• Definition: Chemical equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction, leading to constant concentrations of reactants and products in a closed system.

• Dynamic Nature: At equilibrium, reactions continue to occur, but with no net change in concentrations.

Understanding Equilibrium Expressions

• Equilibrium Constant (K): A numerical value that expresses the ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of their stoichiometric coefficients.

• General Form: For a reaction aA + bB ↔ cC + dD, the equilibrium expression is K =[C]c[D]d/[A]a[B]b.

Key Points in Equilibrium Expressions

• Only Gaseous and Aqueous Species: Include only gases and solutes in equilibrium expressions; omit solids and pure liquids.

• Use of Concentrations: Express concentrations in molarity (M) for aqueous solutions and partial pressure (P) for gases.

• Equilibrium Constant Units: Depending on the reaction, K may have units or be unitless.

• Magnitude of K:

  • Large K (>1): Products are favored at equilibrium.

  • Small K (<1): Reactants are favored at equilibrium.