Electrochemistry

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Electrochemistry
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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 cell uses electrical current to
  drive a non-spont chemical rxn

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Voltaic cell

• 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-

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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

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Batteries

• Dry-cell batteries
• Dont 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

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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

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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

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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!

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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

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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

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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

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Electrolysis

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