(i.e. ∆S becomes more positive). A reaction that results in a net increase in the number of species will in
these terms be favoured. Although this hypothesis has been usefully applied in a number of cases, its
use is necessarily limited by the difficulty of deciding which side of a reaction has the greater disorder.
Conditional Equilibrium Constants
The discussion of equilibrium given above will be useful in developing ideas later in the book.
However, there are cases where a modified approach is needed, particularly when competing equilibria
are being considered. A typical situation might be the formation of a compound between metal ion Mn+
and a ligand L– in an aqueous solution which also contains a weak acid AH. Equation (3.6) summarizes
the equilibria.
It is the reaction in the box that is of analytical importance. The position of equilibrium is conveniently
expressed by a modified or conditional equilibrium constant K'ML in which allowance has been made for
the competing side reactions. The fraction of Mn+ which has not reacted with L– and remains present as
Mn+ is given by
and similarly for L– not reacted with Mn+
(charges have been omitted for simplicity).
The conditional equilibrium constant may be defined in terms of the true thermodynamic equilibrium
constant KML and the appropriate values of α.
Such equilibrium constants enable calculations and deductions to be made for real systems and may be
used to assess the progress of a particular reaction amongst a number of competing or interfering
reactions. From this consideration the possibility of masking interfering reactions also emerges.
Suppose the solution above contains a second metal ion Nn+ which can also react with L–. If the amount
of L– is limited, Nn+ will be in competition with Mn+. Its effect, however, may be masked if A– can be
selected to react