IB Chemistry SL — Reactivity 2.3: The Extent of Chemical Change (Equilibrium & Le Châtelier)

Many reactions are reversible and reach a dynamic equilibrium in a closed system: the forward and reverse reactions carry on, but at equal rates, so concentrations stop changing. The equilibrium constant Kc measures how far the reaction goes, and Le Châtelier's principle predicts how it responds to change.

👆 Watch the rates meet · apply a stress to the Haber equilibrium · read the Kc magnitude meter

1. Dynamic equilibrium R2.3.1

In a closed system, a reversible reaction reaches dynamic equilibrium when the rate of the forward reaction equals the rate of the reverse reaction. At that point the concentrations of reactants and products stay constant (but generally are not equal), and macroscopic properties like colour and pressure are unchanging — yet both reactions are still happening at the molecular level. Watch the two rates approach and meet:

Forward rate falls (reactants used up); reverse rate rises (products build up). Where they meet and stay equal is equilibrium.

Four characteristics to quote: it is reached in a closed system; forward and reverse rates are equal; concentrations remain constant; and it is dynamic (both reactions continue).

2. The equilibrium constant, Kc R2.3.2

For a homogeneous equilibrium aA + bB ⇌ cC + dD, the equilibrium law gives the equilibrium constant as products over reactants, each raised to its coefficient:

Kc = [C]^c[D]^d ⁄ [A]^a[B]^b

For the Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), this is Kc = [NH₃]² ⁄ ([N₂][H₂]³). Pure solids and liquids are left out; only the equilibrium concentrations go in.

3. What the magnitude of Kc tells you R2.3.3

The size of Kc shows how far the reaction proceeds before reaching equilibrium:

reactants favouredproducts favoured

Only temperature changes Kc. Changing concentration or pressure, or adding a catalyst, shifts the position of equilibrium but leaves Kc the same value (at constant temperature).

4. Le Châtelier's principle R2.3.4

Le Châtelier's principle: if a change is imposed on a system at equilibrium, the position of equilibrium shifts to oppose (counteract) the change. Apply a stress to the exothermic Haber equilibrium N₂ + 3H₂ ⇌ 2NH₃　(ΔH = −92 kJ mol⁻¹):

Doing Higher Level? HL extends R2.3 with Kc calculations from equilibrium concentrations, the reaction quotient Q (predicting which way a reaction runs), and the link ΔG° = −RT ln K. → Open the HL lesson

Common mistakes examiners see

At equilibrium, are the concentrations of reactants and products equal?✗ Yes, they become equal. ✓ No — they become constant, not equal. The forward and reverse rates are what become equal.

Does a catalyst change the position of equilibrium or the yield?✗ Yes, it increases the yield. ✓ No — a catalyst speeds up forward and reverse equally, so equilibrium is reached faster but the position and Kc are unchanged.

What changes the value of Kc?✗ Concentration, pressure or a catalyst. ✓ Only temperature changes Kc. The others shift the position of equilibrium but leave Kc the same.

Increasing pressure on N₂ + 3H₂ ⇌ 2NH₃ shifts the equilibrium…✗ Left, towards the reactants. ✓ Right — towards the side with fewer moles of gas (4 mol → 2 mol), opposing the pressure rise.

Raising the temperature of an exothermic reaction does what to the yield?✗ Increases it. ✓ Decreases it — the equilibrium shifts in the endothermic (reverse) direction, and Kc falls.

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The Extent of Chemical Change