The Foundations of Chemistry

ELECTROLYTIC CELLS

In some electrochemical cells nonspontaneouschemical reactions are forced to occur by the
input of electrical energy. This process is called electrolysis.An electrolytic cell consists
of a container for the reaction material with electrodes immersed in the reaction mate-
rial and connected to a source of direct current. Inert electrodes are often used so that
they do not react.

THE ELECTROLYSIS OF MOLTEN SODIUM CHLORIDE

(THE DOWNS CELL)

Solid sodium chloride does not conduct electricity. Its ions vibrate about fixed positions,
but they are not free to move throughout the crystal. Molten (melted) NaCl, however, is
an excellent conductor because its ions are freely mobile. Consider a cell in which a source
of direct current is connected by wires to two inert graphite electrodes (Figure 21-2a).
They are immersed in a container of molten sodium chloride. When the current flows,
we observe the following.

1.A pale green gas, which is chlorine, Cl 2 , is liberated at one electrode.

2.Molten, silvery white metallic sodium, Na, forms at the other electrode and floats

on top of the molten sodium chloride.

21-3

21-3 The Electrolysis of Molten Sodium Chloride (The Downs Cell) 851

Battery

Electrons – +

Ammeter

Voltmeter

Molten NaCl

consists of Na+

and Cl–

Graphite

electrode

(inert)

Graphite electrode

(inert)

Na+ is reduced

(Na+ + le– → Na);

therefore, cathode

Cl– is oxidized

(2Cl– → Cl 2 + 2e–);

therefore, anode

Na(
)

Molten

NaCl Cl 2 (g)

Ring-shaped

iron cathode

Graphite

anode

Iron screen

(a)

(b)

Figure 21-2 (a) Apparatus for electrolysis of molten sodium chloride. (b) The Downs cell,
the apparatus in which molten sodium chloride is commercially electrolyzed to produce
sodium metal and chlorine gas. The liquid Na floats on the denser molten NaCl.

Lysismeans “splitting apart.” In many

electrolytic cells compounds are split

into their constituent elements.

MoltenNaCl, melting point 801°C, is a

clear, colorless liquid that looks like

water.

The metal remains liquid because its

melting point is only 97.8°C. It floats

because it is less dense than the molten

NaCl.