2. Understanding Corrosion

2.5 Free Energy Potentials

A chemical reaction will occur only if there is an overall decrease in the energy of the system and its surroundings during a reaction. Consider examples (A) and (B)

Example A

ZnO(s) + C (s) Zn + CO(g)

DS = +285 J/K-mol             DH = +349.9 kJ/mol

Example B

Fe(s) + 1/2 O2 (g) FeO(s)

DS = -71 J/K-mol               DH = -265.5 kJ/mol

In the above example, both DS and DH work opposite to each other in terms of direction of energy change.

Which one would decide the direction of the reaction?

We must therefore introduce a single function expressing the combined effect of both DS and DH. Of the total enthalpy of the system, only a part is converted to useful work, which is called "Free Energy" (DG). This new function can be defined in terms of DH, DS and T. The relationship between the three is deduced as

DHo - DGo = T DSo

This may be rearranged to give

DGo = DHo - T DSo

The following points may be deduced

  • A reaction is not feasible if DGo is positive.

  • A reaction is at equilibrium if DGo = 0

  • A reaction is feasible if  DGo is negative.
Example:
Calculate DGo for the following reaction at 500 K
          Cu(s) + H2 (g) Cu(s) + H2 (g)
          DHo(500) = -87 kJ/mol
          DSo(500) = +47 J/K-mol
Solution:
          DGo = DHo - TDSo
                  = -87000 - (500 x 47)
                  = -110.5 kJ/mol

DGo is negative, hence the reaction is feasible.