7.8 Representations of Equilibrium

Particulate diagrams at equilibrium

At equilibrium, particulate representations show constant numbers of reactant and product particles over time, not equal numbers. Once equilibrium is reached, the ratio of particles stays fixed even though molecules keep reacting in both directions at the macroscopic level — this is the essence of dynamic equilibrium.

Graphs of concentration vs. time

On a concentration vs. time graph, reactant and product concentrations change rapidly at first, then level off into horizontal lines once equilibrium is established. The lines becoming flat (constant slope of zero) indicates rates of forward and reverse reactions are equal, not that concentrations are equal.

Comparing K to Q using diagrams

Given a particulate diagram, you can count particles to calculate Q and compare it to K to predict the direction of shift needed to reach equilibrium. If Q < K, the reaction proceeds forward (more products form); if Q > K, it shifts backward (more reactants form).

Key terms

Particulate diagram at equilibrium

A visual representation showing constant relative numbers of reactant and product particles over time.

Dynamic equilibrium

State where forward and reverse reactions continue at equal rates, keeping concentrations constant.

Concentration vs. time graph at equilibrium

Concentrations change initially, then level off into horizontal lines once equilibrium is reached.

Using Q from a particulate diagram

Count particles to calculate Q, then compare to K to predict the direction the reaction will shift.

Flat lines on a concentration graph mean...

The forward and reverse reaction rates are equal, so net concentration is no longer changing.

Try an AP-style question

What is K?

1. 2
2. 6
3. 8
4. 0.5