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Electrochemistry - PowerPoint PPT Presentation


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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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electricity
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
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-

Voltaic cell
electrical current
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
batteries
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
batteries1
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
batteries2
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
rechargeable batteries
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!
rechargeable batteries1
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
rechargeable batteries2
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
fuel cell
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
electrolysis
Electrolysis
  • Electrical current used to drive nonspont redox rxn
  • In electrolytic cell
  • Electrolysis of water
  • Metal plating: silver coated on metal, jewelry, etc.