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Voltaic Cells

voltaic (galvanic) cells - electrons transferred through external pathway instead of directly

  • oxidation occurs at anode
  • reduction occurs at cathode
  • half-cells must stay neutral >> cations migrate over to cathode, anions migrate over to anode across salt bridge (won’t react w/ other ions)
  • electrons always flow towards anode

cell EMF (electromotive force) - aka cell potential/voltage

  • Ecell = Ecathode - Eanode
    • positive value >> spontaneous
  • standard hydrogen electrode (SHE) - has reduction potential of 0V
    • used to measure voltage of half-reactions
  • intensive property >> changing coefficients in reaction won’t change value
  • E more positive >> greater tendency to reduce (at cathode)
    • strongest oxidizing agent >> most easily reduced
    • strongest reducing agent >> most easily oxidized
  • G° = -nF(E°)

For the following batteries, determine the anode/cathode and voltage  

  • remember, more positive reduction potential means cathode, more negative potential means anode
  • Li-Cd battery
    • Li reduction potential = -3.05 (anode)
    • Cd reduction potential = -0.40 (cathode)
    • voltage = -0.40 - (-3.05) = 2.65
  • Al-Cd battery
    • Al reduction potential = -1.66
    • Cd reduction potential = -0.40
    • voltage = -0.40 - (-1.66) = 1.26

effect of concentration on cell EMF - depends on Nernst equation

  • E = E° - RT / nF lnQ
    • = E° - 2.303RT / nF logQ
    • = E° - 0.0592 V / n logQ at 298K
  • reactant concentration increase >> emf increase
  • product concentration increase >> emf decrease
Subject: 
Chemistry
Subject X2: 
Chemistry

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