Homogeneous and Heterogeneous Equilibria

Created: Tuesday, 04 October 2011 11:27 | Published: Tuesday, 04 October 2011 11:27 | Written by Super User | Print | Email

Introduction

A system in which all reactants are in same phase is known as homogeneous system.

For example in the reaction N₂(g) + 3H₂(g) → 2NH₃(g), all the reactant and products are in gas phase so this is a homogeneous system.

Homogeneous Equilibria

When in a equilibrium reaction all the reactants and products are in same phase, it is known as homogeneous equilibrium.

Examples of equilibrium in gas phase are given below:

(I) The reactions in which number of moles of products are equal to number of moles of reactants

H₂ + I₂→ 2HI

N₂ + O₂ → 2NO

(II) The reactions in which number of moles of products are not equal to number of moles of reactants

N₂ + 3H₂→ 2NH₃

2 SO₂ + O₂ → 2SO₃
PCl₅→ PCl₃+ Cl₂

Examples of equilibrium in liquid phase are:

CH₃COOH + C₂H₅OH → CH₃COOC₂H₅+ H₂O

Expression for equilibrium constant:

For the reaction of hydrogen and iodine to form hydrogen iodide

H₂(g) + I₂(g) → 2HI (g)

For the reaction of formation of ammonia from hydrogen and oxygen

N₂ + 3H₂ → 2NH₃

Heterogeneous Equilibria

The equilibrium in which the reactants and products of a reaction are present in two or more than two phases, is called a heterogeneous equilibrium.

Some examples of heterogeneous equilibrium are:

CaCO₃(s) → CaO (s) + CO₂ (g)

3Fe (s) + 4H₂O (g) → Fe₃O₄(s) + 4H₂ (g)

H₂O (l) → H₂O(g)

Ag₂O(s) + 2HNO₃(aq) → 2AgNO₃(aq) +H₂O(l)

The position of the heterogeneous equilibrium is independent of the amount of pure solid or pure liquid present in the reaction mixture.

As the concentration of solids and liquids remains almost constant during the reaction they do not appear in the equilibrium expression.

Expression for equilibrium constant:

For the decomposition of calcium carbonate to calcium oxide and carbon dioxide

CaCO₃(s) → CaO (s) + CO₂ (g)

But by convention [CaCO₃ (s)] = 1, [CaO (s)] = 1

Hence, K = [CO₂ (g)]

It is better to express the concentration of a gas in terms of partial pressure, the equilibrium constant of this reaction can be is expressed as

Above equation explains why concentration of CO2 becomes constant after the equilibrium is attained in the decomposition of calcium carbonate in a closed vessel.

(ii) For the equilibrium

H₂O (l) → H₂O(g)

But by convention [H₂O(l)] = 1

Hence, Kc = [H₂O(g)]

Or, in terms of pressure,

This explains why vapour pressure of water is constant at constant temperature.

(iii) In the reaction of silver oxide with nitric acid

Ag₂O(s) + 2HNO₃(aq) → 2AgNO₃(aq) +H₂O(l)

as [Ag₂O(s)] =1, [H₂O (l)]=1

Why vapour pressure of water is is constant at constant temperature?

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