The Foundations of Chemistry

The overall cell reaction produces gaseous H 2 and Cl 2 and an aqueous solution of

NaOH, called caustic soda. Solid NaOH is then obtained by evaporation of the residual

solution. This is the most important commercial preparation of each of these substances.

It is much less expensive than the electrolysis of molten NaCl, because it is not necessary

to heat the solution.

THE ELECTROLYSIS OF AQUEOUS SODIUM SULFATE

In the electrolysis of aqueous sodium sulfate using inert electrodes, we observe the

following.

1.Gaseous H 2 is produced at one electrode. The solution becomes basic around that

electrode.

2.Gaseous O 2 is produced at the other electrode. The solution becomes acidic around

that electrode.

As in the previous example, water is reduced in preference to Naat the cathode.

Observation 2 suggests that water is also preferentially oxidized relative to the sulfate ion,

SO 42 , at the anode (Figure 21-4).

2(2H 2 O 2 e88nH 2 2OH) (reduction, cathode)

2H 2 O88nO 2 4H 4 e (oxidation, anode)

6H 2 O88n2H 2 O 2 4H4OH (overall cell reaction)

4H 2 O

2H 2 O88n2H 2 O 2 (net reaction)

The net result is the electrolysis of water. This occurs because H 2 O is more readily reduced

than Naand more readily oxidized than SO 42 . The ions of Na 2 SO 4 conduct the current

through the solution, but they take no part in the reaction.

21-5

854 CHAPTER 21: Electrochemistry



Not surprisingly, the fluctuations in
commercial prices of these widely used
industrial products—H 2 , Cl 2 , and
NaOH—have often paralleled one
another.

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 21.10, Electrolysis.

Gaseous H 2 is produced from H 2 O
at an illuminated photoelectrode.
Light from the sun may soon be
used to produce hydrogen, the
ultimate clean-burning fuel.

Figure 21-4 The electrolysis of
aqueous Na 2 SO 4 produces H 2 (g) at
the cathode and O 2 at the anode.
Bromthymol blue indicator has been
added to the solution. This indicator
turns blue in the basic solution
near the cathode (where OHis
produced) and yellow in the acidic
solution near the anode (where H
is formed).

Battery

Pt cathode Pt anode

H 2 (g) Na+

SO 4 2–

OH– H+

O 2 (g)

2(H 2 O + 2e– → H 2 (g) + 2OH–)

Reduction

e– e–

2H 2 O → O 2 (g) + 4H+ + 4e–

Oxidation