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

Where Do I Find Ered + Eox ?

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