23.3. Electrolysis http://www.ck12.org
Electrolysis of Molten Sodium Chloride
Molten (liquid) sodium chloride can be electrolyzed to produce sodium metal and chlorine gas. The electrolytic cell
used in this process is called a Down’s cell (Figure23.8).
FIGURE 23.8
A Down’s cell is used for the electrolysis
of molten sodium chloride. Liquid sodium
metal is produced at the cathode, while
chlorine gas is produced at the anode.In a Down’s cell, the liquid sodium ions are reduced at the cathode to liquid sodium metal. At the anode, liquid
chloride ions are oxidized to chlorine gas. The reactions and cell potentials are shown below.
Oxidation (anode): 2Cl−(l)→Cl 2 (g)+2e− E^0 =− 1 .36 V
Reduction (cathode): Na+(l)+e−→Na(l) E^0 =− 2 .71 V
Overall reaction: 2Na+(l)+2Cl−(l)→2Na(l)+Cl 2 (g) E^0 cell=− 4 .07 VThe battery must supply over 4 volts to carry out this electrolysis. This reaction is a major industrial source of
chlorine gas, and it is the primary way to obtain pure sodium metal. Chlorine gas is widely used as a disinfectant,
such as in swimming pools.
Electrolysis of Water
The electrolysis of water produces hydrogen and oxygen gases (Figure23.9). The electrolytic cell consists of a
pair of platinum electrodes immersed in water containing a small amount of an electrolyte, such as H 2 SO 4. The
electrolyte is necessary because pure water does not contain enough ions to effectively conduct a current. At the
anode, water is oxidized to oxygen gas and hydrogen ions. At the cathode, water is reduced to hydrogen gas and
hydroxide ions.
Oxidation (anode): 2H 2 O(l)→O 2 (g)+4H+(aq)+4e− E^0 =− 1 .23 V
Reduction (cathode): 2H 2 O(l)+2e−→H 2 (g)+2OH−(aq) E^0 =− 0 .83 V
Overall reaction: 2H 2 O(l)→O 2 (g)+2H 2 (g) E^0 cell=− 2 .06 V