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
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)+2e−⇌Cd(s)
By Nernst equation,
ECd2+/Cd=E0Cd2+/Cd−0.05912log1[Cd2+]
ECd2+/Cd=−0.40−0.05912log11=−0.40 V
And, for the half - cell reaction,
Hg2Cl2(s)+2e−⇌2Hg(l)+2Cl−(aq)
ECl−/Hg2Cl2/Hg=E0Cl−/Hg2Cl2/Hg−0.05912log[CI−]2
ECl−/Hg2Cl2/Hg=0.28−0.05912×2 log 0.1
ECl−/Hg2Cl2/Hg=0.28−0.05912×2 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