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1 Molar Solids and Liquids in Equilibrium Constant Expressions?

posted Dec 6, 2016, 10:05 PM by Grace Ong   [ updated Dec 6, 2016, 10:06 PM ]
The following post was first posted on Blogger on Wednesday, 3 July 2013.

From time to time, I come across students telling me that they assume the concentrations of solids and liquids to be one when writing equilibrium constant (K) expressions.

One? Well, apparently, that was what these students were taught. This is really puzzling because there is no way the concentration of water in an aqueous solution or the concentration of a solid salt in a solubility equilibrium can be 1 mol dm−3.

I suspect this approach was taken because there was no distinction made between concentration and activity.

A chemical reaction depends on the activity of each species, not their concentrations. In an ideal solution, the number of ions that is available to react is exactly equal to the number of ions that is present in the solution. However reality is seldom ideal, so the number of ions that is actually available to react is less than the number of ions present in the solution. Thus there is a need for an 'effective concentration' or activity to represent the non-ideal situation.

The activity of a chemical species is a dimensionless quantity. For pure solids and liquids, it is assigned a value of one.

Using molar concentrations in K expressions (and in rate equations too) is therefore a simplification. To have a more realistic depiction of a reaction at equilibria, K expressions are sometimes written in terms of activities rather than concentrations. In such situations, solids and liquids are omitted since their activities are equal to one. This omission, however, does not automatically apply to K expressions that are written in terms of concentrations. (I shall elaborate on this in a later post.)

  1. The formal definition of activity as given in the IUPAC Gold Book is a mathematical one. The definition given on Wikipedia, while written in words, isn't that much easier to understand. If you are game enough for stuff that is way beyond the scope of the H2 Chemistry syllabus, I suggest that you get hold of the definitive text on Physical Chemistry by Peter Atkins and Julio de Paula.
  2. For purposes of 'A' Level, it suffices to assume that molar concentrations approximate activities, which is true for dilute aqueous solutions anyway.