I have notice that this problem doesn't provide the data to be used in solving it. But I found the same problem and found a data to be used in classifying it. This are the following
Ni^2+ (aq) + S^2- (aq) ----> + Ni (s) S (s)
Pb^2+ (aq) +H2 (g) ----> Pb (s) +2H^+ (aq)
2Ag^+ (aq) + Cr(s) ---> 2 Ag (s) +Cr^2+ (aq)
Solution
1. Ni2+ +2e- => Ni . . Eo = -0.23 V
. . S2- => S + 2e- . . Eo = +0.51 V
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Ni2+ + S2- => Ni + S . . Eo = +0.28 V . if a positive Eo = a Spontaneous Reaction
2. Pb2+ = 2e- => Pb . .Eo = -0.126 V
. . . H2 ==> 2H+ + 2e- . . .Eo = +0.00 V
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Pb2+ + H2 ==> Pb + 2H+ .Eo = -0.126 V ... a negative Eo = NONSPONTANEOUS
(3) 2Ag+ + 2e- =>2 Ag . .Eo = +0.80 V
. . .Cr ==> Cr2+ + 2e- . . . .Eo = +0.56 V
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2Ag+ + Cr ==> 2Ag + Cr2+ . .Eo = +1.36 V ... a positive Eo= SPONTANEOUS
