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Question

For I2+2e2I, standard reduction potential=+0.54 Volt.
For 2BrBr2+2e, standard oxidation potential=1.09 Volt.
For FeFe2++2e, standard oxidation potential=+0.44 Volt.
Out of the below given reactions, how many reactions are spontaneous in nature (predict from the above data) ?
Br2+2I2Br+I2
Fe+Br2Fe2++2Br
Fe+I2Fe2++2I
I2+2Br2I+Br2

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Solution

We know that,
ΔGo=nFEocell
For a spontanesous reaction, ΔG<0 which is when Eocell>0

(a) Br2+2e2Br EoBr2/Br=1.09 V
2II2+2e EoI/I2=0.54 V
for Br2+2II2+2Br Eo=(1.090.54) V=0.55 V
Hence, this is a spontaneous reaction.

(b)
FeFe2++2e EoFe/Fe2+=0.44 V
Br2+2e2Br EoBr2/Br=1.09 V
Br2+FeFe2++2Br Eo=(0.44+1.09) V=1.53 V
So, this reaction is also spontaneous.

(c)
FeFe2++2e EoFe/Fe2+=0.44 V
I2+2e2I EoI2/I=0.54 V
I2+FeFe2++2I Eo=(0.44+0.54) V=0.98 V
Hence, this is also a spontaneous reaction.

(d)
I2+2e2I EoI2/I=0.54 V
2BrBr2+2e EoBr/Br2=1.09 V
I2+2BrBr2+2I Eo=(0.541.09) V=0.55 V
This reaction is non-spontaneous.

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