The Different Products of The Electrolysis of Aqueous And Molten Sodium Chloride

Industrial Chemistry > 4. The industrial production of sodium hydroxide requires the use of electrolysis > The Different Products of The Electrolysis of Aqueous And Molten Sodium Chloride >

Analyse information from secondary sources to predict and explain the different products of the electrolysis of aqueous and molten sodium chloride

ELECTROLYSIS OF MOLTEN NaCl

  • only sodium and chloride ions present
  • the products formed are chlorine gas and sodium metal
  • only possible reaction at the anode is the oxidation of chloride ions to form chlorine gas:

2Cl– (l) à Cl2 (g) + 2e–

  • only possible reaction at the cathode is the reduction of sodium ions to form sodium metal:

2Na+ (l) + 2e–à 2Na (l)

Full Equation:

2NaCl (l) à Cl2 (g) + 2Na (l)

ELECTROLYSIS OF AQUEOUS NaCl

    • complication is resulted by the presence of water
    • possible anode reactions are oxidation of chloride ions to form chlorine gas and oxidation of water molecules to form H+ and oxygen gas
      • If oxidation of chloride ions happened at anode:

2Cl– (l) à Cl2 (g) + 2e–                                                E = -1.36 V

      • If oxidation of water molecules happened at anode:

2H2O (l)à O2 (g) + 4H+ (aq) + 4e–        E = -1.23 V

    • possible cathode reactions are reduction of sodium ions to form sodium metaland reduction of water molecules to form hydroxide and hydrogen gas
      • If reduction of sodium ions happened at cathode:

Na+ (aq) + e–à Na (s)                                                 E = -2.71 V

      • If reduction of water molecules happened at cathode:

2H2O (l) + 2e–à 2OH– (aq) + H2 (g)                        E = -0.83 V

  • the products formed are hydroxide ions and chlorine and hydrogen gases

Reduction Process

  • The reaction with higher EMF is most likely to proceed as it will lead to more spontaneity

Oxidation Process

  • theoretically, oxidation of water is expected to happen at the anode given that it is more spontaneous than the oxidation of chloride ions
  • however, oxidation of water molecules happens only when a dilute solution of NaCl undergoes electrolysis
  • in electrolytic process, the voltage required for some reactions is considerably higher than what the electrode potential indicates
    • Overvoltage – difference between the electrode potential and the actual voltage required to cause electrolysis
  • the overvoltage for O2 formation is quite high which allows formation of Cl2 for electrolysis of concentrated NaCl
  • in addition, the conditions are different to those in which standard reduction potentials are measured,g. concentrations of Cl– ions is high, not 1 molL-1

Full Equation:

2NaCl (aq) + 2H2O (l) à 2NaOH (aq) + Cl2 (g) + H2 (g)