Question

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

Answer 1

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 = 1/(P_B)^2`

Hope this clarifies!

Warmest regards!