The equation
Co3O4(s) + Co(s) → Co(OH)2(s) + CoO(s)
can be correctly balanced two different ways. Find the two correctly balanced equations. Which is chemically more reasonable?
The two different ways to balance the equation are based on how the redox couples are chosen:
Method 1
Oxidation:
Co(s) + H2O(l) → CoO(s) + 2 H+(aq) + 2 e–
Reduction:
Co3O4(s) + 2 H2O(l) + 2 H+(aq) + 2 e– → 3 Co(OH)2(s)
Net:
Co3O4(s) + Co(s) + 3 H2O(l) → 3 Co(OH)2(s) + CoO(s)
Method 2
Oxidation:
Co(s) + 2 H2O(l) → Co(OH)2(s) + 2 H+(aq) + 2 e–
Reduction:
Co3O4(s) + 2 H+(aq) + 2 e– → 3 CoO(s) + H2O(l)
Net:
Co3O4(s) + Co(s) + H2O(l) → Co(OH)2(s) + 3 CoO(s)
To determine which reaction is most chemically reasonable, the spontaneity of each reaction must be estimated using Gibb's energies. Assuming T = 298 K, then the Gibb's energies of formation can be used directly from the Table of Thermodynamic Quantities.
Method 1:
ΔG° = [3(–454.4) + (–214.2)] – [(–774.0) + (0) + 3(–237.2)] = –91.8 kJ
Method 2:
ΔG° = [(–454.4) + 3(–214.2)] – [(–774.0) + (0) + (–237.2)] = –85.8 kJ
Since the Gibb's energy for method 1 is more negative, this is the more spontaneous reaction.