Heterogeneous chemical equilibrium describes the behaviour of a mixture of chemical substances where the composition of the mix changes over time. This process is different to homogeneous equilibrium, in which the composition of a solvent is constant over time. Consider a beaker of water and two beakers of detergent. The water in the beaker will dissolve the detergent, and as a result, the detergent will get diluted. In heterogeneous chemical equilibrium, the concentrations of reactants and products can change over time and space.

Heterogeneous chemical equilibrium is the balance between the concentrations of the reactants and products of a chemical reaction within a single phase. This means that the chemical equilibrium occurs at a single set of concentrations for both the reactants and products.

Homogeneous chemical equilibrium vs heterogeneous chemical equilibrium:

Heterogeneous chemical equilibrium is a system in which different chemical species are present in different amounts, or stoichiometrically, with respect to one another. This contrasts with homogeneous chemical equilibrium, in which all chemical species are present in the same amount, or homogeneously, with respect to one another. In homogeneous chemical equilibrium, no chemical reactions occur. In contrast, in heterogeneous chemical equilibrium, chemical reactions occur in which a reactant is converted to a product, with the reverse reaction occurring to reconvert the reactant to a reactant.

Factors controlling heterogeneous chemical equilibrium:

Equilibrium constant:

The first factor that controls heterogeneous chemical equilibrium is the equilibrium constant. The equilibrium constant is the ratio of the products to the reactants and is equal to the quotient of the products concentration and the reactants concentration. The equilibrium constant can determine the percentage of products in the mixture at a given temperature. It can also determine the Gibbs free energy or the change in free energy at a given temperature. 

Temperature:

The most crucial factor for achieving heterogeneous chemical equilibrium is temperature. The equilibrium constant is directly proportional to the temperature. As the temperature increases, the equilibrium constant decreases.

The temperature has a particularly strong effect on the free energy of a heterogeneous equilibrium since it causes the molecules to vibrate faster. As a result, their energies become spread out more widely.

Concentration:

In heterogeneous chemical equilibrium, the concentrations of the chemicals in a solution are equal, but the mixtures are not. For example, the mixture becomes more heterogeneous when salt is added to water. The water in the combination still has the same concentration, but the mixture is no longer homogeneous. This can be seen by the formation of little white clumps in the water.

Rate of reaction:

In heterogeneous chemical equilibrium, a reaction occurs in one location while the products and reactants are distributed throughout the solution. The rate of reaction in a homogeneous solution is much faster than the rate of reaction in a heterogeneous solution. This is because the reactants and products are evenly distributed throughout the solution in a homogeneous solution, so the diffusion of the reactants and products is much faster than the diffusion of the reactants and products in a heterogeneous solution.

Amount of reactants:

In heterogeneous chemical equilibrium, the amounts of reactants and products are not the same in every region. This is because the atoms and molecules in a chemical reaction diffuse or move throughout the system. This means that the concentration of one substance will often be higher in one place in the system than another.

Examples of heterogeneous chemical equilibrium:

1. The equilibrium between water and water vapour is a heterogeneous chemical equilibrium. In such a reaction, the reactants are not mixed and are not in contact with each other.
2. When two species react in a solution and the products of the reaction separate into distinct phases, the overall reaction is said to be heterogeneous. The reaction will go to completion only if the forward reaction rate is greater than the reverse reaction rate. In these instances, the equilibrium will be established.
3. The aqueous solution of copper acetate gives a purple precipitate of copper carbonate as the predominant product.

Conclusion:

In heterogeneous chemical equilibrium, the substances involved in the reaction are not uniformly distributed in the reacting mixture. In heterogeneous chemical equilibrium, compounds diffuse towards a common distribution by the interplay of chemical reactions and motion. This phenomenon plays a major role in natural processes such as photosynthesis and is also observed in the atmosphere when pollutants diffuse into the air. 

The mathematical form of heterogeneous chemical equilibrium was discovered by the Polish scientist Stanislas Dehmel who was interested in studying the gas absorption of CO2 by water.