Question

Silver jewellery turns greyish black due to the formation of Ag₂S layer on it. Can this tarnish be removed by dipping tarnished jewellery in an aluminium vessel containing inert electrolytic solution. The standard electrode potential for the half reactions are given below:

Ag₂S + 2e^– → 2Ag + S^2-

E° = -0.71V

and Al³⁺ +3e^– →Al

E° = -1.66V

Answer 1

Tarnish (black colour) will be removed if the following reaction occur.

Al + Ag₂S → Al³⁺ +2Ag + S₂

here E° for this reaction 

= -0.71 - (-1.66) 

= 0.95 V

EMF is positive. ∆G will be negative this reaction will occur and tarnish will be removed.