Why does cell potential chg. when NH3 added to CuSO4? Why does it chg again when Na2S added to first soln?

Question

NH3 & Na2S both added to CuSO4 half rxn. with smaller molarity. Does that make a difference? The other half rxn. was just CuSO4 with the salt bridge connecting.

Both additions made to the half rxn. with smaller molarity. Also need to know which is the anode rxn and which is the cathode rxn since Cu can react with itself?

Answer 1

Any change in the concentration of Cu2+ ions will alter the electrode potential. Yes, copper can "react with itself" - you have two half-cells with different copper-containing species.

1. Cu2+ + 4NH3 ----> [Cu(NH3)4]2+ (dark blue solution)

2. Cu2+ + S2- ----> CuS (dark precipitate)

Both of these lower the concentration of Cu2+ in solution.