Physical Chemistry Third Edition

608 13 Chemical Reaction Mechanisms II: Catalysis and Miscellaneous Topics

For a current density of 0.01 A cm−^2 and a hydrogen ion concentration of

1.0 mol L−^1 , the overpotentials for the reduction of hydrogen ions at several electrodes

are found to be^35

0.035 V for platinized platinum

0.56 V for iron

0.76 V for silver

1.10 V for mercury

Overpotentials for different reactions at the same electrode will also differ from each

other in different ways, and it is even possible to react one substance at a nonzero current

when another substance in the solution would preferentially react at an infinitesimal

current. An example that is exploited inpolarographyis the reduction of a fairly active

metal onto a mercury electrode. The overpotential for production of hydrogen from

water on a mercury electrode is so large that almost no hydrogen is evolved at a potential

difference sufficient to plate out the metal, even if hydrogen would preferentially be

evolved at an infinitesimal current.

PROBLEMS

Section 13.5: Nonequilibrium Electrochemistry

13.33In theHall–Heroult processfor the production of
aluminum, aluminum oxide is dissolved in molten
cryolite, Na 3 AlF 6 , which melts at 1000◦C, and
electrolyzed. The cathode is molten aluminum,
and the anode is made of graphite. The anode product
is carbon dioxide. Write the half-reaction
equations and the cell reaction equation.

13.34Find the activity of chloride ion necessary so
that chlorine instead of oxygen would be evolved
when an infinitesimal current flows at 298.15 K
in the electrolysis cell of Example 13.6.

13.35Find the value of the charge transfer resistance

at 298.15 K for the reduction of hydrogen ion

from a solution of 0.0100 mol L−^1 on a mercury

electrode with an area of 1.00 cm^2. Assume a

value of the exchange current density of

j 0  2. 5 × 10 −^15 Acm−^2.

13.36For the formation of hydrogen gas from a dilute

sulfuric acid solution on a mercury electrode

at 25◦C, the current density was equal to

4. 8 × 10 −^6 Acm−^2 at an overpotential of 0.60 V and

was equal to 3. 7 × 10 −^7 Acm−^2 at an overpotential of

0.50 V. Find the values of the parameters in the Tafel

equation, Eq. (13.5-26).

13.6 Experimental Molecular Study of Chemical

Reaction Mechanisms

The “classical” study of chemical reaction rates involves determination of concen-

trations of reactants or products as a function of time. Such information cannot lead

directly to knowledge of a reaction mechanism. However, there are techniques that

deliver molecular information about a reaction mechanism.

(^35) W. C. Gardiner, Jr.,Rates and Mechanisms of Chemical Reactions, W. A. Benjamin, Inc., New York,
1969, p. 197.