Chemical Equilibrium Foundations

Reversible Reaction

A chemical reaction that can proceed in both the forward direction, where reactants form products, and the reverse direction, where products form reactants.

Reversible Reaction Symbol

The double arrow (⇌) indicates that a chemical reaction can proceed in both the forward and reverse directions.

Forward Reaction

The direction of a reversible reaction in which the substances written as reactants are converted into the substances written as products.

Reverse Reaction

The direction of a reversible reaction in which the substances written as products are converted back into the substances written as reactants.

Forward Reaction Rate

The rate at which reactants are converted into products in a reversible reaction.

Reverse Reaction Rate

The rate at which products are converted back into reactants in a reversible reaction.

Initial State of a Reversible Reaction Starting with Reactants

At the beginning, reactant concentrations are relatively high and product concentrations are relatively low or zero, so the forward reaction initially dominates.

Why Does the Forward Reaction Rate Decrease with Time?

As reactants are consumed, their concentrations decrease, causing the forward reaction rate to decrease.

Why Does the Reverse Reaction Rate Increase with Time?

As products accumulate, their concentrations increase, allowing the reverse reaction to occur increasingly rapidly.

Dynamic Equilibrium

The state of a reversible reaction in which the forward and reverse reactions continue to occur but do so at equal rates.

Condition for Dynamic Equilibrium

Dynamic equilibrium is reached when the forward reaction rate equals the reverse reaction rate.

Forward and Reverse Rates at Equilibrium

At equilibrium, the forward and reverse reaction rates are equal.

Do Chemical Reactions Stop at Equilibrium?

No. Both the forward and reverse reactions continue to occur at equilibrium.

Why Is Chemical Equilibrium Called Dynamic?

It is dynamic because reactants continue forming products and products continue forming reactants even though there is no net change in their concentrations.

Net Change at Equilibrium

There is no net change in reactant or product concentrations because the forward and reverse reactions occur at equal rates.

Concentrations at Equilibrium

The concentrations of reactants and products remain constant with time once equilibrium has been established.

Does Equilibrium Mean Equal Reactant and Product Concentrations?

No. Equilibrium requires equal forward and reverse reaction rates, not equal concentrations of reactants and products.

Constant Concentration vs. Equal Concentration at Equilibrium

At equilibrium, each concentration remains constant with time, but the concentrations of different substances do not have to be equal to one another.

Macroscopic Appearance of Equilibrium

The observable properties of an equilibrium system remain constant because there is no net change in composition.

Microscopic Behavior at Equilibrium

At the molecular level, forward and reverse reactions continue to occur continuously at equal rates.

Equilibrium Rate Condition

Rate of forward reaction = rate of reverse reaction.

Equilibrium Concentration Condition

Reactant and product concentrations remain constant with time, although their numerical values need not be equal.

Establishment of Chemical Equilibrium

A reversible reaction approaches equilibrium as the forward rate decreases and the reverse rate increases until the two rates become equal.

Reaction-Rate Behavior Before Equilibrium

Before equilibrium is reached, the forward and reverse reaction rates are unequal, producing a net change in composition.

Reaction-Rate Behavior at Equilibrium

Once equilibrium is reached, the forward and reverse rates are equal and the macroscopic composition remains constant.

Closed System and Chemical Equilibrium

A reversible reaction can establish and maintain equilibrium when the reacting substances remain within a system rather than being continuously lost.

Physical Equilibrium

A dynamic equilibrium involving opposing physical processes rather than opposing chemical reactions.

Phase Equilibrium

A dynamic equilibrium between opposing phase changes in which the forward and reverse physical processes occur at equal rates.

Liquid–Vapor Equilibrium

A dynamic equilibrium in which the rate of vaporization equals the rate of condensation.

Vaporization at Liquid–Vapor Equilibrium

Liquid molecules continue entering the vapor phase even after equilibrium has been established.

Condensation at Liquid–Vapor Equilibrium

Vapor molecules continue returning to the liquid phase even after equilibrium has been established.

Liquid–Vapor Equilibrium Condition

Rate of vaporization = rate of condensation.

Saturated Solution Equilibrium

A dynamic equilibrium in which dissolution and crystallization occur at equal rates.

Dissolution at Saturation

Solute particles continue leaving the solid and entering solution even when the solution is saturated.

Crystallization at Saturation

Dissolved solute particles continue returning to the solid phase even when the solution is saturated.

Saturated Solution Equilibrium Condition

Rate of dissolution = rate of crystallization.

Chemical Equilibrium vs. Physical Equilibrium

Chemical equilibrium involves opposing chemical reactions, whereas physical equilibrium involves opposing physical processes; both are dynamic states characterized by equal opposing rates.

Central Meaning of Equilibrium

Equilibrium is a dynamic state in which opposing processes occur at equal rates, producing no net macroscopic change.