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Lecture 2511/2/05
Seminar today 5:30
TSB 006
Redox
Oxidation: Cu+ Cu+2 + e-
Reduction Fe3+ + e- Fe+2
Review: How to solve Redox
1) split into half cell reactions
2) Balance non-H, O atoms
3) Balance O with H2O
4) Balance H with H+
5) Balance charge with e-
6) Balance e’s and add equations
Practice: MnO4- + NO2
- Mn2+ + NO3-
Electrical charge (q)
q = moles x F
(coulombs) = moles x (coulombs/mole)
Faradays constant = 9.649 x 104 C/mol e’s
Electrical current (I)
Quantity of charge flowing per second through a circuit
Current = coulombs/sec= moles/sec x coulombs/mole= Ampere (A)
Electrical potential (E)
Measure of the work when an electrical charge moves from one point to another
Work = E x qJoules = volts x coulombs = J/coulombs x coulombs
Free Energy
G = - work = -E x q G = -nFE
Ohm’s Law
Current = Electrical potential /resistance I = E / R (amps) = Volts/Ohms (units) A = V / (units)
Power (P)
Work done per unit time J/s = Watts
P = work/s = E x q / s = E x I
Galvanic (voltaic) Cell
Uses a spontaneous chemical reaction to generate electricity
Need: Redox reaction Substance to be separated so e’s flow through
circuit
)s(Ag)aq(AgNO)aq()NO(Cd)s(Cd 323
One line = phase boundary
Two lines = salt bridge
Voltage Depends on
which elements and compounds are participating in the reaction
concentrations
CurrentDepends on size (# moles) of
elements/compounds
Standard potentials (E°)
All compounds are 1 bar or 1 M
Cell Potential
E°cell = E°cathode - E°anode
E°cell = E°red - E°ox
Standard Hydrogen Electrode (SHE)
Arbitrarily assigned E° = 0 V
Used to establish potential of other half-reactions
Pt(s),H2 (g, 1 atm)|H+ (aq, a=1)||
Standard Reduction Potential
1. Relative to SHE2. Reduction potential3. Reactants and products at standard state4. Independent of # of moles of reactants
and products shown in balanced half-reaction
1. Fe3+ + e- Fe2+ E° = +0.771 V2. 5Fe3+ + 5e- 5Fe2+ E° = +0.771 V
Nernst Equation
Way to express the driving force of a reaction Includes:
force at standard concentrations Modification term for reagent concentrations
Nernst Equation
Qlogn
V05916.0EE
Qlog303.2 Qln
C/mol96490F
K-mol V-Crmol) o J/(K3145.8R
QlnnF
RTEE
o
o
Nernst Equation
Fe3+ + e- Fe2+
]Fe[
]Fe[log
1
V05916.0771.0E
Qlogn
V05916.0EE
3
2
o