The Foundations of Chemistry

whereas reaction with an excess of oxygen gives tetraphosphorus decoxide, P 4 O 10 :

P 4 (s)5O 2 (g)88nP 4 O 10 (s) tetraphosphorus decoxide

Sulfur burns in oxygen to form primarily sulfur dioxide (Figure 6-9) and only very small
amounts of sulfur trioxide.

S 8 (s)8O 2 (g)88n8SO 2 (g) sulfur dioxide (bp 
10.0°C)

S 8 (s)12O 2 (g)88n8SO 3 (
) sulfur trioxide (bp 43.4°C) The production of SO 3 at a reasonable

rate requires the presence of a catalyst.

6-8 Oxygen and the Oxides 259

 5

Nearly all nonmetals exhibit more

than one oxidation state in their

compounds. In general, the most commonoxidation states of a nonmetal are (1) its peri-

odic group number, (2) its periodic group number minus two, and (3) its periodic group

number minus eight. The reactions of nonmetals with a limited amount of oxygen usually

give products that contain the nonmetals (other than oxygen) in lower oxidation states,

usually case (2). Reactions with excess oxygen give products in which the nonmetals

exhibit higher oxidation states, case (1). The examples we have cited are CO and CO 2 ,

P 4 O 6 and P 4 O 10 , and SO 2 and SO 3. The molecular formulas of the oxides are some-

times not easily predictable, but the simplestformulas are. For example, the two most

common oxidation states of phosphorus in molecular compounds are 3 and 5. The

simplest formulas for the corresponding phosphorus oxides therefore are P 2 O 3 and P 2 O 5 ,

respectively. The molecular (true) formulas are twice these, P 4 O 6 and P 4 O 10.

OXIDATION STATES OF NONMETALS

 4

 6

Reactions of Nonmetal Oxides with Water

Nonmetal oxidesare called acid anhydrides(or acidic oxides) because many of them
dissolve in water to form acids with no change in oxidation state of the nonmetal(see Figure
6-8). Several ternary acidscan be prepared by reaction of the appropriate nonmetal oxides
with water. Ternary acids contain three elements, usually H, O, and another nonmetal.

Nonmetal

Oxide 
Water 88nTernary Acid

carbon dioxide CO 2 (g) H 2 O(
) 88nH 2 CO 3 (aq) carbonic acid

sulfur dioxide SO 2 (g) H 2 O(
) 88nH 2 SO 3 (aq) sulfurous acid

sulfur trioxide SO 3 (
) H 2 O(
) 88nH 2 SO 4 (aq) sulfuric acid

dinitrogen pentoxide N 2 O 5 (s) H 2 O(
) 88n2HNO 3 (aq) nitric acid

tetraphosphorus decoxide P 4 O 10 (s) 6H 2 O(
)88n4H 3 PO 4 (aq) phosphoric acid

 4

 4

 6

 5

 5

 4

 4

 6

 5

 5

Nearly all oxides of nonmetals react with water to give solutions of ternary acids. The
oxides of boron and silicon, which are insoluble, are two exceptions.

Figure 6-9 Sulfur burns in oxygen

to form sulfur dioxide.