Monday

Homogeneous vs Heterogeneous Equilibria | chemca

Chemical Equilibrium

Homogeneous and Heterogeneous Equilibria

Classification of equilibrium based on the physical states of reactants and products.

By chemca Team • Updated Jan 2026

Chemical equilibria are classified into two main types based on the physical states (phases) of the participating substances: Homogeneous Equilibrium and Heterogeneous Equilibrium. This distinction is crucial for writing the correct equilibrium constant expression.

1. Homogeneous Equilibrium

In a homogeneous equilibrium, all reactants and products are present in the same phase (e.g., all gaseous or all liquid/aqueous solution).

Case A: Gaseous System

Synthesis of Ammonia (Haber Process):

$N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}$

Since everything is gas, all terms appear in $K_c$ and $K_p$.

$$ K_c = \frac{[NH_3]^2}{[N_2][H_2]^3} $$

Case B: Liquid Solution System

Esterification:

$CH_3COOH_{(l)} + C_2H_5OH_{(l)} \rightleftharpoons CH_3COOC_2H_5_{(l)} + H_2O_{(l)}$

All components are liquids/miscible.

$$ K_c = \frac{[Ester][H_2O]}{[Acid][Alcohol]} $$

2. Heterogeneous Equilibrium

In a heterogeneous equilibrium, reactants and products are present in two or more different phases.

Crucial Rule: The active mass (concentration) of any Pure Solid or Pure Liquid is constant by convention (unity, 1). Therefore, they are omitted from the equilibrium constant expression.

Example 1: Decomposition of Calcium Carbonate

$CaCO_{3(s)} \rightleftharpoons CaO_{(s)} + CO_{2(g)}$

Solids $CaCO_3$ and $CaO$ are ignored.

$$ K_c = [CO_2] $$ $$ K_p = P_{CO_2} $$

Example 2: Reaction of Steam with Iron

$3Fe_{(s)} + 4H_2O_{(g)} \rightleftharpoons Fe_3O_{4(s)} + 4H_{2(g)}$

Ignore $Fe_{(s)}$ and $Fe_3O_{4(s)}$.

$$ K_c = \frac{[H_2]^4}{[H_2O]^4} $$

Example 3: Vapor Pressure of Liquid

$H_2O_{(l)} \rightleftharpoons H_2O_{(g)}$

Ignore Liquid water.

$$ K_p = P_{H_2O} \text{ (Vapor Pressure)} $$

3. Key Differences

Feature	Homogeneous	Heterogeneous
Phases	Single phase	Multiple phases
K Expression	Usually includes all reactants and products	Excludes pure solids and pure liquids
Le Chatelier	Adding any reactant/product shifts equilibrium	Adding pure solid/liquid has NO effect

Knowledge Check

Test your ability to classify and write K expressions

Search This Blog