Lecture_7_Redox

Lecture_7_Redox - Chem 310 Lecture Module 7 Reduction...

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Chem 310 Lecture Module 7 Reduction – Oxidation Properties of TM complexes
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The Basics Oxidation is the gaining of oxygen, loss of hydrogen, or loss of electrons eduction is the loss of oxygen gain of hydrogen or gain of electrons Reduction is the loss of oxygen, gain of hydrogen or gain of electrons - these two reactions complement one another (they are coupled) - net change in oxidation states for ox and red steps must balance 2Mg + O 2MgO 2 Cathode (reduction) : 2Mg 2Mg 2+ + 4e - Anode (oxidation): O 2 + 4e - 2O 2- an electrolytic cell passage of current initiates a redox r n i e to make In an electrolytic cell, passage of current initiates a redox r xn , i.e., to make chlorine from the electrolysis of table salt, or the electrolysis of H 2 O to make H 2 : Na + + e - Na 2H 2 O + 2e H 2 ( g ) + 2OH Cl - ½ Cl 2 + e - 4OH O 2 ( g ) + 2H 2 O + 4e In a galvanic cell, a spontaneous redox r xn occurs to generate an electrical current (due to thermodynamic driving force): ie, an alkaline battery n (s) + 2OH q) ZnO (s) + H (l)+2e Zn (s) + 2OH (aq) 2 O (l) + 2e 2MnO 2 (s) + H 2 O (l) + 2e Mn 2 O 3 (s) + 2OH (aq) In a rechargeable cell, both reactions occur depending on the whether the cell is being charged or discharged (ie, both reactions are readily reversible) LiCoO 2 ÍÎ Li 1-x CoO 2 + x Li + + xe - x Li + + xe - + 6C ÍÎ Li x C 6
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Example of a Galvanic Cell: The Daniell cell Electrical work is done by this system (ie it is spontaneous) Cu 2+ + 2e - Î Cu Zn Î Zn 2+ + 2e - ell voltage is given by the change in Gibbs free energy: Cell voltage is given by the change in Gibbs free energy: Δ G o = -z F E o cell z = # e- transferred, F = Faraday constant, Δ G in J/mol Equilibrium constant K: since Δ G o = - RT ln K ln K = (z F E o cell )/RT for the cell above, E o is 1.10V, so K ~ 10 37 For a cell that is thermo driven, E o is positive; Δ G o is negative; and K >1
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Defining and Using Standard Reduction Potentials Used to evaluate thermodynamic viability of a redox reaction Li is the most powerful reducing agent (easily ox to Li + ) E 0 [Zn 2+ /Zn] vs 2H + /H 2 All potentials defined relative to the standard hydrogen electrode E 0 [Cu 2+ /Cu] vs 2H + /H E o cell = E o red –E o ox Î 1.10V [ ] 2 ometime given reactions don’t Sometime given reactions don t occur due to overpotential being necessary – addntl activation energy needed - ie, O 2 is actually reduced t 1 8V not 1 23V F 2 is the most powerful oxidizing agent (F 2 easily red to F - ) at 1.8V, not 1.23V
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Reference Electrodes Silver chloride/silver electrode: Cl - (aq) AgCl Ag (E o = 0.222V) AgCl + e - Ag(l) + Cl - (aq)
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Lecture_7_Redox - Chem 310 Lecture Module 7 Reduction...

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