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Electrochemistry II: Balancing Equations
Likewise, group 8 contains elements often associated with each other because of position in the periodic table and similarity in chemical properties. The
emphasis in this group is on the oxidized form of the element (the halogen X 2 ).
The same reduced forms of the halogen occur again in group 9, but associated
in a different way. There is no problem with F 2 because it is the strongest
oxidizing agent of all chemicals; it lies at the bottom of every list. You need
concern yourself only with the approximate E° values of the first three; the
order is the same as that in the periodic table.
Groups 6 and 9 lie farthest from the previous experience of students and
require more study than the others. Most chemists think of the oxidizing agents
of group 9 as being very strong oxidizing agents and would classify a large
number of them as having E° values of about 1.35 to 1.50. It pays to know that
the E° of HNO 3 (giving NO) is approximately 1.0, because it is so common.
Note that almost none of the half-reactions in the first eight groups overlap with
group 9. Another help with group 9 is to note that all the halogen-containing
oxidizing agents are reduced to the halide forms. You should probably think of
the first seven groups in terms of the relative strengths of the reducing agents
(those chemicals on the righthand side of the half-reaction), and think of groups
8 and 9 in terms of the relative strengths of the oxidizing agents (those chemicals
on the lefthand side of the half-reaction).
Two major oversimplifications are involved in Table 18-1: (a) only metals are
shown lying above H 2 , and (b) no reactions in basic solution are shown. You
will probably spend far less time memorizing this table, or something similar to
it, than you would spend trying to get comparable information in some other
intuitive, haphazard manner, and your work in memorizing it will be rewarded
by superior predictions. This table is not practical for making predictions about
reactions composed of half-reactions whose E° values lie very close together.
EFFECT OF ACID CONCENTRATION
The E° values of all the half-reactions involving H
+ depend on having H
+
concentration of unit activity; for all other half-reactions, the E° values are
independent of the H
+ concentration. For those that do involve H
+ , the actual
half-reaction potentials (and therefore the strengths as oxidizing or reducing
agents) can be greatly affected by the H
+ concentration. By means of the Nernst
equation (p 275) we can calculate that, in pure water, where [H
+ ] = 10~
7 M,
£ Hz _ H+ = -0.41 volts
Immediately we see that none of the medium active metals (group 4) are
able to react with water to displace H 2 , and most of those in the active metal
group (group 3) would react with little energy change.
Electrochemistry II: Balancing Equations
Likewise, group 8 contains elements often associated with each other because of position in the periodic table and similarity in chemical properties. The
emphasis in this group is on the oxidized form of the element (the halogen X 2 ).
The same reduced forms of the halogen occur again in group 9, but associated
in a different way. There is no problem with F 2 because it is the strongest
oxidizing agent of all chemicals; it lies at the bottom of every list. You need
concern yourself only with the approximate E° values of the first three; the
order is the same as that in the periodic table.
Groups 6 and 9 lie farthest from the previous experience of students and
require more study than the others. Most chemists think of the oxidizing agents
of group 9 as being very strong oxidizing agents and would classify a large
number of them as having E° values of about 1.35 to 1.50. It pays to know that
the E° of HNO 3 (giving NO) is approximately 1.0, because it is so common.
Note that almost none of the half-reactions in the first eight groups overlap with
group 9. Another help with group 9 is to note that all the halogen-containing
oxidizing agents are reduced to the halide forms. You should probably think of
the first seven groups in terms of the relative strengths of the reducing agents
(those chemicals on the righthand side of the half-reaction), and think of groups
8 and 9 in terms of the relative strengths of the oxidizing agents (those chemicals
on the lefthand side of the half-reaction).
Two major oversimplifications are involved in Table 18-1: (a) only metals are
shown lying above H 2 , and (b) no reactions in basic solution are shown. You
will probably spend far less time memorizing this table, or something similar to
it, than you would spend trying to get comparable information in some other
intuitive, haphazard manner, and your work in memorizing it will be rewarded
by superior predictions. This table is not practical for making predictions about
reactions composed of half-reactions whose E° values lie very close together.
EFFECT OF ACID CONCENTRATION
The E° values of all the half-reactions involving H
+ depend on having H
+
concentration of unit activity; for all other half-reactions, the E° values are
independent of the H
+ concentration. For those that do involve H
+ , the actual
half-reaction potentials (and therefore the strengths as oxidizing or reducing
agents) can be greatly affected by the H
+ concentration. By means of the Nernst
equation (p 275) we can calculate that, in pure water, where [H
+ ] = 10~
7 M,
£ Hz _ H+ = -0.41 volts
Immediately we see that none of the medium active metals (group 4) are
able to react with water to displace H 2 , and most of those in the active metal
group (group 3) would react with little energy change.