Electrolysis of Molten, or Liquid, Salts Chemistry Tutorial

Key Concepts

• Melting a salt enables it to conduct electricity.

  non-conductor	→	conductor
  MX(s)	heat →	M+(l) + X-(l)

• The use of inert electrodes, electrodes made of a material that will not take part in the reactions, means the only species present that can take part in the electrolytic cell reactions are the anions and cations of which the salt is composed.
• During the electrolysis of a molten salt:

  (a) Oxidation occurs at the anode (positive electrode).
  (Oxidation, loss of electrons, occurs at the anode)

  (b) Anions (negatively charged ions) migrate to the positive anode (anode).

  (c) Reduction occurs at the cathode (negative electrode).
  (Reduction, gain of electrons, occurs at the cathode)

  (d) Cations (positively charged ions) migrate to the negative cathode (cathode).

• Reactions occurring during electrolysis of a molten, or liquid, salt (MX_(l)) are:

  oxidation at anode (positive electrode)	X-(l)	→	X + e-
  reduction at cathode (negative electrode)	e- + M+(l)	→	M

• Electrolysis is a non-spontaneous reaction:

  E_cell for the electrolytic cell is negative¹.

• Applied emf must be greater than the emf for the cell, ie greater than -E_cell.
• Mass of substance produced electrolytically is proportional to the quantity of electricity flowing.

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Electrolytic Cell for the Electrolysis of a Molten Salt

For the electrolysis of molten salt, MX_(l) using inert electrodes:

• The electrolyte is MX_(l).
• M⁺ are the cations which will be reduced at the cathode.
• X^- are the anions which will be oxidized at the anode.

NOTE: Electrons (e-) flow from anode (positive electrode) to cathode (negative electrode) Anions (negative ions) flow to the anode. Cations (positive ions) flow to the cathode.

EMF (volts) required to drive this non-spontaneous reaction is greater than E_cell for the spontaneous redox reaction.
Required voltage (EMF) > -E_{(electrolytic cell)}

Worked Example : Electrolysis of NaCl_(l)

Sodium chloride, NaCl, is a salt composed of sodium ions, Na⁺, and chloride ions, Cl^-.

Sodium chloride, NaCl, can not conduct electricity when it is present as a solid, NaCl_(s), because the ions are locked into a regular three dimensional lattice are are not free to move.

When heat is applied to the solid sodium chloride, NaCl_(s), it will melt to form liquid sodium chloride, NaCl_(l) :

In the liquid state, the sodium ions (Na⁺_(l)) and chloride ions (Cl^-_(l)) are free to move (they are said to be be mobile), so molten, or liquid, sodium chloride can conduct electricity.

This means it is possible to construct an electrolytic cell using molten sodium chloride as shown in the diagram below:

Note:

• The electrodes must be inert electrodes so that there are no competing reactions in the electrolytic cell.
• Electrons flow from anode (positive electrode) to cathode (negative electrode).
• Anions, Cl^-, flow towards the anode.
• Cations, Na⁺, flow towards the cathode.

Remember, oxidation occurs at the anode and reduction occurs at the anode, so we can write the half-equations for the reactions at the anode and cathode, and therefore the overall redox equation for the redox reaction occurring in this electrolytic cell:

Bubbles of Cl_2(g) would be produced at the anode.
Droplets of Na_(l) would be produced at the cathode.