Based on the sign of e cell, classify these reactions as spontaneous or non spontaneous as written.? assume standard conditions. ni^2+ (aq) + s^2- (aq) > + ni (s) s (s) (nonspontaneous)? pb^2+ (aq) +h2 (g) > pb (s) +2h^+ (aq) (nonspontaneous)? 2ag^+ (aq) + cr(s) > 2 ag (s) +cr^2+ (aq) (spontaneous? ) are these correct?

A electrochemical reaction is said to be spontaneous, if  

Answer 1:
Consider reaction: Ni^2+ (aq) + S^2- (aq) ----> + Ni (s) + S (s) 

The cell representation of above reaction is given by;
    

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: Pb^2+ (aq) +H2 (g) ----> Pb (s) +2H^+ (aq)

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: 2Ag^+ (aq) + Cr(s) ---> 2 Ag (s) +Cr^2+ (aq)

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

The answer is the first option just took the test