A electrochemical reaction is said to be spontaneous, if 
Answer 1:
Consider reaction: <span>Ni^2+ (aq) + S^2- (aq) ----> + Ni (s) + S (s)
The cell representation of above reaction is given by;
</span>
Hence,
we know that,
and
Therefore,
= - 0.25 - (-0.47) = 0.22 v
Since, is positive, hence cell reaction is spontaneous
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Answer 2:
Consider reaction: <span>Pb^2+ (aq) +H2 (g) ----> Pb (s) +2H^+ (aq)
</span>
The cell representation of above reaction is given by;
Hence,
we know that,
and
Therefore, = - 0.126 - 0 = -0.126 v
Since, is negative, hence cell reaction is non-spontaneous.
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Answer 3:
Consider reaction: <span>2Ag^+ (aq) + Cr(s) ---> 2 Ag (s) +Cr^2+ (aq)
</span>
The cell representation of above reaction is given by;
Hence,
we know that,
and
Therefore, = - 0.22 - (-0.913) = 0.693 v
Since, is positive, hence cell reaction is spontaneous
Answer 1:
Consider reaction: <span>Ni^2+ (aq) + S^2- (aq) ----> + Ni (s) + S (s)
The cell representation of above reaction is given by;
</span>
Hence,
we know that,
and
Therefore,
Since,
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Answer 2:
Consider reaction: <span>Pb^2+ (aq) +H2 (g) ----> Pb (s) +2H^+ (aq)
</span>
The cell representation of above reaction is given by;
Hence,
we know that,
and
Therefore,
Since,
....................................................................................................................
Answer 3:
Consider reaction: <span>2Ag^+ (aq) + Cr(s) ---> 2 Ag (s) +Cr^2+ (aq)
</span>
The cell representation of above reaction is given by;
Hence,
we know that,
and
Therefore,
Since,