Heterogeneous Equilibrium is a state of equilibrium in a system where multiple phases coexist. This usually refers to substances existing simultaneously in multiple physical states, such as solid, liquid, and gas.

This article covers the basic ideas of heterogeneous equilibrium, its examples, and reactions along with a few problems based on the concept.

What is Heterogeneous Equilibrium?

Heterogeneous equilibrium occurs when reactants and products of a chemical reaction coexist in distinct phases simultaneously while maintaining a balance between the forward and reverse reactions.

Heterogeneous equilibrium involves substances in different states of matter, such as solid, liquid, or gas. For instance gas-liquid equilibrium in systems like the dissolution of gases in liquids or the adsorption of gases onto solid surfaces.

The equilibrium between water vapour and liquid water in a closed container is also an example of heterogeneous equilibrium.

In a heterogeneous equilibrium, multiple phases exists therefore, the concentrations of reactants and products may not be uniform throughout the system.

Equilibrium Constant

Equilibrium constant (K) for a chemical reaction can be described as the ratio between concentrations of the products to the concentrations of the reactants. It is used to determine chemical behaviour of the reaction.

The rate constants are constant at a given temperature. The ratio of the forward reaction rate constant to the backward reaction rate constants should remain same, and this is referred to as the equilibrium constant (K).

The general form of the equilibrium constant expression for a reaction is:

[Tex]K= \frac{[C]^c. [D]^d} {[A]^a. [B]^b}[/Tex]​

Where:

• [A], [B], [C], and [D] are the molar concentrations of the reactants and products at equilibrium,
• a, b, c, and d are the coefficients of the reactants and products in the balanced chemical equation.

Units of Equilibrium Constant

Equilibrium constant is a dimensionless quantity because it expresses the ratio of product concentrations to the reactant concentrations, each raised to the power of its stoichiometric coefficient. Consequently, when the concentrations (or partial pressures) are divided, the units of the equilibrium constant cancel out making it a dimensionless quantity.

Check, Applications of Equilibrium Constants

Examples of Heterogeneous Equilibrium

Heterogeneous equilibrium involves reactions where reactants and products are present in different phases simultaneously. Here are some examples of heterogeneous equilibrium:

Gas-Liquid Equilibrium

The dissolution of a gas in a liquid, such as the equilibrium between carbon dioxide (CO₂) gas and carbonic acid (H₂CO₃​) in water is an example of gas-liquid equilibrium. The reaction is as follows:

CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq)

In this equilibrium, carbon dioxide gas dissolves in water to form carbonic acid. The concentration of dissolved carbon dioxide in the aqueous phase depends on factors such as temperature and pressure.

Solid-Liquid Equilibrium

Another common example is the solubility equilibrium of soluble salts, such as the equilibrium between solid calcium hydroxide (Ca(OH)₂) and its dissolved ions in water. The reaction for the same is as follows:

Ca(OH)₂(s) ⇌ Ca²⁺(aq) + 2OH⁻(aq)

Here, calcium hydroxide is in equilibrium with its dissolved ions in the aqueous phase. The concentration of dissolved calcium ions and hydroxide ions depends on the solubility product of calcium hydroxide and the ionic strength of the solution.

Solid-Gas Equilibrium

Adsorption-desorption processes on solid surfaces is also an example of heterogeneous equilibrium. Consider the equilibrium between a gas, such as nitrogen (N₂​), and its adsorbed molecules on the surface of a solid catalyst:

N₂​(g) ⇌ N₂​(ads)

In this equilibrium, nitrogen gas is adsorbed onto the solid surface, and the extent of adsorption depends on factors such as pressure, temperature, and the nature of the solid surface.

Read More,

• Difference Between Heterogeneous and Homogenous Equilibrium
• Factors Affecting Chemical Equilibrium
• Law of Chemical Equilibrium and Equilibrium Constant

NCERT Problem on Heterogeneous Equilibrium

Problem 1: The value of K_p for the reaction, CO₂ (g) + C (s) ⇆ 2CO (g) is 3.0 at 1000 K. If initially P_CO2 = 0.48 bar and P_CO = 0 bar and pure graphite is present, calculate the equilibrium partial pressures of CO and CO₂.

Solution:

For the reaction, let ‘x’ be the decrease in pressure of CO₂, then CO₂(g) + C(s) ⇆ 2CO(g)

Initial pressure of CO₂: 0.48 bar

At equilibrium: (0.48 – x) bar

Initial pressure of CO is 0 bar

At equilibrium the pressure is 2x bar

Now, K_p = p²_CO /p_CO2

K_p = (2x)²/(0.48 – x) = 3 (given)

4x² = 3(0.48 – x)

4x² = 1.44 – 3x

4x² + 3x – 1.44 = 0

a = 4, b = 3, c = –1.44

[Tex]x = \frac{{-b \pm \sqrt{b^2 – 4ac}}}{{2a}}[/Tex]

= [–3 ± √(3)2– 4(4)(–1.44)]/2 × 4 = (–3 ± 5.66)/8

= (–3 + 5.66)/8 (as value of x cannot be negative hence we neglect that value)

x = 2.66/8 = 0.33

The equilibrium partial pressures are,

p_CO = 2x = 2 × 0.33 = 0.66 bar

p_CO2 = 0.48 – x = 0.48 – 0.33 = 0.15 bar

Heterogeneous Equilibrium FAQs

What is heterogeneous equilibria?

Heterogeneous equilibria refer to chemical equilibria in systems where reactants and products are present in different phases.

Is the formation of water heterogeneous equilibrium?

No, the formation of water (H₂O) is an example of a homogeneous equilibrium, not a heterogeneous equilibrium.

What is homogeneous and heterogeneous equilibrium?

In a homogeneous equilibrium, all the reactants and products exist in the same phase whereas in a heterogeneous equilibrium, the reactants and products are present in different phases.

What is an example of a heterogeneous reaction?

An example of a heterogeneous reaction is the catalytic oxidation of carbon monoxide (CO) to carbon dioxide (CO₂) using a platinum (Pt) catalyst. The reaction is given below:

2CO(g)+O₂(g) → 2CO₂(g)

What is heterogeneous equilibrium constant?

The term heterogeneous equilibrium constant refers to the equilibrium constant K for a reaction that involves substances in different phases.