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Electrolysis of Molten, or Liquid, Salts Tutorial

Key Concepts

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Electrolytic Cell for the Electrolysis of a Molten Salt

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

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.

Electrolytic Cell Reactions
anode:
(oxidation)
X- X + e-   E1
cathode:
(reduction)
M+ + e- M   E2
 
cell:
(REDOX)
X- + M+ X + M   Ecell = E1 + E2
  Ecell = -V
  Ecell is negative

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

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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) :

solid heat
liquid
NaCl(s) heat
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:

Na+

← Cl-

Note:

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:

Electrolytic Cell Reactions
anode:
(oxidation)
Cl- ½Cl2(g) + e-
cathode:
(reduction)
Na+ + e- Na
 
cell:
(REDOX)
Cl- + Na+ ½Cl2(g) + Na

Bubbles of Cl2(g) would be produced at the anode.
Droplets of Na(l) would be produced at the cathode.

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1. The species are NOT present in their standard states so that the electrode potentials (E) are NOT standard electrode potentials (Eo).