7.1 Introduction to Equilibrium

What is chemical equilibrium?

Equilibrium occurs when the forward and reverse rates of a reversible reaction become equal, so the concentrations of reactants and products stay constant over time. This is a dynamic state — the reactions haven't stopped, they're just occurring at the same rate in both directions. It occurs in a closed system.

Reaction quotient Q vs. equilibrium constant K

The reaction quotient (Q) has the same expression as K ([products]^coeff / [reactants]^coeff) but can be calculated at *any* point in a reaction. At equilibrium, Q = K. Comparing Q to K tells you which direction a reaction will shift: if Q < K, the reaction proceeds forward; if Q > K, it proceeds in reverse.

Macroscopic constancy, microscopic motion

At equilibrium, macroscopic properties like color, pressure, and concentration appear unchanging. But at the molecular level, molecules continue to react — particles are still colliding and converting between reactants and products. This is why equilibrium is dynamic, not static.

Key terms

Chemical equilibrium

State where forward and reverse reaction rates are equal, so concentrations of reactants/products remain constant.

Dynamic equilibrium

Equilibrium where reactions continue occurring in both directions at equal rates, even though overall concentrations don't change.

Reaction quotient (Q)

Same expression as K, but can be calculated at any point in a reaction (not just at equilibrium).

Q < K

Reaction shifts forward (toward products) to reach equilibrium.

Q > K

Reaction shifts in reverse (toward reactants) to reach equilibrium.

Try an AP-style question

What is true at chemical equilibrium?

1. The forward and reverse rates are equal
2. Reactant and product concentrations are equal
3. Only the forward reaction still occurs
4. All reactions have stopped completely