Thermodynamics Solutions: #1

1.* (1997 F 2) Inorganic redox chemistry can be complicated. For example, reactions which involve the exchange of more than one electron are often very slow and sometimes do not take place, even when they are downhill from a thermodynamic point of view. This is due to the fact that electron-transfer redox precesses take place by one-electron steps. Consider the following problem: the oxidation of Tl0 to Tl3+ by Fe3+. This reaction is very slow. Use the standard redox potentials given below to evaluate the proposed reaction:

3Fe3+ + Tl0 ---> 3Fe2++ Tl3+

Eo values:

Tl3+/Tl1+ E0 = +1.25V
Tl1+/Tl0 E0 = -0.336V
Tl3+/Tl0 E0 = +0.72V
Fe3+/Fe2+ E0 = +0.77V

A. Evaluate the thermodynamic feasibility of the proposed reaction.

E0 (Fe3+/Fe2+) - E0 (Tl3+/Tl0) = +.77 - .72 = +0.05

Therefore the reaction should go spontaneously from left to right as written.

B. Provide a rational for the fact that the proposed reaction is too slow to observe.

Electron transfer processes typically take place by one electron steps. The first one electron step Tl0 ---> Tl+1 —(-0.336V) is okay, but then we are forced to do a two electron step which is unfavorable: Tl+1 ->Tl+3 —(+1.25) = -1.25V.