The standard reduction potentials for Zn2+/Zn, Ni2+ / Ni and Fe2+ / Fe are –0.76, –0.23 and –0.44 V respectively.
The reaction X + Y2+ X2+ + Y will be spontaneous when :
X = Ni, Y = Fe
X = Ni, Y = Zn
X = Fe, Y = Zn
X = Zn, Y = Ni
For a spontaneous reaction, Eo must be positive.
So, Eo = Eo reduced constituent - Eo oxidized constituent
Eo = (-0.23) - (-0.76)
[We get this by maximizing EoR and minimizing EoO]
Alternatively, we can also solve this qualitatively. Elements with a low value of SRP are good reducing agents and therefore, can be easily oxidized. Also, elements with a low negative value of SRP are good oxidizing agents and can be reduced easily. Keeping the above in mind, we can clearly see that and the reaction is:
Therefore, option(4) is correct