23.4 Electrode potentials

• A half-cell contains the chemical species present in a redox half-equation
• The equilibrium in a half-cell is written so that the forward reaction shows reduction and the reverse reaction shows oxidation
• An ion half-cell is a solution containing ions of the same element in different oxidation states

  An inert platinum electrode is used to transport electrons
• Electrode potential is the tendency for electrons to be gained and for reduction to take place
• The standard electrode potential is the e.m.f of a half-cell connected to a standard hydrogen half-cell under standard conditions of 298K, 1 mol dm-3 and 100kPa
• To measure a standard electrode potential, the half-cell is connected to a standard hydrogen electrode

  • the two electrodes are connected by a wire to allow a controlled flow of electrons through a voltmeter
  • solutions area connected by a salt bridge allowing the flow of ions
  • salt bridge contains a concentrated solution of an electrolyte that does not react with either solution
• The more negative the standard electrode potential value

  • the greater the tendency to lose electrons and undergo oxidation
  • the less the tendency to gain electrons and undergo reduction
• Electrons flow along the wire from the more negative half-cell to the less negative half-cell
• The equation of the negative electrode is reversed and added to the other equation for the overall cell reaction
• The standard electrode potential can be calculated by positive value - more negative value