Latimer Diagrams

A Latimer diagram provides us a concise way of presenting a great deal of information about the various oxidation states of the elements. Below is a Latimer diagram for chlorine in acid solution. The potentials that are given are the reduction potential for going from the species at the left of a line to the one at the right of the line.

The arrow connecting ClO4- and ClO3- represents the half-reaction

You can easily use a Latimer diagram to determine the reduction potential for half-reactions between non-adjacent species. This procedure is different from what you are accustomed to doing when you add two half-reactions in order to generate an overall reaction with no excess electrons. In that case, you simply add the E0's for the two half reactions. In the case of making a new half-reaction, a reaction that has electrons on one side or the other, you need to remember that the relation between free energy and Eo is D Go = -nF Eo. Therefore when you add two half-reactions where the electrons do not cancel, the potential of the resultant reaction is given by

As an example, to go from HClO to Cl- the potential would be given by

Eo = (1.63+1.36)/2 = 1.50V

You can also use Latimer diagrams to predict whether or not a particular form of an element will be stable in solution, or will undergo a disproportionation reaction. Look at a species in a Latimer diagram. If the potential going to the right is more positive than that going to the left, the species is unstable and will undergo, slowly or quickly, a disproportionation reaction. For example, looking at the diagram above you can see that ClO2 will disproportionate to HClO and ClO3-. The Latimer diagram will not tell us if the disproportionation will be fast or slow, that is kinetics.

Here is a Latimer diagram for manganese in acidic solution.

Answer the following based on this diagram:

1. Write the full half-reaction for each step in the diagram.
2. Calculate the potential for going from MnO4- to Mn2+.
3. Which species, if any, will undergo disproportionation in solution?
4. The potential given here are for reactions in the standard state, which means that [H+] =1M, among other things. Recalculate the Latimer diagram at pH=3.

 William F. Coleman wcoleman@wellesley.edu Dept. of Chemistry, Wellesley College Date Created: February 16, 2003 Last Modified: March 16, 2005 Expires: September 15, 2014 Copyright 2003 by William F. Coleman