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

1 Answer

Answer:

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!