1.11 Physical Chemistry (Redox Equilibria)

• Electrochemical cells


  • A cell has two half–cells
  • The two half cells have to be connected with a salt bridge
  • Simple half cells will consist of a metal (acts an electrode) and a solution of a compound containing that metal (eg Cu and CuSO4)
  • These two half cells will produce a small voltage if connected into a circuit (i.e. become a Battery or cell)
• Salt bridge


  • The salt bridge is used to connect up the circuit
  • The free moving ions conduct the charge
  • A salt bridge is usually made from a piece of filter paper (or material) soaked in a salt solution, usually potassium nitrate
  • The salt should be unreactive with the electrodes and electrode solutions
• Why does a voltage form?
• Why use a high resistance voltmeter?

  • The voltmeter needs to be of very high resistance to stop the current from flowing in the circuit
  • In this state it is possible to measure the maximum possible potential difference (E)
  • The reactions will not be occurring because the very high resistance voltmeter stops the current from flowing
• What happens if current is allowed to flow?
• Cell Diagrams


  • Electrochemical cells can be represented by a cell diagram
  • The solid vertical line represents the boundary between phases e.g. solid (electrode) and solution (electrolyte)
  • The double line represents the salt bridge between the two half cells
  • The voltage produced is indicated
  • The more positive half cell is written on the right if possible (but this is not essential)
• Systems that do not include metals

  • If a system does not include a metal that can act as an electrode, then a platinum electrode must be used and included in the cell diagram
  • A platinum electrode is used because it is unreactive and can conduct electricity
• Measuring the electrode potential of a cell
• The Standard Hydrogen Electrode


  • The potential of all electrodes are measured by comparing their potential to that of the standard hydrogen electrode
  • The standard hydrogen electrode (SHE) is assigned the potential of 0 volts
  • The hydrogen electrode equilibrium is: H2 (g) 2H+ (aq) + 2e-
  • To make the electrode a standard reference electrode some conditions apply: 1. Hydrogen gas at pressure of 100kPa, 2. Solution containing the hydrogen ion at 1.0 mol dm-3, 3. Temperature at 298K, 4. Platinum electrode
• Secondary standards


  • The standard hydrogen electrode is difficult to use, so often a different standard is used which is easier to use
  • These other standards are themselves calibrated against the SHE
  • The common ones are: silver / silver chloride, calomel electrode
• Standard Electrode Potentials


  • The standard conditions are: all ion solutions at 1 mol dm-3, temperature 298 K, gases at 100 kPa pressure, no current flowing
  • When an electrode system is connected to the hydrogen electrode system, and standard conditions apply the potential difference measured is called the standard electrode potential, E
  • Standard electrode potentials are found in data books and are quoted as: Li+(aq) | Li (s) E= -3.03V, or as half equations: Li+ (aq) + e- Li (s) E= -3.03V
• Calculating the EMF of a cell
• Using series of standard electrode potentials
• O2(g) + 4H+(aq) + 4e– → 2H2O(I) Eo+1.23V


  • F2(g) + 2e– → 2F–(aq) Eo +2.87V
• Fluorine reacts with water

  Fluorine is a stronger oxidising agent than oxygen
• Cl2(aq) + 2e– → 2Cl–(aq) Eo+1.36V

  • 2HOCl(aq) + 2H+(aq) + 2e– → Cl2(aq) + 2H2O(I) Eo+1.64V
• Chlorine should undergo a redox reaction with water


  Chlorine is a stronger oxidising agent than water
• H2O2(aq) + 2H+(aq) + 2e– → 2H2O(I) Eo +1.77V


  Hydrogen peroxide is reduced, with the oxidation state of oxygen decreasing from +1 to -2
• Oxidation number


  The charge on an atom in a compound
• Calculating Ecell from standard electrode potentials

  Ecell = Ered - Eox
• Eo


  Standard electrode potential
• Fluorine reacts with water


  Fluorine has a higher oxidation number than oxygen in water
• Chlorine should undergo a redox reaction with water


  Chlorine has a higher oxidation number than oxygen in water
• Hydrogen gas is bubbled into a solution containing a mixture of iron(II) and iron(III) ions


  Fe3+ will be reduced to Fe2+ by H2 oxidising to H+
• Ecell is a measure of how far from equilibrium the cell reaction lies
• The more positive the Ecell the more likely the reaction is to occur
• If current is allowed to flow, the cell reaction will occur and the Ecell will fall to zero as the reaction proceeds and the reactant concentrations drop