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Question

Construct galvanic cells from the following pairs of half cells and calculate their emf at 25oC.

Cd2+(aq,1 M)∣Cd(s); E0Cd2+/Cd=−0.40 V

Hg2Cl2(s)∣Hg(l)∣Cl−(aq,0.1M);E0Cl−/Hg2Cl2/Hg=0.28 V

A
Ecell=0.01 V
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B
Ecell=0.739 V
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C
Ecell=0.01 V
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D
Ecell=0.739 V
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Solution

From the standard reduction potential values we can see Hg2Cl2(s) will act as cathode in the galvanic cell. Applying nernst eq. for individual half cells.
For the half - cell reaction,
Cd2+(aq)+2eCd(s)
By Nernst equation,
ECd2+/Cd=E0Cd2+/Cd0.05912log1[Cd2+]
ECd2+/Cd=0.400.05912log11=0.40 V
And, for the half - cell reaction,
Hg2Cl2(s)+2e2Hg(l)+2Cl(aq)
ECl/Hg2Cl2/Hg=E0Cl/Hg2Cl2/Hg0.05912log[CI]2
ECl/Hg2Cl2/Hg=0.280.05912×2 log 0.1
ECl/Hg2Cl2/Hg=0.280.05912×2 log 10
ECl/Hg2Cl2/Hg=0.28+0.05912×2
ECl/Hg2Cl2/Hg=0.28+0.0591=0.339 V
The cell may be represented as
Cd(s)Cd2+(aq)Cl(aq)Hg2Cl2(s)Hg(l)
Ecell= reduction potential of RHS electrode reduction potential of LHS electrode
Ecell=0.339(0.40)=0.739 V


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