An electrochemical cell is set up consisting of a copper electrode immersed in a solution of copper nitrate in one half cell and a zinc electrode immersed in solution of zinc nitrate. A small amount of sodium sulfide is added to the Cu(s)/ Cu 2+ (aq) half cell. What would you expect to observe? account for these observations.
Copper electrode in a solution of copper nitrate `(E^o=0.34 V)` and a zinc electrode immersed in solution of zinc nitrate `(E^o=-0.76 V) ` constitute an elctrochemical cell in which copper serves as the positive electrode or cathode and zinc, as the negative electrode, or anode.
The `E^o` values depend, inter alia, upon the concentration of the electrolyte solutions.
Copper sulphide has a very small value of `K_(SP)` and as such, when sulphide ions are added in the form of Na2S, CuS gets precipitated resulting in a fall in the concentration of `Cu^(2+)` ions in solution.
This will decrease the formal potential of cathode, according to the Nernst equation for the `Cu^(2+)` /`Cu` (s) system:
This fall in `E^o_(cath)` would result in a decrease in `E_(cell)` .
Thus, addition of a small amount of sodium sulfide to the Cu(s)/`Cu^(2+)` (aq) half-cell would result in a fall in the cell potential.