chapter 23

• electricity


  flow of electrons, in redox reactions we can control the flow of electrons between chemical species, this is the basis of cells and batteries
• voltaic cell


  converts chemical energy into electrical energy
• half cell


  contains the chemical species present in a redox half equation
• a voltaic cell can be made by connecting together 2 half cells, which then allows electrons to flow, in the cell the chemicals in the 2 half cells must be kept apart so electrons don't flow in an uncontrolled way and electrical energy is still released
• to form a circuit and measure the potential using a voltmeter, half cells must be connected by a salt bridge, a wire cannot be used to complete the circuit because that would create another half cell

  
• metal / metal ion half cells

  simplest half cell consists of a metal rod dipped into solution of its aqueous metal ion, at the phase boundary, where metal is in contact with its ions, an equilibrium is set up and by convection, the equilibrium will show the forward reaction being reduced and the backwards reaction being oxidised

  
• in an isolated half cell, there is no net transfer of electrons either into or out of the metal, when 2 half cells are connected, the direction of electron flow depends on the relative tendency of each electrode to release electrons
• ion half cells

  contain ions of the same element in different oxidation states e.g a half cell can be made from a mix of aqueous iron 2 and iron 3 ions, here there is no metal to transport electrons either in or out, so an inert metal electrode made out of platinum is used

  
• electrode potentials


  in an operating cell:

  • electrode with more reactive metal loses electrons and is oxidised = negative electrode
  • electrode with the less reactive metal gains electrons and is reduced = positive electrode
• standard electrode potential


  standard chosen is a half cell containing H2 gas and a solution containing H+ ions, an inert platinum electrode is used to allow electrons in and out of the half cell

  
• standard electrode potential conditions

  • solutions have to have a concentration of exactly 1 moldm-3
  • temperature is 298K / 25C
  • pressure is 100 kPa / 1 bar