# When writing an equilibrium constant expression, do I include aqueous reactants and products or only gases?

May 22, 2018

Yes aqueous reactants and products are included in equilibrium constant expressions

#### Explanation:

We actually have 2 types of equilibrium constants, ${K}_{c}$ and ${K}_{p}$

${K}_{c}$ is in terms of concentration, so will include aqueous species, mixture of liquids and gases.

${K}_{p}$ is in terms of partial pressure of gases, so will naturally apply to gases only.

Let's have an example to differentiate ${K}_{c}$ and ${K}_{p}$ and make things clearer.

For the below reaction:

A(aq) + 2B(g) = 3C(s) + 4D(aq)

${K}_{c}$ is in terms of concentration so all the terms, except C which is a solid, are included.

Concentration of a solid is equal to its density which is always a constant term, so it'll just be part of K and therefore not in the K expression.

Therefore K_c = [D]^4/([A][B]^2

For ${K}_{p}$ we only include the partial pressure of gases, so only partial pressure of B, ${P}_{B}$, will be included.

Therefore ${K}_{p} = \frac{1}{{P}_{B}} ^ 2$

Hope this clarifies!

Warmest regards!