This page explains what is meant by an equilibrium constant, introducing equilibrium constants expressed in terms of concentrations, Kc. It assumes that you are familiar with the concept of a dynamic equilibrium, and know what is meant by the terms “homogeneous” and “heterogeneous” as applied to chemical reactions.

We need to look at two different types of equilibria (homogeneous and heterogeneous) separately, because the equilibrium constants are defined differently.

  • A homogeneous equilibrium has everything present in the same phase. The usual examples include reactions where everything is a gas, or everything is present in the same solution.
  • A heterogeneous equilibrium has things present in more than one phase. The usual examples include reactions involving solids and gases, or solids and liquids.

 Kc in homogeneous equilibria

This is the more straightforward case. It applies where everything in the equilibrium mixture is present as a gas, or everything is present in the same solution.

A good example of a gaseous homogeneous equilibrium is the conversion of sulphur dioxide to sulphur trioxide at the heart of the Contact Process:

A commonly used liquid example is the esterification reaction between an organic acid and an alcohol – for example:

Writing an expression for Kc

We are going to look at a general case with the equation:


No state symbols have been given, but they will be all (g), or all (l), or all (aq) if the reaction was between substances in solution in water.

If you allow this reaction to reach equilibrium and then measure the equilibrium concentrations of everything, you can combine these concentrations into an expression known as an equilibrium constant.

The equilibrium constant always has the same value (provided you don’t change the temperature), irrespective of the amounts of A, B, C and D you started with. It is also unaffected by a change in pressure or whether or not you are using a catalyst.



Organic chemistry activity.


Molecules with the same molecular formula can have different structures, this is called isomerism. Structural isomers can belong to different homologous series.

  1.  The alkene with the molecular formula C4H8 can form three structural isomers.

(a)          Draw the structural formulae of the three isomers.

(b)         Give the IUPAC names of the three isomers.

  1. An organic compound with the molecular formula C3H6O2 can similarly form three structural isomers.

(a)            Draw the structural formulae of the three isomers

(b)           Give the IUPAC names for the three isomers.

  1. There are two structural isomers for the organic compound with the molecular formula C2H4O2.

(a)           Write down the structural formulae of these two isomers and next to each, the IUPAC name.

  1. An organic compound has the following molecular formula:C5H12O.

(a)           Write down the structural formulae for three of its structural isomers.

(b)           Write down the IUPAC names of the structural isomers.

  • Cartoons of structural isomers (chemistry)
    Cartoons of structural isomers (chemistry) (Photo credit: Wikipedia)

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