# Consider this equilibrium: I2(s) + H2O(l) ⇌ H+(aq) + I-(aq) + HOI(aq). What is the Keq expression for this equilibrium?

May 27, 2014

The expression is ${K}_{e q}$ = [H⁺][I⁻][HOI]

We often consider a generic reaction of the form

aA +bB ⇌ cC +dD

The equilibrium constant expression for this reaction is

${K}_{e q} = \frac{{\left[C\right]}^{c} {\left[D\right]}^{d}}{{\left[A\right]}^{a} {\left[B\right]}^{b}}$

Notice that it is always "products" over "reactants". Each component A, B, C, and D is raised to the power of its coefficient a, b, c, and d in the balanced equation.

The concentration of a solid or of a liquid does not change during a reaction. We omit it from the equilibrium constant expression.

Consider the equation

I₂(s) + H₂O(l) ⇌ H⁺(aq) + I⁻(aq) + HOI(aq)

I₂ is a solid, and H₂O is a liquid. Their concentrations do not change during the reaction. We omit them from the ${K}_{e q}$ expression.

All the products are in solution. They can change their concentrations. We keep them in the ${K}_{e q}$ expression.

Thus, the ${K}_{e q}$ expression is

${K}_{e q}$ = [H⁺][I⁻][HOI].