Question

What are the oxidation and reduction reactions of gold plating on copper?

Answer 1

Gold would be reduced, since it is moving from an ion to a solid. So:
\[Au ^{3+} + 3 e^- \rightarrow Au\]
Then copper is being oxidized to provide those extra electrons:
\[Cu \rightarrow Cu^{2+} = 2e^-\]

Answer 2

My research indicated though, that in in electroplating, the metal that is oxidised is the metal that is being used to coat the other object. This would suggest that Gold would be reduced. Could that happen?

Answer 3

In a standard electroplating, yes, you would both oxidize and reduce the same metal. The goal is to remove the metal, atom by atom, from a solid piece and deposit it, atom by atom, in a very smooth and thin coat somewhere else. The purpose of the oxidation is to get the metal into solution, and the purpose of the reduction is to get it out again -- on top of the object to be electroplated.

So, for example, you could have an electrolytic solution (e.g. a solutionof sodiun cyanide), a copper object, and a gold bar. You run an electric current from the gold bar to the copper object. The current strips electrons from the gold bar, oxidizing the Au to Au+3 cations, which dissolve in the electrolyte. They then drift over to the cathode (the copper object) and there the electric current supplies electrons to reduce the Au+3 cations to Au metal, which coats the copper in a very thin layer of gold, atom by atom.

You use copper for the object to be coated because copper conducts electricity, because it's more active than Au so the Au+3 cannot oxidize it, and because gold and copper form similar crystal structures (ccp with lattice constants 3.6 A for copper and 4.1 A for gold), so that the gold layer will nestle in nicely to the underlying copper structure.