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last chapter! Chapter 19: Electron Transfer Reactions

last chapter! Chapter 19: Electron Transfer Reactions. Main Concepts: Redox Reactions consist of two halves, Ox idation and Red uction Reaction Potential, E rxn , in Redox Reactions Nernst Equation adjusts E o rxn to make E rxn

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last chapter! Chapter 19: Electron Transfer Reactions

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  1. last chapter! Chapter 19: Electron Transfer Reactions • Main Concepts: • Redox Reactions consist of two halves, • Oxidation and Reduction • Reaction Potential, Erxn, in Redox Reactions • Nernst Equation adjusts Eorxn to make Erxn • Electrochemical Cells (in two flavors) used to get useful work from redox reactions

  2. Electron Transfer Reactions • Reaction Potential, Erxn, in Redox Reactions • How does this relate to Free Energy? • How do you write redox reactions? • How can you predict spontaneous redox reaction? Let’s watch one in action!

  3. Electron Transfer Reactions: contrast to Acid/Base AH + B  A- + BH+ thermodynamics Acid: H+ donor K: extent of forward rxn Base: H+ acceptor DG = -RTlnK DG < 0, spont. acid strength: Ka base strength: Kb Acid/base reactions occur in pairs: need an acid and a base, a H+ donor and a H+ acceptor

  4. Electron Transfer Reactions: parallels to Acid/Base Xred + Yox Xox + Yredthermodynamics Xred = reductant: e- donor Erxn , “EMF”, potential Yox = oxidant: e- acceptor DG < 0, spont. DG = -n F Erxn reductant strength: Ered oxidant strength: Eox Redox reactions occur in pairs: need a reductant and an oxidant, an e- donor and an e- acceptor

  5. Electron Transfer Reactions: contrast to Acid/Base AH + B  A- + BH+ Acid: H+ donor K: extent of forward rxn Base: H+ acceptor DG = -RTlnK DG < 0, spont. acid strength: Ka base strength: Kb Xred + Yox Xox + Yred Xred = reductant: e- donor Erxn , EMF, potential Yox = oxidant: e- acceptor DG < 0, spont. DG = -n F Erxn reductant strength: Ered oxidant strength: Eox

  6. Electron Transfer Reactions • Let’s see one in action! • Then answer these questions: • What was the reaction? • What was the electron transfer? • What was the reductant? • What was the oxidant? • What is the reduction half reaction? • What is the oxidation half reaction?

  7. Electron Transfer Reactions • Further questions: • What is the sum of the ox and red half reactions? • Was it spontaneous? • What is the sign of DG? • What is the sign of Erxn , the reaction potential? • What is the magnitude of Erxn ?

  8. Determining Reaction Potential by Calculation Erxn = Ered + Eox

  9. Where Do I Find Ered + Eox?

  10. Determining Reaction Potential by Calculation Erxn = Ered + Eox Ag+ + e-  Ag Ered = +0.799 V Cu(2+) + 2e-  Cu Ered = +0.337 V Cu  Cu(2+) + 2e- Eox =-0.337 V 2 Ag+ + Cu  2Ag + Cu(2+) Erxn = +0.462 V Erxn = Ered+Eox = 0.799 -0.337 = 0.462 V

  11. Determining Free Energy from Erxn 2 Ag+ + Cu  2Ag + Cu(2+) Erxn = +0.462 V DG = -n F Erxn DG = -2 x (9.65 x 104 C/mol) e- x Erxn DG = - 89,166 J = -89 kJ 1 volt = 1 Joule/Coulomb

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