http://www.ck12.org Chapter 23. Electrochemistry
FIGURE 23.12
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.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. 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): 4H 2 O(l)+4e−→2H 2 (g)+4OH−(aq) E^0 =− 0 .83 V
Overall reaction: 2H 2 O(l)→O 2 (g)+2H 2 (g) E^0 cell=− 2 .06 VThe hydrogen ion and hydroxide ions produced in each reaction combine to form water, resulting in the net reaction
shown above. The added electrolyte is not consumed in the reaction. In this process, the hydrogen in the water is
reduced (from +1 to 0) and the oxygen is oxidized (from -2 to 0). This process was discovered in 1800 by William
Nicholson and Johann Ritter. One modern-day application is the generation of oxygen for use in sealed vehicles,
such as spacecrafts and submarines.
Electrolysis of Aqueous Sodium Chloride
Earlier we examined the electrolysis of molten sodium chloride. It may be logical to assume that the electrolysis of
aqueous sodium chloride, called brine, would yield the same result by the same reactions. However, the reduction
reaction that occurs at the cathode does not produce sodium metal, because the water is reduced instead. This is
because the reduction potential for water is only−0.83 V compared to−2.71 V for the reduction of sodium ions.
This makes the reduction of water preferable, because its reduction potential is less negative. Chlorine gas is still
produced at the anode, just as in the electrolysis of molten NaCl.