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LECTURE 27 Electrochemical Reactions

LECTURE 27 Electrochemical Reactions - SUMMARY FROM LAST...

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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+ . The 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 Δ E o =E o Ox -E o Red or alternatively by the change in free energy of the reaction Δ G. These two measures are related through Δ G = -nF Δ E o . Δ 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 In a electrochemical cell Cu is oxidized by Cl 2 : Cl 2 + 2e - Æ 2 Cl - Cu Æ Cu 2+ + 2e - ELECTROCHEMISTRY Hence the spontaneous reaction is: Cl 2 + Cu Æ Cu 2+ + 2 Cl - This information can be obtained by breaking up the reaction into two half reaction and balancing these half reactions. The relative standard potential E o of two redox couples reacting together tells in which direction a spontaneous reaction will proceed. Example: Cu/Cu 2+ E o =0.34 V > Cl - /Cl 2 is E o =1.36 V The standard potential E o doesn’t tell anything about the stoichiometry of the reaction and the number of electron transferred.
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Dr. P. Lucas U of A MSE 110 Balancing redox half-reactions Two principles should be followed to balance an equation: ELECTROCHEMISTRY
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LECTURE 27 Electrochemical Reactions - SUMMARY FROM LAST...

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