JEE MAIN - Chemistry (2012 - No. 8)
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+ $$\to$$ X2+ + Y will be spontaneous when :
X + Y2+ $$\to$$ X2+ + Y will be spontaneous when :
X = Ni, Y = Fe
X = Ni, Y = Zn
X = Fe, Y = Zn
X = Zn, Y = Ni
Explanation
For a spontaneous reaction $$\Delta G$$ must be $$-ve$$
Since $$\Delta G = - nF{E^ \circ }$$
Hence for $$\Delta G$$ to be $$-ve$$ $$\Delta {E^ \circ }$$ has to be positive.
Which is possible when $$X = Zn,Y = Ni$$
$$Zn + N{i^{ + + }}\buildrel \, \over \longrightarrow Z{n^{ + + }} + Ni$$
$$E_{Zn/Z{n^{ + 2}}}^ \circ + E_{N{i^{2 + }}/Ni}^ \circ $$
$$ = 0.76 + \left( { - 0.23} \right) = + 0.53$$ (positive).
Since $$\Delta G = - nF{E^ \circ }$$
Hence for $$\Delta G$$ to be $$-ve$$ $$\Delta {E^ \circ }$$ has to be positive.
Which is possible when $$X = Zn,Y = Ni$$
$$Zn + N{i^{ + + }}\buildrel \, \over \longrightarrow Z{n^{ + + }} + Ni$$
$$E_{Zn/Z{n^{ + 2}}}^ \circ + E_{N{i^{2 + }}/Ni}^ \circ $$
$$ = 0.76 + \left( { - 0.23} \right) = + 0.53$$ (positive).
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