|MadSci Network: Chemistry|
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: http://goldbook.iupac.org/S05917.html) 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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