Question

Thermodynamics: Copper reacts with oxygen in air to form two possible oxides:

2Cu(s) + (1/2)O2(g) -> Cu2O(s)

2Cu(s) + O2(g) -> 2 CuO(s)

Determine which of the three solids, Cu, Cu2O, or CuO, is stable in atmospheric-pressure air (21% mol O2), over the temperature range from 300 K to the melting point of copper.

Answer 1

In actual practice, cuprous oxide forms as a red oxide in air at temperatures below about 300 C. At higher temperatures , formation of black cupric oxide, CuO is favored.

since ΔG for this reaction Cu₂O+0.5O₂=2CuO is a huge -113.4 kJ/mol at 298K, i.e you need ~10−40 bar O₂ (or less) for Cu₂O to be Stable (i.e never at r.t.). At high T , you may get a state where Cu(I) oxide is stabler, but in the presence of excess Oxygen, it will be oxidized to Cu(II) oxide. Of course, kinetic effects may allow Cu₂O to survive, but if you heat copper to relatively high T, with excess O₂, you'll get CuO (with some Cu(I) contamination).