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Electrochemistry II: Electrolytic Cells 97

TYPES OF ELECTROLYTIC CELLS

Electrolysis of Molten NaCl
1. Determine whether or not the electrolysis involves (a) a molten
(melted) or (b) an aqueous substance. In this example, the
electrolyte is molten sodium chloride.
2. Identify all species present. If the electrolysis involves a molten
substance, this step is easy, since the species present are simply
the ions of the substance, i.e. water is not involved. Molten
sodium chloride contains equal numbers of sodium Naf cations
and chloride C1- anions, respectively i.e.
NaCl(1) + Na+(l) + Cl-(,)
3. Having identified all species present, determine which species are
attracted to the cathode, and which species are attracted to the
anode, remembering that the cathode is negative and the anode is
positive (‘CNAP’), and also that like charges repel each other
and unlike charges attract one another.
Cathode -ve Na+ Anode +ve C1-
4. Determine the two respective half-reactions, recalling that reduc-
tion takes place at the cathode and oxidation takes place at the
anode (‘CROA’).

Cathode reaction: Na+(l) + e --+ Nao(l)
Anode reaction: Cl-(,) -+ Cl,,,, + e

In the electrolysis of molten sodium chloride, a pool of sodium
metal deposits at the cathode, and bubbles of chlorine gas form
at the anode.
5. Draw the cell (Figure 7.4). The porous separator permits the
diffusion of ions from one side of the cell to the other, and
prevents the sodium produced at the cathode reacting with the
chlorine produced at the anode. The experimental set-up above is
used commercially in the Downs cell for the electrolysis of molten
sodium chloride.