7.2 Direction of Reversible Reactions

Reversible reactions reach equilibrium

In a reversible reaction, reactants form products and products re-form reactants simultaneously. Over time, the forward and reverse reaction rates become equal, and concentrations of reactants and products stop changing. This state is dynamic equilibrium — reactions still occur, but no net change is observed macroscopically.

Direction depends on Q vs K

The reaction quotient (Q) has the same form as the equilibrium constant expression but uses current concentrations. Comparing Q to K predicts direction: if Q < K, the reaction shifts forward (toward products); if Q > K, it shifts reverse (toward reactants); if Q = K, the system is at equilibrium.

Equilibrium is dynamic, not static

At equilibrium, both forward and reverse reactions continue at equal rates, so molecules keep converting back and forth even though macroscopic properties (concentration, color, pressure) stay constant. This distinguishes equilibrium from a reaction that has simply stopped.

Key terms

Reversible reaction

A reaction that can proceed in both the forward and reverse directions.

Dynamic equilibrium

A state where forward and reverse reaction rates are equal, so concentrations remain constant even though reactions continue.

Reaction quotient (Q)

An expression with the same form as K, calculated using current (non-equilibrium) concentrations, used to predict reaction direction.

If Q < K

Reaction shifts forward, forming more products.

If Q > K

Reaction shifts in reverse, forming more reactants.

Try an AP-style question

If Q < K, which way does the reaction proceed?

1. It stops
2. It doesn't shift
3. Forward, toward products
4. Reverse, toward reactants