L24 - Oxidation-reduction reactions are characterized by...

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Dr. P. Lucas U of A MSE 110 Electrochemistry Oxidation-reduction reactions are characterized by the transfer of electrons from one reactants to the other. They can be divided in two half reactions. An oxidation correspond to a loss of electrons Fe Æ Fe 3+ + 3 e - A reduction correspond to a gain of electrons O 2 + 4e - Æ 2 O 2- An element can switch from the oxidized to the reduced form as it looses or gain electrons and it is then defined as a redox couple Cu/Cu 2+ . •T h e standard redox potential E o quantifies the relative ability of each redox couple to give or take electrons. The larger the standard redox potential E o the more oxidant the species. A reaction involving two redox couples can be conducted in a controlled fashion in an electrochemical cell . The driving force of that reaction is given by the difference in standard redox potential E o =E o Ox -E o Red or alternatively by thechange in free energy of the reaction G. These two measures are related through G = -nF E o . E o =E o Ox -E o Red is the voltage produced by the electrochemical cell in which E o Ox is the standard potential of the oxidant (species gaining electrons) and E o Red is the standard potential of the reductant (species loosing electrons). The standard redox potential of species are measured in an electrochemical cell in reference to the redox couple H + /H 2 which by definition has potential E o = 0. SUMMARY FROM LAST CLASS
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Dr. P. Lucas U of A MSE 110 Redox reactions ELECTROCHEMISTRY The standard potential E o of redox couples tells in which direction a spontaneous reaction will proceed. •E x a m p l e : Cu/Cu 2+ E o =0.34 V > Cl - /Cl 2 is E o =1.36 V In a electrochemical cell Cu is oxidized by Cl 2 : Cl 2 + 2e - Æ 2 Cl - Cu Æ Cu 2+ + 2e - Hence the spontaneous reaction is: Cl 2 + Cu Æ Cu 2+ + 2 Cl - The standard potential E o doesn’t tells anything about the stoichiometry of the reaction and the number of electron transferred. This information can be obtained by breaking up the reaction into two half reaction and balancing these half reactions.
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Dr. P. Lucas U of A MSE 110 Balancing redox half-reactions ELECTROCHEMISTRY
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This note was uploaded on 11/19/2009 for the course MSE 110 taught by Professor Lucas during the Spring '08 term at Arizona.

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L24 - Oxidation-reduction reactions are characterized by...

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