Writing Your Own Half-Reactions
293
DETERMINATION OF OXIDATION STATE
The general method of balancing electron-transfer equations requires that halfreaction equations be available. Short lists of common half-reactions, similar to
Table 17-1, are given in most textbooks, and chemistry handbooks have extensive lists. However, no list can provide all possible half-reactions, and it is not
practical to carry lists in your pocket for instant reference. The practical alternative is to learn to make your own half-reaction equations. There is only one
prerequisite for this approach: you must know the oxidation states of the
oxidized and reduced forms of the substances involved in the electron-transfer
reaction. In Chapter 8 you learned the charges on the ions of the most common
elements; now we review the method of determining the charge (the oxidation
state) of an element when it is combined in a radical.
PROBLEM:
What is the charge of Cr in the Cr 2 O?~ ion?
SOLUTION:
It will be convenient to remember that, whenever oxygen is combined with other
elements, it always has a charge of -2 unless it is in a peroxide (in which case it is
- 1). Similarly, it will be useful to know that, whenever hydrogen is combined with
other elements, it always has a charge of + 1 unless it is a hydride (in which case it
is -1). Peroxides and hydrides are not common.
The total charge (C) on an ion is the sum of the charges of the atoms that
compose it. If we let z be the charge of a given element in the ion, and n be the
number of atoms of that element in the ion, then
C = n l z 1 + « 2 z 2 + • • • = 2«jZj
If we apply this equation to the Cr 2 Of~ ion whose charge is -2, we have
-2 = (2)(z cr ) + (7)(-2)
2z cr = 14 - 2 = 12
Zcr = +6 = charge on Cr
WRITING YOUR OWN HALF-REACTIONS
The second simplest situation that exists for balancing electron-transfer equations is the one in which the principal oxidation and reduction products are
given, and you know (or are told) that the reaction actually takes place. All you
need do is to write your own half-reactions, and then proceed as illustrated in
the first problem.
293
DETERMINATION OF OXIDATION STATE
The general method of balancing electron-transfer equations requires that halfreaction equations be available. Short lists of common half-reactions, similar to
Table 17-1, are given in most textbooks, and chemistry handbooks have extensive lists. However, no list can provide all possible half-reactions, and it is not
practical to carry lists in your pocket for instant reference. The practical alternative is to learn to make your own half-reaction equations. There is only one
prerequisite for this approach: you must know the oxidation states of the
oxidized and reduced forms of the substances involved in the electron-transfer
reaction. In Chapter 8 you learned the charges on the ions of the most common
elements; now we review the method of determining the charge (the oxidation
state) of an element when it is combined in a radical.
PROBLEM:
What is the charge of Cr in the Cr 2 O?~ ion?
SOLUTION:
It will be convenient to remember that, whenever oxygen is combined with other
elements, it always has a charge of -2 unless it is in a peroxide (in which case it is
- 1). Similarly, it will be useful to know that, whenever hydrogen is combined with
other elements, it always has a charge of + 1 unless it is a hydride (in which case it
is -1). Peroxides and hydrides are not common.
The total charge (C) on an ion is the sum of the charges of the atoms that
compose it. If we let z be the charge of a given element in the ion, and n be the
number of atoms of that element in the ion, then
C = n l z 1 + « 2 z 2 + • • • = 2«jZj
If we apply this equation to the Cr 2 Of~ ion whose charge is -2, we have
-2 = (2)(z cr ) + (7)(-2)
2z cr = 14 - 2 = 12
Zcr = +6 = charge on Cr
WRITING YOUR OWN HALF-REACTIONS
The second simplest situation that exists for balancing electron-transfer equations is the one in which the principal oxidation and reduction products are
given, and you know (or are told) that the reaction actually takes place. All you
need do is to write your own half-reactions, and then proceed as illustrated in
the first problem.