Platinum Metals Rev., 1968, 12, (4), 138
Thermochemistry of the Platinum Metals and their Compounds
The word “thermochemistry” may seem to have an unduly academic ring to it, far removed from the world of the experimental chemist, and indeed in the case of the platinum metals the information on the subject is so widely scattered in the literature (and often in obscure journals) that it may seem to have little practical relevance. However, when properly handled, such data on the stabilities of the elements and their compounds can be helpful to the experimentalist for they afford a quantitative basis to the very fundamental problem of whether a given chemical reaction will proceed or not.
The appearance of a comprehensive and critical review on the thermochemistry and oxidation potentials of the six platinum group metals and their compounds is therefore timely and welcome. This, prepared by R. N. Goldberg and L. G. Hepler (1) at the Carnegie-Mellon University, Pittsburgh, brings together a mass of previously uncorrelated information, more particularly concerned with the aqueous solution chemistry of the compounds. It is the purpose of this note to indicate the scope of the review, the information that it presents and the use that may be made of it.
For each element there is a brief but well-informed discussion of its descriptive chemistry, followed by a table of thermodynamic data for the metal and its compounds; the approach here is selective and critical, and the authors clearly indicate the value they attach to the reported results. Each section also includes data on oxidation potentials, equilibrium constants and related matter, while oxidation potential diagrams are given for various metal oxidation states in acidic and in basic solutions. The general arrangement of material is very similar to that in Latimer’s well-known book (2) and to anyone acquainted with that work the present review should cause no difficulties.
For each metal and such compounds for which data are available the following three quantities are listed (all refer to the standard temperature of 298°K):
The ΔH°f values are obtained from calorimetric or vapour pressure data, S° by calculation from specific heat or spectroscopic measurements, and ΔG°f from the equation
where T = 298°K and S is the difference between S° values of the reaction products and reactants.
For species in aqueous solution, ΔG°f values are difficult to measure accurately but may be obtained by measurements of the standard electrode potential E° of the reaction under consideration:
(where n is the number of electrons involved in the cell reaction and E° the standard oxidation potential, in volts; ΔG° is then in Kcals/mole). The sign convention used here for E° is (regrettably) not the one recommended by the IUPAC convention but is that used in Latimer’s book and a number of other texts; e.g. for the cell reaction
All the cell reactions are written as oxidations; i.e. with the electrons on the right-hand side.
is also used to calculate ΔG° for some reactions (T=298°K, and K is the equilibrium constant of the reaction).
From these data ΔG°f values are tabulated for a number of compounds in the solid and (occasionally) gaseous states and for aqueous solutions. This standard free energy of formation is the most useful thermodynamic quantity for practical application; if it is negative, as in
the reaction should proceed spontaneously in the direction indicated. However, this tells us nothing of the rate of such a reaction, merely that it is thermodynamically possible. If ΔG°f were large and positive the reaction could not proceed spontaneously, with or without a catalyst. Of course, by proceeding to non-standard states of reaction the balance may be changed. It is also possible to calculate equilibrium constants for reactions involving the compounds quoted by using these ΔG°f values together with other readily available data.
Potential diagrams are given for the various oxidation states of the elements, and these provide a useful shorthand way of summarising information on the relative stabilities in solution of their various oxidation states.
In short, this review provides at least a foundation for the more quantitative aspects of platinum metal chemistry. It should stimulate further research in a field which, for the platinum group elements, has received less attention than it deserves.
- 1R. N. Goldberg and L. C. Hepler, Thermochemistry and Oxidation Potentials of the Platinum Group Metals and their Compounds, Chem. Rev., 1968, 68, 229 .
- 2W. M. Latimer, Oxidation Potentials, Prentice Hall, Second edition, 1959 .