The saturated calomel electrode is a half cell that is often used as a reference

Question

The saturated calomel electrode is a half cell that is often used as a reference for measuring the reduction potentials of other half cells (calomel is an old name for mercury(1) chloride, Hg2C2). The half-cell reaction and reduction potential for the electrode are (saturated aqueous KCl solution) E= 0.241 V When a saturated calomel electrode is connected to a Co+ I Co half cell in which the concentration of Co* is 0.050 M, the measured cell potential at 298 K is 0.561 V and the cobalt half cell is the anode. (Show all work) (a) What is E for this Co Co half cell? (b) Write the net cell reaction. (c) What is Eo for a Co2* Co half cell?

Explanation / Answer

(a) Co (s) --------> Co+2 (l) + 2e- ; E = +0.28 V
(b) Hg2Cl2 (s) + 2e- <-----> 2 Hg (l) + 2 Cl- ; E = + 0.241 V (reduction at cathode)
Co (s) ----------> Co+2 (l) + 2e- ; E = +0.28 V (Oxidation at anode)
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Hg2Cl2 (s) +  Co (s) ---------> 2 Hg (l) + 2 Cl- + Co+2 (l) ; Eocell = 0.521 V
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(c) Use Nernst equation to solve for Eo
  Eo = Eocell - 0.0591/n log [oxidized / reduced]
At 298 K Eocell = 0.561 V ; n = 2
Eo = 0.561 - 0.0591/2 log [Co+2 / 1]
Eo = 0.561 - (-0.038445) = 0.5994 V
  

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