Page 96 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 FUNDAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

equation may be written in the form:

This enables us to calculate the effect of change in the ratio [Mn04]/[MnZ+]
at any hydrogen ion concentration, other factors being maintained constant. In
this system, however, difficulties are experienced in the calculation owing to the
fact that the reduction products of the permanganate ion Vary at different
hydrogen ion concentrations. In other cases no such difficulties arise, and the
calculation may be employed with confidence. Thus in the reaction:

It is now possible to calculate the equilibrium constants of oxidation-reduction
reactions, and thus to determine whether such reactions can find application in
quantitative analysis. Consider first the simple reaction:

The equilibrium constant is given by:
[Cl-]' x [Fe3+]'
= K
[Cl,] x [Fe2+]'
The reaction may be regarded as taking place in a voltaic cell, the two half-cells
being a C12,2C1- system and a Fe3+,FeZ+ system. The reaction is allowed to
proceed to equilibrium, and the total voltage or e.m.f. of the ce11 will then be zero,
i.e. the potentials of the two electrodes will be equal:
0.0591 [Cl,] 0.0591 [Fe3+]
= E~I + -108-
EE2,2c1- + -
2 l 0 8 ~ 1 [FeZ+]
Now EC;2,2CI- = 1.36 volts and Egl+,Fe2+ 0.75 volt, hence
=
[Fe3+]' x [Cl-]' 0.61
-- = 20.67 = log K
-
log [Fe2+]' x [Cl2] 0.02965

The large value of the equilibrium constant signifies that the reaction will proceed
from left to right almost to completion, i.e. an iron(I1) Salt is almost completely
oxidised by chlorine.
Consider now the more complex reaction:

The equilibrium constant K is given by: