CHEM 132, Chapter 17

• Electrochemistry


  The interconversion of electrical and chemical energy
• Electrochemical processes


  1. Oxidation-reduction reactions

  2. Energy released by a spontaneous reaction is converted to electricity

  3. Electrical energy is used to cause a non spontaneous reaction to occur
• Oxidation number


  • Free elements have an oxidation number of zero
  • In monotomic ions, the oxidation number is equal to the charge on the ion
  • The oxidation number of oxygen is usually -2
  • The oxidation number of hydrogen is +1 except when its bonded to metals in binary compounds (then it is -1)
• Oxidation


  The loss of electrons
• Reduction


  The gain of electrons
• Reducing agent


  An electron donor (reactant is oxidized)
• Oxidizing agent


  An electron acceptor (reactant that is reduced)
• Conversion of chemical energy to electrical energy

  Through redox reactions
• Spontaneous redox reactions


  Produce a current that can be used to do work
• Useful work


  Only done when reactants are separated, which forces electron transfer through a wire
• Oxidation


  Occurs at the anode
• Reduction


  Occurs at the cathode
• Cathode and anode


  Connected by conductive wire
• Circuit


  Closed with salt bridge, a tube filled with strong electrolyte in a matrix
• Cell potential


  The energy difference between half reactions (aka electromotive force)
• Galvanic cell


  Spontaneous (Enot cell >0)
• Line notation


  Shorthand to describe electrochemical cells
• Concentrations of aqueous solutions should be specified in notation
• Work


  • Done when electrons are transferred through a wire with a potential difference between the ends
  • Work flowing out of system is negative
  • Cell potential and work have opposite signs
• Concentration cells


  • Cells where both compartments have the same components but different concentrations
  • Difference in concentration is the only factor that produces voltage
  • Voltages are usually small