AP Chemistry Unit 7: Equilibrium Notes

Introduction

Equilibrium is dynamic: forward and reverse molecular events continue while their rates are equal and macroscopic composition is constant.

The equilibrium constant describes composition at a stated temperature. The reaction quotient uses the same expression to diagnose the direction of net change before equilibrium.

ICE tables, particle models, and solubility expressions must all preserve stoichiometry, mass, and charge. Disturbance analysis begins with the immediate physical change, then uses Q versus K.

Current exam guidance: Aligned to the current 2026–27 AP Chemistry course framework and exam scope.

7.1

Introduction to Equilibrium

What remains active when a reversible system reaches equilibrium?

Short answer

Forward and reverse molecular events continue at equal rates, so concentrations stay constant even though individual particles keep reacting.

Know firstReversible reactions · Rate · Concentration

Constant composition comes from equal opposing rates

In a closed reversible system, reactants form products while products reform reactants. Early in the process one direction may dominate, but changing concentrations alter collision frequencies until the two rates become equal. At that point there is no net composition change.

Equal rates do not mean equal concentrations. The equilibrium mixture can strongly favor either side, and particles do not stop. A flat concentration graph is macroscopic evidence of balanced traffic, not molecular inactivity.

Chemistry model

Equilibrium is dynamic

Move time and watch forward and reverse rates become equal while concentrations become constant, not necessarily equal.

Time4.0 min

Time: 4.0 min
Forward rate: 0.589
Reverse rate: 0.461
Equal-rate limit: 0.500

forward ratereverse rate

at equilibrium: rateforward = ratereverse

Read this figure without the graphic

Equilibrium is dynamic. The figure connects forward rate, reverse rate, constant concentrations so the relationship can be read without relying on color alone.

Representation	What to read
forward rate	starting evidence or quantity
reverse rate	intermediate relationship
constant concentrations	result or conclusion

Three snapshots of one reversible system

State	Forward rate	Reverse rate	Net change
Before equilibrium	0.080 M s^-1	0.020 M s^-1	toward products
At equilibrium	0.050 M s^-1	0.050 M s^-1	zero
At equilibrium later	0.050 M s^-1	0.050 M s^-1	zero

Keep these three ideas

Equilibrium is dynamic.
Equal rates create constant composition.
Equilibrium concentrations need not be equal.

Keep reading Unit 7: Equilibrium

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7.1 Introduction to EquilibriumRead

7.2 Direction of Reversible Reactions

7.3 Reaction Quotient and Equilibrium Constant

7.4 Calculating the Equilibrium Constant

7.5 Magnitude of the Equilibrium Constant

7.6 Properties of the Equilibrium Constant

7.7 Calculating Equilibrium Concentrations

7.8 Representations of Equilibrium

7.9 Introduction to Le Châtelier’s Principle

7.10 Reaction Quotient and Le Châtelier’s Principle

7.11 Introduction to Solubility Equilibria

7.12 Common-Ion Effect

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