Question

Tarnished silver contains AgS. Can this tarnish be removed by placing tarnished silverware in an aluminium pan containing an inert electrolytic solution such as NaCl? The standard electrode potential for half reaction:

\[\ce{Ag2S_{(s)} + 2e- -> 2Ag_{(s)} + S^{2-}}\] is −0.71 V and for \[\ce{Al^{3+} + 3e- -> 2Al_{(s)}}\] is −1.66 V.

Answer 1

Yes, tarnish on silver (which contains Ag₂S) can be removed by placing the tarnished silverware in an aluminium pan containing an inert electrolyte such as NaCl. The process occurs through an electrochemical reaction in which aluminium acts as the reducing agent and silver sulphide is reduced back to metallic silver.

The relevant half-reactions are:

Reduction (cathode):

\[\ce{Ag2S_{(s)} + 2e- -> 2Ag_{(s)} + S^{2-}}\]  (E° = −0.71 V)

Oxidation (anode):

\[\ce{Al^{3+} + 3e- -> 2Al_{(s)}}\]  (E° = +1.66 V)

The standard cell potential is:

`E_"cell"^° = E_"cathode"^° - E_"anode"^°`

= (−0.71) − (−1.66)

= −0.71 + 1.66

= +0.95 V

Since the value of `E_"cell"^°` is positive, the redox reaction is feasible, and hence the silver sulphide tarnish is removed.