Reference Electrodes The E° value for the following reaction is 0.446 V relative
ID: 1067283 • Letter: R
Question
Reference Electrodes
The E° value for the following reaction is 0.446 V relative to the standard hydrogen electrode (SHE):
A chemist wishes to determine the concentration of CrO42- electrochemically. A cell is constructed consisting of a saturated calomel electrode (SCE; which has a reduction potential of 0.242 V relative to the SHE) and a silver wire coated with Ag2CrO4 and suspended in a CrO42- solution.
What is the potential of this cell at 25°C when [CrO42-] = 1.00 M?
Calculate G for the full-cell reaction at 25°C when [CrO42-] = 1.00 M?
What is the potential of this cell at 25°C when [CrO42-] = 5.80×10-3 M?
For a solution of unknown [CrO42-], the measured potential for the cell at 25°C is 0.303 V. What is [CrO42-] (in mol/L)?
Explanation / Answer
Given reaction is
Ag2CrO4 (s) + 2e- ----------------> 2 Ag (s) + CrO42- (aq)
No of electrons transferred n = 2
Given that Eo = 0.446 V
a) What is the potential of this cell at 25°C when [CrO42-] = 1.00 M?
Ecell = Eo - (0.059/n) log [CrO42-]
= 0.446 V - (0.059/2) log [1 M ]
= 0.446 - 0
= + 0.446 V
Therefore,
cell potential = + 0.446 V
b) Calculate G for the full-cell reaction at 25°C when [CrO42-] = 1.00 M?
From above, Ecell = + 0.446 V
G = -nFE
= - 2x 96485 x 0.446
= - 86.1 J
G = - 86.1 J
c) What is the potential of this cell at 25°C when [CrO42-] = 5.80×10-3 M?
Ecell = Eo - (0.059/n) log [CrO42-]
= 0.446 - (0.059/2) log [ 5.80×10-3]
= 0.51 V
Therefore,
cell potential = + 0.51 V
d) Given that Ecell = 0.303 V
[CrO42-] = ?
Ecell = Eo - (0.059/n) log [CrO42-]
0.303 = 0.446 - (0.059/2) log [CrO42-]
[CrO42-] = 7.0 x 104 M
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