CK-12-Chemistry Intermediate

http://www.ck12.org Chapter 23. Electrochemistry

In order to obtain the overall reaction, the reduction half-reaction was multiplied by two to equalize the electrons.
The hydrogen ion and hydroxide ions produced in each reaction combine to form water. The added electrolyte is not
consumed in the reaction.

FIGURE 23.9

Apparatus for the production of hydrogen and oxygen gases by the

electrolysis of water.

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.

Oxidation (anode): 2Cl−(aq)→Cl 2 (g)+2e− E^0 =− 1 .36 V

Reduction (cathode): 2H 2 O(l)+2e−→H 2 (g)+2OH−(aq) E^0 =− 0 .83 V

Overall reaction: 2Cl−(aq)→O 2 (g)+2H 2 (g) E^0 cell=− 2 .06 V

Since the hydroxide ion is also a product of the net reaction, the important chemical sodium hydroxide (NaOH) is
obtained by evaporating the water after the hydrolysis is complete.

Electroplating

Many decorative objects like jewelry are manufactured with the aid of an electrolytic process.Electroplatingisa
process in which a metal ion is reduced in an electrolytic cell to deposit the solid metal onto a surface. Shown below
(Figure23.10) is a cell in which silver metal is to be plated onto a stainless steel spoon.

The cell consists of a solution of silver nitrate and a strip of silver, which acts as the anode. The spoon is the cathode.
The anode is connected to the positive electrode of a battery, while the spoon is connected to the negative electrode.

When the circuit is closed, silver metal from the anode is oxidized, allowing silver ions to enter the solution.

Anode: Ag(s)→Ag+(aq)+e−