Lecture 25 11/2/05 Seminar today 5:30 TSB 006

Redox Oxidation:Cu +  Cu +2 + e - ReductionFe 3+ + e -  Fe +2

Review: How to solve Redox 1) split into half cell reactions 2) Balance non-H, O atoms 3) Balance O with H 2 O 4) Balance H with H + 5) Balance charge with e - 6) Balance e’s and add equations Practice: MnO NO 2 -  Mn 2+ + NO 3 -

Electrical charge (q) q = moles x F (coulombs) = moles x (coulombs/mole) Faradays constant = x 10 4 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 q Joules = 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

One line = phase boundary Two lines = salt bridge

Voltage  Depends on which elements and compounds are participating in the reaction concentrations Current  Depends 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),H 2 (g, 1 atm)|H + (aq, a=1)||

Standard Reduction Potential 1. Relative to SHE 2. Reduction potential 3. Reactants and products at standard state 4. Independent of # of moles of reactants and products shown in balanced half-reaction 1. Fe 3+ + e -  Fe 2+ E° = V 2. 5Fe e -  5Fe 2+ E° = V

Nernst Equation Way to express the driving force of a reaction  Includes: force at standard concentrations Modification term for reagent concentrations

Nernst Equation

Fe 3+ + e -  Fe 2+