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 in polarography is 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.
P R O B L E M S
Section 13.5: Nonequilibrium Electrochemistry
13.33 In the Hall–Heroult process for 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.
13.34 Find 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.
13.35 Find 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 .
13.36 For 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 concentrations 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.
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 in polarography is 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.
P R O B L E M S
Section 13.5: Nonequilibrium Electrochemistry
13.33 In the Hall–Heroult process for 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.
13.34 Find 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.
13.35 Find 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 .
13.36 For 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 concentrations 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.