Lecture 25 3/28/07

Secondary Batteries (rechargeable) NiCad E° = 1.229 V Cathode: 2NiO(OH) (s) + 2H2O + 2e-  2Ni(OH)2 (s) + 2OH- Anode: Cd(s) + 2OH- -  Cd(OH)2 (s) + 2e-

Secondary Batteries (rechargeable) NiMH E° = 1.4 V Cathode: NiO(OH) (s) + H2O + e-  Ni(OH)2 (s) + OH- Anode: MH(s) + OH- -  M (s) + H2O + e-

H2 (g) + 2 AgCl (s)  2H+ (aq) + 2 Cl- (aq) + 2Ag(s) This reaction occurs in a cell with H2 (g) pressure of 1.0 atm and [Cl-] = 1.0 M at 25˚C; the measured Ecell = 0.34 V. What is the pH of the solution? E˚(cathode) = 0.222V

Sn(s) + Pb2+ (aq)  Sn2+ (aq) + Pb(s) Ecell = 0.01V for a galvanic cell with this reaction at 25˚C. Sn(s) + Pb2+ (aq)  Sn2+ (aq) + Pb(s) What is the equilibrium constant for the reaction?

Electrolysis of molten salts Cathode: 2Na+ (l) + 2e-  2Na(l) E° = -2.71 V Anode: 2Cl- (l)  Cl2 (g) + 2e- E° = 1.36 V Net: 2NaCl (l)  Cl2 (g) + 2Na(l) E° =

Electrolysis of aqueous solutions What would happen if you put electricity into a solution of KI? Possible oxidation reactions? Possible reduction reactions? Actual Reaction?

General rules in aqueous solutions Reduction 6H2O + 2e-  H2 (g) + 2OH- E° = -0.8277 V Oxidation 6H2O  O2 (g) + 4H3O+ + 4e- E° = 1.229 V

Counting electrons

1.50 amps flow through a Ag+(aq) solution for 15.0 min. What mass of Ag metal is deposited?

Pb(s) + HSO4-(aq)  PbSO4(s) + H+(aq) + 2e- The anode reaction in a lead storage battery is: Pb(s) + HSO4-(aq)  PbSO4(s) + H+(aq) + 2e- If a battery delivers 1.50 amp, and you have 454 g of Pb, how long will the battery last?

Recap Balancing Redox reactions Electrochemical cells Batteries Standard Reduction Potential table Corrosion Nernst equation Electrolysis Molten aqueous Counting electrons

Acid Mine Drainage REDOX in the real world FeS2 (pyrite) + O2  SO42- + Fe2+

www.state.sd.us/denr/DES/mining/P000106.jpg http://www.nelpi.org/tarcreek/slides/5s.html

2Ag+ (aq) + Hg (l) ↔ 2Ag(s) + Hg2+ (aq) Example Calculate the equilibrium constant for: 2Ag+ (aq) + Hg (l) ↔ 2Ag(s) + Hg2+ (aq) E° (V) Hg2+ (aq) + 2e-  Hg (l) 0.855 Ag+ (aq) + e-  Ag(s) 0.7994

Primary Batteries (non-rechargeable) Oxyride battery E° = 1.7 V Cathode: NiOOH + H2O + e-  Ni(OH)2 + OH- (1) Anode: MnO2 (aq) + H2O + e-  MnO(OH) (s) + OH- New battery Vacuum pouring technology Should last 2X longer than alkaline battery

Primary Batteries (non-rechargeable) Oxyride battery Not responsible for this one E° = 1.7 V Cathode: NiOOH, MnO2 Anode: Zinc New battery Vacuum pouring technology Should last 2X longer than alkaline battery