1. The potential of an iron electrode when polarized as cathode at 0.001 A/cm2 i
ID: 548914 • Letter: 1
Question
1. The potential of an iron electrode when polarized as cathode at 0.001 A/cm2 is-0.916 V versus IN calomel half-cell. The pH of the elec- trolyte is 4.0. What is the value of the hydrogen overvoltage? Ans-0.40 V Notes: Assume Patm, T298 K. Assume that there is no Fe2 in the electrolyte; therefore, when Fe is cathodically polarized Fe will not plate out. The only cathodic reaction that can occur, therefore, is hydrogen evolution. H2 Hints: (over potential) = (actual)-,, H The cathode potential, (actual) , is in volts vs the 1 N calomel reference electrode. When you calculate 11.H. , it will be in volts vs the standard hydrogen electrode. To get the over potential, , the potentials (actual) and 11, H, must be taken with respect to the same reference electrode Ans for Htm,--0.236 VsHE Don't forget to calculate the hydrogen overvoltage (also called the over potential). 3. The potential of a platinum anode at which oxygen evolves in an elec- trolyte of pH 10 is 1.30 V with respect to the saturated calomel elec- trode. What is the value of oxygen overvoltage? Ans. 0.90 v Notes: Assume P1 atm Hints: (over potential) = (actual)-0.OH. The platinum anode potential, (actual), is in volts vs the saturated calomel electrode When you calculate the over potential, , the potentials (actual) and %/OH must be taken with respect to the same reference electrode , it will be in volts vs the standard hydrogen electrode. To get . Ans for 020 :0.637 VsHE Don't forget to calculate the oxygen overvoltageExplanation / Answer
SOLUTION:
1.
Hydrogen overvoltage by Tafel equation:
E0=a+blog I
E0=Overpotential or hydrogen overvoltage
a,b are constants
I=current density
slope from Tafel curve comes as:
E0=a+(2x2.303RT)/Flog I
=a+(2x2.303x8.314x298)/96485.33log0.001=-0354=approx -0.4
Therfore hydrogen overvoltage is -0.4 V
2.
Over potential = (phi) actual - (phi)O2/OH-
=1.3-0.401
=0.9 Volt
For phi_O2/OH- refer table to find it's values. I have used from the table available on Wikipedia.
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