The equilibrium constant expression for this reaction is $K_{eq} = \frac{[HOCl]^{2}}{[Cl_{2}]^{2}}$ . This is an example of a heterogeneous equilibrium wherein the states of the reactants and products are different. In such a case, only the concentration of the gaseous and aqueous substances are included in the equilibrium constant expression.

Further Explanation:

The equilibrium constant expression is the ratio of the concentration of the products and the concentration of reactants.

The guidelines in writing the equilibrium constant expressions are as follows:

Write molar concentration of each product in the numerator of the Keq expression.
Write the coefficient of the substance as the exponent of the molar concentration of the substance in the Keq expression.
Write the molar concentration of each reactant raised to its coefficient in the denominator.

Note: Pure substances (i.e. solids and liquids) are not included in the equilibrium constant expression as their concentrations are constant.

The numerical equivalent of the Keq expression is called the equilibrium constant. It has a specific value for a given temperature. The equilibrium constant provides information about the spontaneity and progress of an equilibrium reaction.

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