Electrode potentials

• A voltaic cell is a type of electrochemical cell which converts chemical energy into electrical energy
• the conversion of chemical energy takes place in the modern cells and batteries that power such devices as mobile phones
• to make a voltaic cell:

  • as electrical energy results from movement of electrons
  • you need chemical reactions that transfer electrons from one species to another
  • redox reactions
• A half-cell contains the chemical species present in a redox half-equation
• a voltaic cell can be made by connecting together two different half-cells, which then allows electrons to flow. In the cell, the chemicals in the two half-cells must be kept apart - if allowed to mix, electrons would flow in an uncontrolled way and heat energy would be released rather than chemical energy
• the simplest half-cell consists of a metal rod dipped into a solution of its aqueous metal ion - this is represented using a vertical line for the phase boundary between the aqueous solution and the metal

  • Zn2+ (aq)/ZN (s)
  • Cu2+ (aq)/ Cu (s)
• at the phase boundary where the metal is in contact with its ions, an equilibrium will be set up. by convention, the equilibrium in a half-cell is written so that the forward reaction shows reduction and the reverse reaction shows oxidation
• 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
• an ion/ion half-cell contains ions of the same element in different oxidation states e.g. a half-cell can be made containing a mixture of aqueous iron (ii) and iron (iii) ions

  redox equilibrium is:

  Fe3+ (aq) +e- (reversible arrows) Fe2+ (aq)
• in an ion/ion half-cell there is no metal to transport electrons either into or out of the half-cell, so an inert metal electrode made of platinum is used
• How do you know which electrode has a greater tendency to gain or lose electrons?

  in a cell with 2 metal/metal ion half-cells connected, the more reactive metal releases electrons more readily and is oxidised
• in an operating cell:

  • the electrode with the more reactive metal loses electrons and is oxidised - this is the negative electrode
  • the electrode with the less reactive metal gains electrodes and is reduced -this is the positive electrode
• the tendency to be reduced and gain electrons is measured as a standard electrode potential
• the standard chosen is a half-cell containing hydrogen gas, H2 and a solution containing H+ (aq) ions

  an inert platinum electrode is used to allow electrons into and out of the half-cell
• by definition, the standard conditions used are:

  • solutions have a concentration of exactly 1moldm-3
  • temp = 298K
  • pressure is 100kPa (1 bar)
• the standard electrode potential is the e.m.f of a half-cell connected to a standard hydrogen half-cell under std conditions of 298K, solution concentrations of 1moldm-3 and pressure 100kPa
• by definition, the std electrode potential of a std hydrogen electrode is exactly 0V
• the sign of a standard electrode potential shows the sign of the half cell connected to the standard hydrogen electrode and shows the relative tendency to gain electrons compared with the hydrogen cell