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