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Chapter 8 (CIC) and Chapter 20 (CTCS)

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

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Chapter 8 (CIC) and Chapter 20 (CTCS)

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  1. 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

  2. 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) + H2O(l) + e-  Ni(OH)2 (s) + OH-(aq) NiOOH(s) + H2O(l) + Cd(s) Ni(OH)2(s) + Cd(OH)2(s)

  3. 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?

  4. What Was Seen in Lab? 2 Ag+(aq) + Cu(s)  2 Ag(s) + Cu2+(aq) Eºcell = 0.46 V • Standard State (1 M for solutions) • Standard Hydrogen Electrode 2 H+(aq) + 2 e-  H2(g) (1 atm) Eºred  0.00 V • Can’t have just a reduction reaction! 2 H+(aq) + Cu(s)  H2(g) + Cu2+(aq) Eºcell = -0.34 V • Not spontaneous! H2(g) + Cu2+(aq)  2 H+(aq) + Cu(s) Eºcell = 0.34 V

  5. What did you find for the reaction of H+ with Zn? Pb? Zn  Zn2+ + 2 e- Eºox = 0.76 V Pb  Pb2+ + 2 e- Eºox = 0.126 V • See Table 20.1 or Appendix E for Eºred Q: Calculate the standard emf for Cu2+ reacting with iodide to yield the elements. Describe what happens at the anode? What is the oxidizing agent?

  6. Q: Consider the acidification of MnO4- to Mn2+ and oxygen gas. What is the standard emf and is the reaction spontaneous?

  7. 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!

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