MadSci Network: Chemistry

Re: Re: Why do I find 3 different standard potentials for the electrolysis of water

Date: Sun Feb 22 15:25:33 2009
Posted By: Cesar Prado-Fdez,
Area of science: Chemistry
ID: 1234793962.Ch

The standard reduction potential is the reduction potential (Electromotive
force for the reduction of the specie) at the standard conditions. This is:

"When we  say  that a  reaction  takes place at  standard  conditions, we
mean  the  following:
1. The  temperature  is  the  standard  thermodynamic  temperature,  25°C,
 unless  stated otherwise.
2. All reactants and products are at unit activity. This means that:
a. Any solution species that takes part in the reaction is at a
concentration of exactly 1 M;
b. Any gas that takes part in the reaction is at a pressure of exactly 100 kPa;
c. Any other substance that takes part in the reaction is pure."
(IUPAC and General Chemistry. Whitten, Peck & Davis)

The electromotive force or potential is never an absolute number but it has
to be defined versus a reference point. The reference point selected is the
standard hydrogen electrode (SHE) which is given a value of absolute 0.00 V
at al temperatures.
(IUPAC gold book:

The electrolysis of water consist in two reactions one oxidation (ocurring
at the anode) and a reduction (at the other electrode, the cathode)

Acidic media:
Ox   2 H2O ---> O2 + 4 H+ +2 e-    -(-1.23) V
Red: 2 H+ + 2 e- ---> H2              0.00  V
                                      1.23  V Overall potential

Basic media:
Ox:  4 OH− ---> O2 + 2 H2O + 4e−      0.40  V
Red: 2 H2O + 2e− ---> H2 + 2OH−       0.83  V
                                      1.23  V Overall potential

As the SHE is quite difficult to maintain other reference electrodes, as
long as they give out a defined constant potential. This is the case of the
calomel saturated (KCl) electrode, SCE, which gives a constant potental of
0.24 V. 

Using this electrode instead of the SHE will make you to add an extra 0.24V
to the value expected for the reaction, thus giving 1.48V

Potentials are taken from CRC Handbook of Chemistry and Physics, 85th ed. 2004

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