Topic 5: Chemical Equilibrium - Equilibrium Constants (K)

Chemical Equilibrium

A state in which the concentrations of reactants and products no longer change because the rates of the forward and reverse reactions are equal

Reversible Reaction

A reaction in which reactants form products and products can reform reactants, indicated by the double arrow (⇌)

• Example: H₂O (l) ⇌ H₂O (g)

Dynamic Equilibrium

A condition where the forward and reverse reactions occur at equal rates, so concentrations remain constant but reactions continue at the molecular level

Closed System

A system where no substances can enter or leave, allowing equilibrium to be established

Equilibrium Constant (K)

A fixed ratio of product concentrations to reactant concentrations at equilibrium for a given reaction and temperature

Kc

The equilibrium constant expressed in terms of concentrations (mol/L)

Temperature and K

The only factor that changes the equilibrium constant K is temperature

Equilibrium Position

Not affected by initial concentrations, the system shifts to re-establish the K ratio

Equilibrium Expression

An equation showing how K is calculated from the concentrations of products and reactants at equilibrium

General Equilibrium Expression

Where: aA + bB ⇌ dD + eE

K_c = [D]^d[E]^e/[A]^a[B]^b

• Products in the numerator

• Reactants in the denominator

Stoichiometric Coefficients

Used as powers on the concentration terms in the K expression

Heterogeneous Equilibrium

A reaction with species in multiple phases (solid, liquid, gas, aqueous)

• Example: CaCO₃(s) ⇌ CaO(s) + CO₂(g)

Solids and Liquids in Equilibrium Expressions

Pure solids and liquids are excluded from the K expression because their concentrations do not change

Large K (K > 1)

Products are favored; high product concentration and low reactant concentration

Small K (K < 1)

Reactants are favored; low product concentration and high reactant concentration

K = 1

Neither direction is favored; significant amounts of both reactants and products

Reversing a Reaction

The new equilibrium constant is the reciprocal of the original: K_c' = 1/K_c

• Example: If PCl₅ ⇌ PCl₃ + Cl₂ is reversed,

  • K_c = [PCl₃][Cl₂]/[PCl₅]

Changing Coefficients

If all coefficients in the balanced equation are multiplied by n, the new equilibrium constant becomes Kⁿ