We know that for homogeneous mixtures the equilibrium constant is determined using both concentration of reactants and of products. For heterogeneous equilibrium involving a gas and a liquid or a solid, we only use the concentration of the gas because the concentration of liquids and gases are constants, and can thus be omitted from the equation. But what of the heterogeneous equilibrium between a solid and a liquid? Both are incompressible, so what do you do?



1 Answer 1


You're right, they don't really have concentrations or pressure to work with, which would lead to 0/0 craziness.

I think we can go back a step and look at how the equilibrium constant is defined. When you derive the equilibrium constant you don't use pressure or concentrations. You actually use the substances' activities which we can relate to change in Standard Gibbs Free Energy for the reaction (which is easy to find with tabulated data):

gstar vs k

For a full derivation refer to Wikipedia's article on the equilibrium constant.

  • $\begingroup$ Ok, so I haven't learned about Gibbs Energy yet. Is it safe to assume that for my exams we won't be tested on this? $\endgroup$
    – user11629
    Mar 9, 2014 at 3:43
  • $\begingroup$ I suppose...? Haha the equation is commonplace in high school chemistry but the connection wasn't made in my coursework until physical chemistry. (college upper div chemistry) $\endgroup$
    – rch
    Mar 10, 2014 at 4:30

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