Chemical & Ionic Equilibrium

Syllabus Unit: Physical Chemistry

Why it matters

Chapter overview

Most reactions don't run to completion — they settle into a dynamic balance where forward and reverse rates match. This topic quantifies that balance through the law of mass action and the equilibrium constant, predicts how it responds to disturbance via Le Chatelier's principle, and extends the same ideas to ionic equilibria in water: acid-base behaviour, pH, buffer solutions, and the solubility product governing sparingly soluble salts.

Chemistry Syllabus Structure Physical Chemistry ~33% weightage

Live Animation1 / 4

1Forward Ratekf[A]^a[B]^b

2Reverse Ratekr[C]^c[D]^d

3Rates Become EqualDynamic equilibrium reached

4Kc = kf/kr= [C]^c[D]^d / [A]^a[B]^b

1Law of Mass Action & Equilibrium Constant

At equilibrium, forward and reverse reactions haven't stopped — they're racing at exactly matched speeds.

Learning Objectives

State the law of mass action and write the equilibrium constant expression Kc for a general reaction.
Relate Kc and Kp via Kp = Kc(RT)^Δn and identify when the two are equal.
Interpret the magnitude of K: K>>1 products favoured, K<<1 reactants favoured, K≈1 comparable amounts.
Explain why pure solids and liquids are omitted from the equilibrium constant expression.

Key Terms

Law of Mass ActionAt equilibrium, the ratio of product to reactant concentrations (each raised to its stoichiometric coefficient) is constant, K, at a given temperature.

Equilibrium Constant (Kc)The value of the mass-action ratio at equilibrium, expressed using molar concentrations.

KpThe equilibrium constant expressed in partial pressures of gaseous species; Kp = Kc(RT)^Δn.

Reaction Quotient (Q)The mass-action ratio at any point in a reaction, not necessarily equilibrium; comparing Q to K predicts the reaction's direction.

Many reactions never truly finish — they reach a **dynamic equilibrium**, where the forward and reverse reactions continue at exactly equal rates, leaving concentrations constant but not zero.

Law of mass action

For aA + bB ⇌ cC + dD: Kc = [C]^c[D]^d / [A]^a[B]^b

Kc and Kp

Kp = Kc(RT)^Δn, where Δn = (moles of gaseous products) - (moles of gaseous reactants). If Δn = 0, Kp = Kc.

Reading the size of K

K >> 1 — equilibrium lies far to the right; mostly products at equilibrium.
K << 1 — equilibrium lies far to the left; mostly reactants at equilibrium.
K ≈ 1 — significant amounts of both reactants and products at equilibrium.

Exam TipThe reaction quotient Q uses the same expression as K but at any concentrations, not necessarily equilibrium ones: Q < K means the reaction proceeds forward, Q > K means it proceeds in reverse, Q = K means it's already at equilibrium.

Common MistakePure solids and liquids are never written in the K expression, since their 'concentration' (density/molar mass) is constant regardless of amount — e.g. for CaCO3(s) ⇌ CaO(s) + CO2(g), Kc = [CO2] only.