Equilibrium expressions and calculations

    Use equilibrium concentrations or partial pressures, with stoichiometry to find missing amounts.

  4. The Haber process

    Balance equilibrium yield, reaction speed and operating cost.

Scope and references

Learning outcomes and sources

9. Chemical Equilibria. Use the outcome map to find the explanation for a particular syllabus requirement.

See the learning outcome map

  1. 9(a) Explain reversibility and dynamic equilibrium through rates.

    • Forward and reverse reactions
    • Equal nonzero rates
    • Constant but not necessarily equal amounts
    • Closed system

    Equal rates give constant composition, not equal amounts

  2. 9(b) Apply Le Chatelier Principle to disturbances.

    • Concentration
    • Pressure with stated constraints
    • Temperature and reaction enthalpy

    A disturbed equilibrium responds against the imposed change

  3. 9(c) Distinguish changes in K from changes in composition.

    • Concentration, pressure, temperature
    • Catalyst
    • Temperature dependence of K

    Temperature changes the constant; other disturbances change the route back to it

  4. 9(d) Deduce concentration and partial-pressure expressions.

    • Kc
    • Kp
    • Coefficients as powers and treatment of phases
    • Kp-Kc relationship not required

    Build K from the written equation and equilibrium quantities

  5. 9(e) Calculate Kc or Kp from equilibrium data.

    • Equilibrium concentrations
    • Equilibrium partial pressures
    • Stoichiometry and units

    Build K from the written equation and equilibrium quantitiesUse stoichiometry to complete the equilibrium row

  6. 9(f) Calculate quantities present at equilibrium.

    • Initial/change/equilibrium amounts
    • Given equilibrium constant and data
    • No quadratic-equation solution required

    Use stoichiometry to complete the equilibrium row

  7. 9(g) Explain industrial Haber-process conditions.

    • Equilibrium yield and reaction-rate compromise
    • Pressure, temperature, catalyst
    • Product removal and recycling

    A useful ammonia process needs both yield and speed