The Foundations of Chemistry

for predicting which of several possible oxidations or reductions is favored. The half-
reactions and overall cell reaction for this electrolysis are

2Cl88nCl 2  2 e (oxidation, anode)

2H 2 O 2 e88n2OHH 2 (reduction, cathode)

2H 2 O2Cl88n2OHH 2 Cl 2 (overall cell reaction as

net ionic equation)

2Na88n 2Na (spectator ions)

2H 2 O 2NaCl 88n 2NaOHH 2 Cl 2 (overall cell reaction as

formula unit equation)

The cell is illustrated in Figure 21-3. As before, the electrons flow from the anode ()
through the wire to the cathode ().

We will omit the notation that

indicates states of substances—(s), (
),

(g), and (aq)—except where states are

not obvious. This abbreviates writing

equations.

The electrolysis of the aqueous

solution of KI, another Group IA–

Group VIIA salt. At the cathode

(left), water is reduced to H 2 (g)

and OHions, turning the

phenolphthalein indicator pink.

The characteristic brownish color

of aqueous I 2 appears at the anode

(right).

Battery

Pt cathode Pt anode

H 2 (g) Na+

Na+

OH– Cl–

Cl 2 (g)

2H 2 O + 2e– → H 2 (g) + 2OH–

Reduction

e– e–

2Cl– → Cl 2 (g) + 2e–

Oxidation

Figure 21-3 Electrolysis of

aqueous NaCl solution. Although

several reactions occur at both the

anode and the cathode, the net

result is the production of H 2 (g) and

NaOH at the cathode and Cl 2 (g)

at the anode. A few drops of

phenolphthalein indicator were

added to the solution. The solution

turns pink at the cathode, where

OHions are formed.

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