solids and liquids in K expression

[Kaylee Nezwek 1D]

Hi! In our first lecture, Dr. Lavelle said that solids and liquids are not included in the K expression. However, this confused me in lecture four, when we did the example of 2 H2O (l) -> H3O+ (aq) + OH- (aq) and we still used the K expression to solve for the concentrations of H3O+ and OH-? Would anyone be able to explain when we omit solids and liquids from the K expression and when we don't? Thanks!

[Audra Mcleod 3G]

Hello!
We never include liquids and solids in the equilibrium equations. In the chemical reaction you mentioned, the equilibrium constant equation was [H30+][OH-] not [H30+][OH-] / [H20]. This is mainly because:
1. solids do not have concentrations
2. Liquids/solvents are often present in such large quantities that their changes are negligible (they will appear on both sides of the equation, and thus cancel out in the equilibrium constant equation).

[Kaylee Nezwek 1D]

Ah, thank you! So just to clarify, if a substance is purely a solid/liquid then we would not include it in the K expression, but aqueous solutions are okay to include?

[Lea Baskin Monk 1F]

Yes, exactly! Include aqueous solutions (aq) but never solids (s) or liquids (l).

[Audra Mcleod 3G]

Ah, thank you! So just to clarify, if a substance is purely a solid/liquid then we would not include it in the K expression, but aqueous solutions are okay to include?

Yes! We definitely use aqueous solutions in the equilibrium constant equation.

[Crystal Pan 2G]

We do not use solids or liquids because they do not have concentration. We will include all gases, and aqueous solutions as well because they have concentration (we can calculate their molarity)

[Anh Trinh 1J]

Molar concentration of a pure substance (solid or liquid) does not change in a reaction, therefore solids and liquids are not included in K expressions. H2O is omitted because the change in solvent concentration is insignificant. For a gas, we use its partial pressure.

[kentbui1d]

For liquids (like water for example) there's so much of it that it's more or less the same compared from before and after. For solids, the concentrations don't change. So in the end both solids and liquids are ignored.