Thermodynamics Solutions: #23

23.* (1994 3 4) When a trace of FeSO4 is added to an aqueous solution of H2O2, the solution foams; a gas evolves. When a trace of ZnSO4 is added to another aqueous solution of H2O2, no reaction occurs.

A. Write equations showing the reactions that occur when FeSO4 is present:

The reaction steps must be in this order because the H+'s are required for the second step to occur.

B. Show that the reaction has a negative free energy change.

-0.69V

+0.77V

+1.78V

-0.77V

+1.09V

DG°=-nFE°

Because E°>0, DG°<0.

The reaction is spontaneous.

C. Explain the role of FeSO4 and why ZnSO4 has no effect.

FeSO4 acts as a catalyst for this reaction but ZnSO4 does not. Only a trace of FeSO4 is required because, as a catalyst, it is regenerated. With a catalyst the first step is spontaneous:

-0.69V

+0.77V

––––

+0.08V

The positive E° makes the first step spontaneous.

With Zn:

-0.69V

-0.76V

––––

-1.45V

The negative E° makes the first step non-spontaneous.