redox_reactions_s09 - Oxidation-reduction (redox) reactions...

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Unformatted text preview: Oxidation-reduction (redox) reactions Reactions in which there are changes in oxidation state (oxidation number) between reactants and products 2 MnO 4- + 10 Br- + 16 H + 2 Mn 2+ + 5 Br 2 + 8 H 2 O One reactant must be oxidized (lose electrons, the reductant or reducing agent ) and another must be reduced (gain electrons, the oxidant or oxidizing agent ) In some instances a single reactant will be both oxidized and reduced (disproportionation) in other cases two species containing the same element in different oxidation states will combine to give a single product with an intermediate oxidation state (comproportionation). Oxidation-reduction (redox) reactions, contd Redox reactions may be separated into oxidation and reduction half-reactions. When half-reaction reagents can be isolated into separate compartments connected by a conduit for ion migration, electron flow through an external circuit can be utilized to perform work (basis for batteries and electrochemical cells). The potential (E o ) of an electrochemical cell is the sum of the potentials of the reduction and oxidation half-reactions The potential for a cell or half-reaction is related to the free energy change for the redox reaction through the relationship G o = -n E o where n is the number of electrons transferred and is the Faraday (9.65 x 10 4 J V-1 mol-1 = 9.65 x 10 4 C mol-1 ) An electrochemical cell Zn (s) = Zn 2+ + 2 e- E o = -(-0.76) V Cu 2+ + 2 e- = Cu (s) E o = 0.34 V Zn (s) + Cu 2+ = Zn 2+ + Cu (s) E o (cell) = (0.76 + 0.34) =1.10 V G o = -n E o anode (oxidation) cathode (reduction) Cu 2+ Cu Zn Zn 2+ anode (oxidation) on left cathode (reduction) on right Galvanic vs electrolytic cells For a Galvanic Cell , a spontaneous reaction takes place. E > 0 (positive) G < 0 (negative) For the Galvanic cell, the cell performs electrical work on the surroundings (acts as a battery) A positive voltage means a rxn is thermodynamically favored and thus reactants are unstable For an Electrolytic Cell , a non-spontaneous reaction takes place....
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This note was uploaded on 05/23/2011 for the course CHEMISTRY 1311 taught by Professor Cox during the Spring '10 term at Georgia Institute of Technology.

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redox_reactions_s09 - Oxidation-reduction (redox) reactions...

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