last chapter! chapter 19: electron transfer reactions main concepts: 1.redox reactions consist of...
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last chapter!
Chapter 19: Electron Transfer Reactions
Main Concepts:
1.Redox Reactions consist of two halves,Oxidation and Reduction
2.Reaction Potential, Erxn, in Redox Reactions
3.Nernst Equation adjusts Eorxn to make Erxn
4.Electrochemical Cells (in two flavors) used to get useful work from redox reactions
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!
Electron Transfer Reactions: contrast to Acid/BaseAH + B A- + BH+
thermodynamics
Acid: H+ donor K: extent of forward rxn
Base: H+ acceptor G = -RTlnKG < 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
Electron Transfer Reactions: parallels to Acid/Base
Xred + Yox Xox + Yred thermodynamics
Xred = reductant: e- donor Erxn , “EMF”, potential
Yox = oxidant: e- acceptor G < 0, spont.G = -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
Electron Transfer Reactions: contrast to Acid/BaseAH + B A- + BH+Acid: H+ donor K: extent of forward
rxnBase: H+ acceptor G = -RTlnKG < 0, spont.
acid strength: Ka
base strength: Kb
Xred + Yox Xox + Yred
Xred = reductant: e- donor Erxn , EMF, potential
Yox = oxidant: e- acceptor G < 0, spont.G = -n F Erxn
reductant strength: Ered
oxidant strength: Eox
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?
Electron Transfer Reactions
Further questions:• What is the sum of the ox and red half
reactions?
• Was it spontaneous?
• What is the sign of G?
• What is the sign of Erxn , the reaction potential?
• What is the magnitude of Erxn ?
Determining Reaction Potential by Calculation
Erxn = Ered + Eox
Ag+ + e- Ag Ered = +0.799 VCu(2+) + 2e- Cu Ered = +0.337 VCu 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
Determining Free Energy from Erxn
2 Ag+ + Cu 2Ag + Cu(2+) Erxn = +0.462 V
G = -n F Erxn
G = -2 x (9.65 x 104 C/mol) e- x Erxn
G = - 89,166 J = -89 kJ
1 volt = 1 Joule/Coulomb