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2.3.1—Dynamic equilibrium

Syllabus
First assessment 2025
Objective
2.3.1
Level
HL

Dynamic Equilibrium

Dynamic equilibrium occurs in a closed system when the forward and reverse processes continue at equal rates. Macroscopic amounts remain constant, but reactants and products need not be equal in amount.

The same rate-balance idea applies to physical equilibria such as vaporization and condensation as well as to reversible chemical reactions.

In a sealed liquid–vapour system at equilibrium, molecules continue evaporating and condensing at equal rates, so pressure and amounts are constant on average. Equal rates do not mean equal concentrations, and opening the system can prevent equilibrium by allowing matter to escape.

Recognizing Dynamic Equilibrium

Assessment in practice

Representative question

Question 1

[Maximum number: 1]

Ammonia is manufactured by the Haber process.

N2( g)+3H2( g)⇌2NH3( g)ΔHr⊖=−92.0 kJ mol−1\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g}) \quad \Delta H_{\mathrm{r}}^{\ominus}=-92.0 \mathrm{~kJ} \mathrm{~mol}^{-1}

Outline what is meant by dynamic equilibrium.

Extent of Chemical Change Summary

Retrieve the route: define dynamic equilibrium, write K, interpret its magnitude, predict Le Châtelier shifts, compare Q with K, solve a RICE table, and connect K with ΔG.

Check closed-system and equal-rate language, exponents and direction, whether a change affects K, current versus equilibrium concentrations, stoichiometric x changes, and kelvin/unit consistency in ΔG calculations.