Lecture 273/31/06

E° F 2 (g) + 2e - ↔ 2F Ag + + e - ↔ Ag (s)+0.80 Cu e - ↔ Cu (s)+0.34 Zn e - ↔ Zn (s)-0.76 Quiz 1. Consider these half-reactions 1. Which is the strongest oxidizing agent? 2. Will F 2 (g) oxidize Ag(s) to Ag + ? 3. Will Zn 2+ reduce Ag + to Ag(s)? 4. Which ions can be reduced by Cu(s)? 2. What is the oxidation number of sulfur: 1. SO H 2 S

Eº vs. E

Nernst Equation

Calculate the voltage delivered by a voltaic cell using the following reaction if all dissolved species are 0.015M. Fe 2+ + H 2 O 2  Fe 3+ + H 2 O

Nernst Equation and Equilbrium When a cell is at equilibrium, E cell = 0

Primary Batteries (non-rechargeable) Dry Cell or Zinc-carbon Invented by Georges Leclanche in 1866 E° = 1.5 V Anode:Zn(s)  Zn 2+ (aq) + 2e - Cathode2NH 4 + (aq) + 2e -  2NH 3 (g) + H 2 (g) To keep gas from building up: Zn 2+ (aq) + 2NH 3 (g) + 2Cl - (aq)  Zn(NH 3 ) 2 Cl 2 (s) 2MnO 2 (s) + H 2 (g)  Mn 2 O 3 (s) + H 2 O (l) Short shelf life zinc since the reaction slowly continues even if battery not being used

Primary Batteries (non-rechargeable) Alkaline battery E° = 1.54 V AnodeZn(s) + 2OH -  ZnO (aq) + H 2 O + 2e - CathodeMnO 2 (aq) + H 2 O + e -  MnO(OH) (s) + OH - Longer shelf life No gas formation Higher concentrations so last longer than dry cell

Secondary Batteries (rechargeable) Lead battery E° = 2 V Cathode: PbO 2 (s) + 4H + + SO e -  PbSO 4 (s) + 2H 2 O Anode: Pb(s) + SO 4 2-  PbSO 4 (s) + 2e - Put 6 in series to get 12 V

Fuel Cell (Proton Exchange Membrane – PEM) E° = 0.7 V Anode:H 2 (g)  2H + + 2e - Cathode:O 2 (g) + 2H 2 O + 4e -  4OH - Net: O 2 (g) + 2H 2 (g)  2H 2 O Big difference between fuel cell and battery is that the reactants can be replenished with fuel cell