From these observations we can deduce the processes of the cell. Chlorine must be
produced by oxidation of Clions, and the electrode at which this happens must be the
anode. Metallic sodium is produced by reduction of Naions at the cathode, where elec-
trons are being forced into the cell.2Cl88nCl 2 (g) 2 e (oxidation, anode half-reaction)
2[Nae88nNa()] (reduction, cathode half-reaction)
2Na2Cl88n2Na()Cl 2 (g) (overall cell reaction)2NaCl()The formation of metallic Na and gaseous Cl 2 from NaCl is nonspontaneousexcept at
temperatures very much higher than 801°C. The direct current (dc) source must supply
electrical energy to force this reaction to occur. Electrons are used in the cathode half-
reaction (reduction) and produced in the anode half-reaction (oxidation). They therefore
travel through the wire from anodeto cathode.The dc source forces electrons to flow
nonspontaneously from the positive electrode to the negative electrode. The anode is the
positive electrode and the cathode the negative electrode in all electrolytic cells.Figure 21-2a
is a simplified diagram of the cell.
Sodium and chlorine must not be allowed to come in contact with each other because
they react spontaneously, rapidly, and explosively to form sodium chloride. Figure 21-2b
shows the Downs cell that is used for the industrial electrolysis of sodium chloride. The
Downs cell is expensive to run, mainly because of the cost of construction, the cost of the
electricity, and the cost of heating the NaCl to melt it. Nevertheless, electrolysis of a
molten sodium salt is the most practical means by which metallic Na can be obtained,
owing to its extremely high reactivity. Once liberated by the electrolysis, the liquid Na
metal is drained off, cooled, and cast into blocks. These must be stored in an inert envi-
ronment (e.g., in mineral oil) to prevent reaction with O 2 or other components of the
atmosphere.
Electrolysis of NaCl in the Downs cell is the main commercial method of producing
metallic sodium. The Cl 2 gas produced in the Downs cell is cooled, compressed, and
marketed. This partially offsets the expense of producing metallic sodium, but most chlo-
rine is produced by the cheaper electrolysis of aqueous NaCl.THE ELECTROLYSIS OF AQUEOUS SODIUM CHLORIDE
Consider the electrolysis of a moderately concentrated solution of NaCl in water, using
inert electrodes. The following experimental observations are made when a sufficiently
high voltage is applied across the electrodes of a suitable cell.1.H 2 gas is liberated at one electrode. The solution becomes basic in that vicinity.
2.Cl 2 gas is liberated at the other electrode.Chloride ions are obviously being oxidized to Cl 2 in this cell, as they were in the elec-
trolysis of molten NaCl. But Naions are not reduced to metallic Na. Instead, gaseous
H 2 and aqueous OHions are produced by reduction of H 2 O molecules at the cathode.
Water is more easily reduced than Naions. This is primarily because the reduction of
Nawould produce the very active metal Na, whereas the reduction of H 2 O produces
the more stable products H 2 (g) and OH(aq). The active metals Li, K, Ca, and Na (Table
4-12) displace H 2 from aqueous solutions, so we do not expect these metals to be produced
in aqueous solution. Later in this chapter (Section 21-14) we learn the quantitative basis21-4
852 CHAPTER 21: Electrochemistry
In this chapter, as in Chapters 4 and
11, we often use red type to emphasize
reduction and blue type to emphasize
oxidation.
The direction of spontaneousflow for
negatively charged particles is from
negative to positive.