Page 216 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 14] OXIDATION AND REDUCTION 205

H He

Constant,
equal to Steps of 2
group Steps of 1
number

+3 mostly, rarely +2 or +4
Steps of 1 +3 mostly
First part Last part
Fig. 14-1. Possible oxidation numbers

EXAMPLE 14.12. Determine the formula of two oxides of sulfur.

Ans. The oxygen must exist in a −2 oxidation state, because it is more electronegative than sulfur. Therefore, sulfur
must exist in two different positive oxidation states in the two compounds. Its maximum oxidation state is +6,
corresponding to its position in periodic group VIA. It also has an oxidation state of +4, which is 2 less than its
maximum (see Fig. 14-1). The formulas therefore are SO 3 and SO 2 .

14.4. OXIDATION NUMBERS IN INORGANIC NOMENCLATURE
In Chap. 6 we placed Roman numerals at the ends of names of metals to distinguish the charges on monatomic
cations. It is really the oxidation number that is in parentheses. This nomenclature system is called the Stock
system. For monatomic ions, the oxidation number is equal to the charge. For other cations, again the oxidation
2+
number is used in the name. For example, Hg 2 is named mercury(I) ion. Its charge is 2+; the oxidation number
+
of each atom is +1. Oxidation numbers are also used for other cations, such as dioxovanadium(V) ion, VO 2 .
The prefix oxo- stands for oxygen. Oxidation numbers can be used with nonmetal-nonmetal compounds, as in
sulfur(VI) oxide for SO 3 , but the older system using prefixes (Table 6-2) is still used more often.
EXAMPLE 14.13. Name the following according to the Stock system: (a) NiCl 2 ,(b)UO 2 SO 4 , and (c)P 2 O 5 .
Ans. (a) Nickel(II) chloride (b) dioxouranium(VI) sulfate (c) phosphorus(V) oxide

14.5. BALANCING OXIDATION-REDUCTION EQUATIONS
In every reaction in which the oxidation number of an element (or more than one) in one reactant goes up, an
element (or more than one) in some reactant must go down in oxidation number. An increase in oxidation number
is called an oxidation. A decrease in oxidation number is called a reduction. The term redox (the first letters
of reduction and oxidation) is often used as a synonym for oxidation-reduction. The total change in oxidation
number (change in each atom times number of atoms) must be the same in the oxidation as in the reduction,
because the number of electrons whose “control” is transferred from one species must be the same as the number
transferred to the other. The species that causes another to be reduced is called the reducing agent; in the process,
it is oxidized. The species that causes the oxidation is called the oxidizing agent; in the process, it is reduced.

EXAMPLE 14.14. (a) In drying dishes, a dish towel could be termed a drying agent, and the dish a wetting agent. What
happens to the towel and to the dish? (b) In its reaction with CrCl 2 ,Cl 2 is the oxidizing agent and CrCl 2 is the reducing agent.
What happens to Cl 2 and to Cr ?
2+
Ans. (a) The towel, the drying agent, gets wet. The dish, the wetting agent, gets dry.
(b) 2 CrCl 2 + Cl 2 −→ 2 CrCl 3
The oxidizing agent, Cl 2 , is reduced to Cl . The oxidation number goes from 0 to −1. The reducing agent, Cr ,is
2+
−
oxidized. Its oxidation number goes from +2to +3. Just as the water must go somewhere in part a, the electrons must go
somewhere in part b.