Chapter 8 (CIC) and Chapter 20 (CTCS) Read in CTCS Chapter 20.4,5 Problems in CTCS: 20.21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43a, 45a

Electrochemical Cell Converts a spontaneous chemical reaction into electrical energy Typically 90% efficient as opposed to the 40% efficiency found in the combustion of fossil fuels Calculator battery (NiCad cell) Cd (s) + 2OH - (aq)  Cd(OH) 2 (s) + 2e - NiOOH (s) + H 2 O (l) + e -  Ni(OH) 2 (s) + OH - (aq) NiOOH (s) + H 2 O (l) + Cd (s)  Ni(OH) 2 (s) + Cd(OH) 2 (s)

Why do Cells Work? Approximately Volts or emf, (electromotive force), or cell potential 1 V = J/C A spontaneous reaction always has a (+) emf value Q: What sign did ΔH typically have for a spontaneous reaction?

What Was Seen in Lab? 2 Ag + (aq) + Cu (s)  2 Ag (s) + Cu 2+ (aq) Eº cell = 0.46 V Standard State (1 M for solutions) Standard Hydrogen Electrode 2 H + (aq) + 2 e -  H 2 (g) (1 atm) Eº red  0.00 V Can’t have just a reduction reaction! 2 H + (aq) + Cu (s)  H 2 (g) + Cu 2+ (aq) Eº cell = V Not spontaneous! H 2 (g) + Cu 2+ (aq)  2 H + (aq) + Cu (s) Eº cell = 0.34 V

What did you find for the reaction of H + with Zn? Pb? Zn  Zn e - Eº ox = 0.76 V Pb  Pb e - Eº ox = V See Table 20.1 or Appendix E for Eº red Q: Calculate the standard emf for Cu 2+ reacting with iodide to yield the elements. Describe what happens at the anode? What is the oxidizing agent?

Q: Consider the acidification of MnO 4 - to Mn 2+ and oxygen gas. What is the standard emf and is the reaction spontaneous?

Spontaneity A spontaneous reaction has Eº + A nonspontaneous reaction has Eº - ΔGº = -nFEº –ΔG is Gibbs Free Energy –n is the number of e - in the balanced reaction – F is the Faraday constant (96,500 C/mol or 96,500 J/V mol) ΔGº = ΔHº -TΔSº –T is the temperature (Kelvin) –ΔS is the entropy Clearly, ΔH is not the whole story to spontaneity!