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An electrochemical cell is set up consisting of a copper electrode immersed in a...

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nerdygirl-1996 | Student, Undergraduate | (Level 1) Honors

Posted August 12, 2013 at 7:50 AM via web

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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.

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llltkl | College Teacher | (Level 3) Valedictorian

Posted August 12, 2013 at 4:27 PM (Answer #1)

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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.

`E_(cell)=E^o_(cath)-E^o_(anod)=0.34-(-0.76)=1.1 V`

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:

`E=E^o-0.059/2log[Cu]/[Cu^(2+)]`

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.

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