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Learn9701Paper 2 · AS Level Structured QuestionsEquilibriaEquilibrium and Kc
Equilibria · Subtopic 1 of 2

Equilibrium and Kc

At dynamic equilibrium in a closed system, forward and reverse rates are equal and concentrations are constant. Le Chatelier's principle: if conditions change, the equilibrium shifts to oppose the change. For aA+bB⇌cC+dDa\mathrm{A}+b\mathrm{B}\rightleftharpoons c\mathrm{C}+d\mathrm{D}, Kc=[C]c[D]d[A]a[B]bK_c=\frac{[\mathrm{C}]^c[\mathrm{D}]^d}{[\mathrm{A}]^a[\mathrm{B}]^b} (products over reactants, powers = coefficients). Use an ICE table to find equilibrium concentrations. Only temperature changes KcK_c: for exothermic forward reactions, raising TT lowers KcK_c. Concentration and pressure changes shift the position but not KcK_c; catalysts change neither.

Cambridge 9701 Paper 1Cambridge 9701 Paper 2Edexcel YCH11 WCH12AQA 7405 Paper 1≈ 50 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe dynamic equilibrium and apply Le Chatelier's principle.
  • Write KcK_c expressions with units.
  • Calculate KcK_c and equilibrium amounts using ICE tables.
  • Explain the effect of conditions on KcK_c.
Pause and think

If you add more reactant to an equilibrium, the position shifts right. Does KcK_c change?

Section 2 of 14

Core idea

Start herePushing back

Squeeze an equilibrium and it pushes back. Heat it and it tries to cool down. Le Chatelier's principle predicts how, and KcK_c tells you how far.

The big idea

KcK_c is a fixed ratio of product to reactant concentrations at a given temperature. Changing concentration or pressure shifts the position to restore KcK_c; changing temperature changes KcK_c itself.

1. ICE tables

Worked exampleHydrogen iodide

1.00 mol H₂ and 1.00 mol I₂ in 1.00 dm³; at equilibrium 1.56 mol HI.

Goal
KcK_c.
Strategy
Initial, change, equilibrium.
  1. I: 1.00, 1.00, 0\text{I}:\ 1.00,\ 1.00,\ 0
    Initial moles.
  2. C: −0.78, −0.78, +1.56\text{C}:\ -0.78,\ -0.78,\ +1.56
    Ratio 1 : 1 : 2.
  3. E: 0.22, 0.22, 1.56\text{E}:\ 0.22,\ 0.22,\ 1.56
    Equilibrium.
  4. Kc=1.5620.22×0.22=50K_c=\frac{1.56^2}{0.22\times0.22}=50
    No units.
AnswerKc≈50K_c\approx50.
✓ CheckLarge value: equilibrium lies to the right ✓.

2. What changes KcK_c?

Effects on position and Kc

Only temperature changes Kc.

Exam tipUnits of KcK_c Write concentration units in the expression and cancel. When moles of gas are equal on both sides, KcK_c has no units.
Pause and think

If KcK_c for a reaction is 0.001, what does that tell you?

Summary

  • Le Chatelier.
  • KcK_c expression and units.
  • ICE tables.
  • Only TT changes KcK_c.
Key words
Dynamic equilibrium
Forward and reverse rates equal; concentrations constant.
Equilibrium constant
Ratio of product to reactant concentrations at equilibrium.
Le Chatelier's principle
A system at equilibrium opposes any change.
HistoryLe Chatelier. Henri Le Chatelier published his principle in 1884. He later tried to synthesise ammonia but abandoned the work after an explosion, years before Haber succeeded.
Fun factCarbon monoxide poisoning. CO binds to haemoglobin over 200 times more strongly than O₂. Treatment with pure oxygen shifts the equilibrium back, releasing the CO.
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