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Electrochemistry

Electrochemistry. Electricity. Movt of electrons Movt of electrons through wire connecting 2 half-reactions  electrochemical cell Also called voltaic or galvanic cell Cell produces current from spont rxn Ex: copper in soln of AgNO 3 is spont

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Electrochemistry

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  1. Electrochemistry

  2. Electricity • Movt of electrons • Movt of electrons through wire connecting 2 half-reactions  electrochemical cell • Also called voltaic or galvanic cell • Cell produces current from spont rxn • Ex: copper in soln of AgNO3 is spont • Electrolytic celluses electrical current to drive a non-spont chemical rxn

  3. Solid Zn in zinc ion soln = half-cell Likewise, Cu/Cu ion soln Wire attached to each solid Salt bridge = 1. contains electrolytes, 2. connects 2 half-cells, 3. anions flow to neutralize accumulated cations at anode and vice-versa (completes circuit) “An Ox” = anode oxidation Has negative charge cuz releases e- “Red Cat” = reduction cathode Has positive charge cuz takes up e- Voltaic cell

  4. Electrical current • Measured in amperes (A) • 1 A = 6.242 x 1018 e-/s • Electric current driven by diff in potential E/unit of charge • Pot diff (electromotive force or emf) = volt (V) • 1 V = J/C

  5. Batteries • Dry-cell batteries • Don’t contain large amts of water • Duracell • Anode: Zn oxidized • Cathode: 2MnO2(s) + 2NH4+(aq) + 2e- Mn2O3(s) + 2NH3(g) + H2O(l) • Cathode is Carbon-rod immersed in moist (acidic) paste of MnO2 that houses NH4Cl • 1.5 V

  6. Batteries • More common dry-cell type: alkaline battery • Anode: Zn(s) + 2OH-(aq) Zn(OH)2(s) + 2e- • Cathode: • 2MnO2(s) + 2H2O(l) + 2e-2MnO(OH)(s) + 2OH-(aq) • Longer shelf-life, “live” longer • Cathode in basic paste

  7. Batteries • Lead-acid storage batteries • In cars • 6 electrochemical cells (2V) in series • Anode: Pb(s) + HSO4-(aq) PbSO4(s) + H+(aq) + 2e- • Cathode: • PbO2(s) + HSO4-(aq) + 3H+(aq) + 2e-  PbSO4(s) + 2H2O(l) • In 30% soln of sulfuric acid • If dead, due to excess PbSO4 covering electrode surfaces • Re-charge (reverse rxn)  converts PbSO4 to Pb and PbO2

  8. Rechargeable batteries • Ni-Cd • Anode: Cd(s) + 2OH-(aq) Cd(OH)2(s) + 2e- • Cathode: • 2NiO(OH)(s) + 2H2O(l) + 2e-  2Ni(OH)2(s) + 2OH-(aq) • KOH, usually, used • 1.30 V • Reverse rxn recharges battery • Excess recharging  electrolysis of water • EXPLOSION!!! • Muhahahaha!

  9. Rechargeable batteries • Cd toxic • Ni-MH • Hybrid car batteries: high energy density • Same cathode rxn • Anode: MH(s) + OH-(aq) M(s) + H2O(l) + e- • Commonly, M = AB5, where A is rare earth mixture of La, Ce, Nd, Pr, and B is Ni, Co, Mn, and/or Mn • Very few use AB2, where A = Ti and/or V

  10. Rechargeable batteries • Anode made of graphite w/incorporated Li-ions btwn carbon layers • Ions spontaneously migrate to cathode • Cathode = LiCoO2 or LiMn2O4 • Transition metal reduced • Used in laptop computers, cell phones, digital cameras • Light weight and high E density

  11. Fuel cell • Reactants flow through battery • Undergo redox rxn • Generate electricity • Hydrogen-oxygen fuel cell • Anode: 2H2(g) + 4OH-(aq) 4H2O(l) + 4e- • Cathode: O2(g) + 2H2O(l) + 4e-  4OH-(aq) • Used in space-shuttle program

  12. Electrolysis • Electrical current used to drive nonspont redox rxn • In electrolytic cell • Electrolysis of water • Metal plating: silver coated on metal, jewelry, etc.

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