16A1.Redox equlibria

• Oxidation


  Loss of electrons
• Reduction


  Gain of electrons
• Oxidation number


  Increases when an element is oxidised
• Reduction


  Decreases when an element is reduced
• The terms 'standard electrode potential' and 'standard redox potential' mean the same thing
• The reactions involved in the measurement of a standard electrode potential are redox reactions
• Standard electrode potentials are sometimes referred to as standard redox potentials and can be listed as an electrochemical series
• Standard conditions for measuring standard electrode potentials

  • 298 K temperature
  • 100 kPa pressure of gases
  • 1.00 mol dm³ concentration of ions
• Standard hydrogen electrode


  Reference electrode consisting of hydrogen gas at 100 kPa bubbling over platinum foil in 1 mol dm³ hydrochloric acid at 298 K
• Measuring a standard electrode potential

  1. Connect standard hydrogen electrode to metal electrode

  2. Use high-resistance voltmeter to measure potential difference

  3. Sign convention: negative sign indicates metal electrode is negative compared to hydrogen electrode
• Standard electrode potential of standard hydrogen electrode is defined as 0.00 V
• Some standard electrode potentials

  • Ag+(aq) + e- = Ag(s), E° = +0.80 V
  • Al3+(aq) + 3e- = Al(s), E° = -1.66 V
  • Cr3+(aq) + 3e- = Cr(s), E° = -0.74 V
  • Fe2+(aq) + 2e- = Fe(s), E° = -0.44 V
• Standard electrode potential is a 'sign invariant quantity', meaning the sign remains the same regardless of how the half-cell reaction is represented
• Negative sign (-)


  Indicates the metal electrode is negative with respect to the hydrogen electrode