7.1 Equilibrium

Equilibrium Approach

Particles flash and convert as a reaction runs toward equilibrium. Concentration curves build above, and a live Q vs. K readout shows when equilibrium is reached.

Conc vs TimeRate CurvesParticle Sim5 Reactions
Topic 7.1

Introduction to Equilibrium

Explain the relationship between the occurrence of a reversible chemical or physical process, and the establishment of equilibrium, to experimental observations.

Many observable processes are reversible: evaporation and condensation of water, absorption and desorption of a gas, dissolution and precipitation of a salt. Important reversible chemical processes include the transfer of protons in acid–base reactions (Unit 8) and the transfer of electrons in redox reactions (Unit 9).

When equilibrium is reached, no observable changes occur. Reactants and products are simultaneously present, and the concentrations or partial pressures of all species remain constant.

But EK 7.1.A.3 is the sentence that matters most: the equilibrium state is dynamic. The forward and reverse processes continue to occur at equal rates, resulting in no net observable change. Nothing has stopped; the two directions simply cancel.

Two consequences students constantly get wrong:

  • Constant does not mean equal. At equilibrium [reactants] and [products] stop changing, but there is no reason for them to be equal to each other. K tells you the ratio, and it is rarely 1.
  • Equilibrium is not "half-reacted". A reaction with a huge K sits almost entirely on the product side and is still at equilibrium.

EK 7.1.A.4 asks you to read graphs: plots of concentration, partial pressure, or reaction rate versus time show equilibrium being established. On a concentration plot, the curves level off. On a rate plot, the forward rate falls and the reverse rate rises until they meet — and that meeting point is the moment the concentration curves go flat.

Key points

  • Equilibrium is dynamic: both directions continue at equal rates.
  • Concentrations become constant, not equal.
  • Equilibrium can be approached from either the reactant side or the product side and reaches the same K.
  • On a rate-vs-time graph, equilibrium is where the two curves meet.

Common mistakes

  • "The reaction stops at equilibrium." The single most penalized misconception in Unit 7.
  • "Concentrations are equal at equilibrium." They are constant, not equal.
  • Assuming equilibrium requires starting with reactants. Start with pure product and you reach the same equilibrium state.
  • Confusing equilibrium with completion. A large K means nearly complete, but the reverse reaction still runs.

Worked example

A sealed flask contains liquid water and water vapor at constant temperature. The water level stops changing. Explain what is happening at the particulate level, and predict what would happen to the vapor pressure if more liquid water were added.

At the particulate level: molecules at the liquid surface with enough kinetic energy continue to escape into the vapor phase, and vapor molecules continue to strike the surface and be captured. The water level stops changing because the rate of evaporation has become equal to the rate of condensation, not because either process has stopped. This is a dynamic equilibrium.

Adding more liquid water: the vapor pressure would be unchanged. Vapor pressure depends only on temperature and the identity of the liquid, not on how much liquid is present. Adding liquid increases the surface area, which raises both the evaporation and condensation rates equally, so the equilibrium position — and therefore the vapor pressure — is unaffected.

This mirrors why pure liquids and solids are excluded from equilibrium expressions (7.3).

Full notes for topic 7.1 →