Lecture 19,20. Electrode Potentials Titrations M

Lecture 19,20. Electrode Potentials Titrations M - Nernst...

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Unformatted text preview: Nernst Equation Nernst Half Reaction Full Reaction Ecell = Eright reduction (cathode) - Eleft oxidation (anode) Gary D. Christian, Analytical Chemistry, 6th ed., John Wiley & Sons, Inc., U.S.A., 2004. Wiley Nernst Equation Nernst Half Reaction Full Reaction Ecell = Eright reduction (cathode) What should I correct What here to make this equation perfect? equation - Eleft oxidation (anode) Gary D. Christian, Analytical Chemistry, 6th ed., John Wiley & Sons, Inc., U.S.A., 2004. Wiley Daniel C. Harris, Exploring Chemical Analysis, 4th ed., W.H. Daniel Freeman and Company, New York, 2009. Freeman Standard Reduction Potential (E0) Standard Gary D. Christian, Analytical Chemistry, 6th ed., John Wiley & Sons, Inc., U.S.A., 2004. Wiley 59.16 mV 1 log 12 = 1 ( aCu 2+ ) æ1 ö 59.16 mV logç ça ÷ ÷ 1 è Cu 2+ ø 59.16 mV 1 1 log 1 2 aCu 2+ 12 = 0 E = E1 1 Cu 2+ / Cu o 2 2 - 59.16 mV 1 59.16 mV 1 1 0 log = E 1 2+ 1 o log = 12 Cu / Cu 1 1 2 aCu 2+ 2 2 ( aCu 2+ ) 0 E Cu 2+ / Cu o - 59.16 mV 1 log 2 aCu 2+ 0 therefore : E 1 2 Cu 2+ 1 / Cu o 2 0 = E Cu 2+ / Cu o Standard Reduction Potential (E0) Standard Potential when a half cell is connected to a Potential Standard Hydrogen Electrode (SHE). Standard All solutions are 1 M. All gases are 1 bar Daniel C. Harris, Exploring Chemical Analysis, 3rd ed., W.H. Freeman and Company, New York, 2005. Freeman Standard Reduction Potential (E0) Standard (see Appendix C for more redox potentials) Daniel C. Harris, Exploring Chemical Analysis, 3rd ed., W.H. Freeman and Company, New York, 2005. Freeman Reference Electrodes Reference Indicator Electrode responds to analyte concentration. to Reference electrode maintains a fixed (reference) potential fixed Daniel C. Harris, Exploring Chemical Analysis, 3rd ed., W.H. Freeman and Company, New York, 2005. Freeman Standard Hydrogen Electrode (SHE) Standard Difficult to handle: Difficult * Pt surface is easily contaminated. Pt * H2 pressure depends on tube immersion depth. * aHCl = 1 M and not [H+] = 1 M Silver-Silver Chloride Electrode Silver-Silver AgCl(s) + eAg(s) + ClE0 = +0.222 V E(sat. KCl) = +0.197 V +0.197 Daniel C. Harris, Exploring Chemical Analysis, 3rd ed., W.H. Freeman and Company, New York, 2005. Freeman Calomel Electrode Calomel ½ Hg2Cl2 (s) + e(s) Hg(l) + ClE0 = +0.268 V E(sat. KCl) = +0.241 V +0.241 Daniel C. Harris, Exploring Chemical Analysis, 3rd ed., W.H. Freeman and Company, New York, 2005. Freeman Voltage Conversions Between Different Reference Electrodes Reference Daniel C. Harris, Exploring Chemical Analysis, 3rd ed., W.H. Freeman and Company, New York, 2005. Freeman Voltage Conversions Between Different Reference Electrodes Different QuickTimeª and a decompressor are needed to see this picture. QuickTimeª and a decompressor are needed to see this picture. QuickTimeª and a decompressor are needed to see this picture. What is the Voltage of this Cell? What Daniel C. Harris, Exploring Chemical Analysis, 3rd ed., W.H. Freeman and Company, New York, 2005. Freeman Balancing Equations Balancing Example: • Working electrode reaction: Working • Reference half cell: SHE Reference (1) Establish what is reduced/oxidized. Establish Determine the number of e- in Determine both reactions. (1) Multiply reactions with integers (1) Multiply to cancel e-. to Fe 3 + + e _ Û Fe 2 + Fe 3 + + e _ Û Fe 2 + H2 Û 2 H+ + 2 e_ 2 Fe 3 + + 2 e _ Û 2 Fe 2 + H2 Û 2 H+ + 2 e_ 2 Fe 3 + + H 2 Û 2 Fe 2 + + 2 H + (1) Balance O (adding H2O), then H (adding H+), then ), other elements => No need here other (1) Check for errors. Do the charges and the number of Check atoms on the left equal the charges on the right? atoms Balancing Equations Balancing (1) Establish what is reduced/ oxidized. Determine the number oxidized. of e- in both reactions. of (1) Multiply reactions with integers (1) Multiply to cancel e-. to (1) Balance O (adding H2O), O), then H (adding H+), then ), then other elements. then Fe 3 + + e _ Û Fe 2 + Mn 7 + + 5 e _ Û Mn 2 + 5 Fe 2 + Û 5 Fe 3 + + 5 e _ Mn 7 + + 5 e _ Mn 2 + 5 Fe 2 + Û 5 Fe 3 + + 5 e _ MnO-4 + 5 e _ Û Mn 2 + + 4 H 2O 5 Fe 2 + Û 5 Fe 3 + + 5 e _ MnO-4 + 8 H + + 5 e _ Û Mn 2 + + 4 H 2O 5 Fe 2 + + MnO-4 + 8 H + Û 5 Fe 3 + + Mn 2 + + 4 H 2O (1) Check for errors. Do the charges and the number of Check atoms on the left equal the charges on the right? atoms 5 Fe 2 + Û 5 Fe 3 + + 5 e _ MnO-4 + 8 H + + 5 e _ Û Mn 2 + + 4 H 2O Example: Example: Left half cell aFe2+ = 0.1 M aFe3+ = 0.01 M Right half cell aMn2+ = 0.01 M aMnO4- = 0.001 M pH = 4.00 5 Fe 2 + Û 5 Fe 3 + + 5 e _ MnO-4 + 8 H + + 5 e _ Û Mn 2 + + 4 H 2O Where is the equilibrium if we replace the voltmeter with a wire and let the cell completely discharge? and …or simply mix the oxidizing and reducing reagent in the same or beaker? beaker? E0 and the Equilibrium Constant If E ≠ 0 V then the reaction is not at equilibrium. If E0 = E+0 – E-0 K = 10 n E0 0.05916 V Daniel C. Harris, Exploring Chemical Analysis, 3rd ed., W.H. Freeman and Company, New York, 2005. Freeman Nernst Equation Nernst What is E? If E is negative the reaction goes in the opposite direction Daniel C. Harris, Exploring Chemical Analysis, 3rd ed., W.H. Freeman and Company, New York, 2005. Freeman Redox Titrations Redox Daniel C. Harris, Exploring Chemical Analysis, 4th ed., W.H. Freeman and Company, New York, 2009. Freeman Redox Titrations Redox Daniel C. Harris, Exploring Chemical Analysis, 4th ed., W.H. Freeman and Company, New York, 2009. Freeman ...
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