Answer
See explanation below.
Work Step by Step
Oxidation reaction Cd(s) --> Cd2+(aq) + 2 e- -E° = +0.40 V
Reduction: Cr2O7 2-(aq) + 14 H+ (aq) + 6 e- --> 2 Cr3+(aq) + 7 H2O(l) E° = +1.33 V
Complete reaction is:
Cr2O7 2-(aq) + 3 Cd(s) + 14 H+ (aq) --> 2 Cr3+(aq) + 3 Cd2+(aq) + 7 H2O(l) Ecell=1.73V
Reaction is spontaneous as E cell is positive
Second part.
Oxidation reaction is Pb(s) --> Pb2+(aq) + 2 e- -E° = +0.13 V
Reduction: Ni2+(aq) + 2 e- --> Ni(s) E° = -0.25 V
Complete reaction is Pb(s) + Ni2+(aq) --> Pb2+(aq) + Ni(s) o E cell = -0.12 V
Reaction is not spontaneous, as E cell is negative.
