R2.3 How far? The extent of chemical changeIB Chemistry HL: Revision notes

Section 1

Dynamic equilibrium and Kc

In a closed system, dynamic equilibrium is reached when the forward and backward rates are equal; concentrations and macroscopic properties stay constant. Physical systems (e.g. liquid and vapour in a sealed flask) behave the same way.

For aA + bB ⇌ cC + dD (homogeneous): Kc = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ). K ≫ 1: mostly products; K ≪ 1: mostly reactants. K changes only with temperature. For the reverse reaction, K′ = 1/K.

Key termsdynamic equilibriumequilibrium constant

Section 2

Le Châtelier's principle

A system at equilibrium shifts to partly oppose a change. Adding reactant or removing product shifts right; higher pressure shifts towards fewer gas molecules (no shift if equal); higher temperature favours the endothermic direction. Only temperature changes K. A catalyst speeds both directions equally, so equilibrium is reached faster with no change in position or K.

Key termsLe Châtelier's principle

Common mistake

Halving the volume of H₂ + I₂ ⇌ 2HI does not shift the position, but the colour still darkens because [I₂] doubles.

Section 3

The reaction quotient, Q (HL)

The reaction quotient, Q, uses the same expression as K but with concentrations at any moment, not necessarily at equilibrium.

Q < K: too few products → net forward reaction.
Q > K: too many products → net reverse reaction.
Q = K: at equilibrium.

Q explains Le Châtelier: a concentration or pressure change alters Q (K fixed); a temperature change alters K (Q momentarily fixed). Either way, the system reacts until Q = K.

Key termsreaction quotient

Exam tip

Compressing a gas mixture multiplies each concentration by the same factor; count the powers on top and bottom to see which way Q moves.

Section 4

Equilibrium calculations (HL)

Use an ICE approach: Initial, Change, Equilibrium concentrations.

Convert amounts to concentrations (divide by volume).
Let x be the change; use the stoichiometry for the other species.
Substitute equilibrium values into K and solve.

If K is a perfect square ratio, take square roots. If K is very small, assume x is negligible compared with initial concentrations, and check. Given one equilibrium amount, work back to find the others and then K.

Key termsICE table

Common mistake

Using moles instead of concentrations; this only works when the moles cancel (equal total powers top and bottom).

Section 5

K and Gibbs energy (HL)

Both K and ΔG° measure the position of equilibrium: $\Delta G^\circ = -RT\ln K$ (data booklet).

ΔG° < 0 → K > 1, products favoured.
ΔG° = 0 → K = 1.
ΔG° > 0 → K < 1, reactants favoured.

Convert ΔG° to J mol⁻¹ before using R = 8.31 J K⁻¹ mol⁻¹. For an exothermic reaction, K falls as temperature rises.

Key termsGibbs energy change

Must know

Equal rates, constant concentrations, closed system.
Kc from coefficients; reverse K = 1/K; only temperature changes K.
(HL) Q < K forward, Q > K reverse.
(HL) ICE tables for equilibrium concentrations.
(HL) ΔG° = −RT ln K; negative ΔG° means K > 1.

That's the notes covered.

Carry on to the next subtopic.