Heterogeneous Equilibria

Introduction

A state of chemical equilibrium occurs when the rates of forward and backward reactions are equal. When a chemical reaction is in equilibrium, the concentrations of reactants and products are either constant or do not tend to fluctuate over time. Based on the states of reactants and products at equilibrium, chemical equilibrium can be divided into two categories –

• The Homogeneous Equilibrium

• The Heterogeneous Equilibrium

The prefixes ‘homo’ and ‘hetero’ come from Greek terms that mean ‘similar’ and ‘different,’ respectively.

Heterogeneous Equilibrium:

As the name implies, Heterogeneous equilibrium is an equilibrium system in which reactants and products exist in two or more states of matter.  For example, in the equilibrium system of ice and water, the reaction is expressed as follows:

H2O(s) ⇌ H2O(l) 

The state of the reactant and product are then different in the above equation. Water exists in the system as both a solid and a liquid. This is called a heterogeneous equilibrium system because the states or phases of the reactant and product are different.

Let’s start with a detailed discussion of heterogeneous equilibrium, then a brief discussion of the homogeneous system.

In heterogeneous equilibrium, the reactants and products are present in distinct phases. Hence the equilibrium constant expression is expressed differently than in homogeneous equilibrium.

The equilibrium constants for reactions containing components in the same phase must be computed differently than those for reactions containing compounds in separate stages. Homogeneous reactions are the former, while heterogeneous reactions are the latter.

Equilibrium Constant:

The ratio of a reaction’s products to its reactants, measured at equilibrium, is the equilibrium constant K for a given reaction. 

aA +  bB  ⇌ cC + dD

The equilibrium constant K is the reaction quotient measured at equilibrium.

The ratio of products to reactants at equilibrium is used to calculate the equilibrium constant (K). Pure solids and liquids are not considered when determining equilibrium constants because their concentration remains constant during the process.

Examples of Heterogeneous Equilibrium:

A few frequent examples of chemical reactions that occur at heterogeneous equilibrium are described below-

• At room temperature, bromine exists as a liquid. It converts rapidly to vapours and has a reddish-brown colour in both phases. The rate of bromine evaporation and condensation of bromine vapours is the same at equilibrium. At equilibrium, the equation can be written as follows:

Br2 (l)  ⇌ Br2 (g)

It is a heterogeneous equilibrium system because bromine exists in different phases as a reactant and product, and the system is in equilibrium. 

• The reaction between carbon dioxide and carbon is another example of a heterogeneous equilibrium system. Gaseous carbon monoxide is produced when gaseous carbon dioxide combines with solid carbon. It is an equilibrium reaction in which the forward and backward rates of reaction are equal. The following is the equation-

CO2 (g) + C (s) ⇌ 2CO (g)

Because the reactants and products in the equilibrium process mentioned above are present in two phases, gas or solid, it is a heterogeneous equilibrium reaction.

• At equilibrium, the reaction between PCl3(l)  and chlorine gas is likewise an example of heterogeneous equilibrium. Because phosphorus trichloride exists in a liquid state in this equilibrium system, chlorine participates as a gas, and the resulting phosphorus pentachloride exists in a solid-state. The equation is as follows:

PCl3 (l) +Cl2 (g) ⇌  PCl5 (s)

• At equilibrium, the reaction between solid Fe2O3 and hydrogen gas is likewise an example of heterogeneous equilibrium. Because Fe2O3 is present in the solid-state and hydrogen is present in the gaseous phase in this equilibrium system, iron and water are produced in the solid and gaseous phases, respectively. The equation is as follows:

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

The reaction of steam with red-hot carbon is another example of heterogeneous equilibrium. The equation is as follows:

H2O (g) + C (s) ⇌  H2 (g) + CO (g)

As you can see, the reaction is in equilibrium, and the gaseous phase contains steam, carbon monoxide, and hydrogen, while the solid phase contains red hot carbon. As a result, it exemplifies heterogeneous equilibrium.

• When a solid calcium carbonate decomposition reaction reaches equilibrium, it represents a good illustration of heterogeneous equilibrium. The equation is as follows:

CaCO3 (s)  ⇌   CaO (s) + CO2 (g)

Because calcium carbonate and calcium oxide are present in solid form while carbon dioxide is present in gaseous form in the above equilibrium process, it is an example of a heterogeneous equilibrium reaction.

Homogeneous Equilibrium Reactions 

When a process reaches equilibrium and all of its reactants and products are in the same phase or state of matter, it is referred to as a homogeneous equilibrium reaction. For instance, consider the reaction between carbon monoxide and hydrogen. The equation is as follows:

CO (g) + 2 H2 (g) ⇌  CH3OH (g)

Carbon monoxide, hydrogen, and methanol are all present in a gaseous state in the foregoing reaction, and the reaction is at equilibrium. Hence it is called homogeneous equilibrium.

Conclusion

A heterogeneous equilibrium occurs when a system’s state of equilibrium contains components from multiple phases. For example, if we put ice and water in a container at a temperature at the same time, both ice and water are present in a state of equilibrium. This concludes our discussion of “Heterogeneous Equilibrium.”