The Foundations of Chemistry

Figure 6-7 Iron powder burns

brilliantly to form iron(III) oxide,

Fe 2 O 3.

6-8 Oxygen and the Oxides 257

For example, the equations for the reactions of calcium and oxygen are

2Ca(s)O 2 (g)88n2CaO(s) calcium oxide (mp 2580°C)

Ca(s)O 2 (g)88nCaO 2 (s) calcium peroxide (decomposes at 275°C)

The other metals, with the exceptions noted previously (Au, Pd, and Pt), react with
oxygen to form solid metal oxides. Many metals to the right of Group IIA show variable
oxidation states, so they may form several oxides. For example, iron combines with oxygen
in the following series of reactions to form three different oxides (Figure 6-7).

heat

2Fe(s)O 2 (g)888n 2FeO(s) iron(II) oxide

heat

6FeO(s)O 2 (g)888n2Fe 3 O 4 (s) magnetic iron oxide (a mixed oxide)

heat

4Fe 3 O 4 (s)O 2 (g)888n6Fe 2 O 3 (s) iron(III) oxide

Copper reacts with a limited amount of oxygen to form red Cu 2 O, whereas with excess
oxygen it forms black CuO.

heat

4Cu(s)O 2 (g)888n2Cu 2 O(s) copper(I) oxide

heat

2Cu(s)O 2 (g)888n 2CuO(s) copper(II) oxide

Metals that exhibit variable oxidation states react with a limited amount of oxygen

to give oxides with lower oxidation states (such as FeO and Cu 2 O). They react with

an excess of oxygen to give oxides with higher oxidation states (such as Fe 2 O 3 and

CuO).

Reactions of Metal Oxides with Water

Oxides of metals are called basic anhydrides(or basic oxides) because many of them
combine with water to form bases with no change in oxidation state of the metal (Figure
6-8). “Anhydride” means “without water”; in a sense, the metal oxide is a hydroxide base
with the water “removed.” Metal oxides that are soluble in water react to produce the
corresponding hydroxides.

Figure 6-8 The normal oxides of the representative elements in their maximum oxidation
states. Acidic oxides (acid anhydrides) are shaded red, amphoteric oxides are shaded purple,
and basic oxides (basic anhydrides) are shaded blue. An amphotericoxide is one that shows
some acidic and some basic properties.

Increasing base character

Increasing acidic character

IA IIA IIIA IVA VA VIA VIIA

Li 2 O

Na 2 O

K 2 OGaCaO 2 O 3 GeO 2

Al 2 O 3 SiO 2 P 4 O 10 SO 3

SeO 3

MgO Cl 2 O 7

Rb 2 OIn 2 O 3 SnO 2 Sb 2 O 5

As 2 O 5

TeO 3 I 2 O 7

Br 2 O 7

SrO

Cs 2 OTlBaO 2 O 3 PbO 2 Bi 2 O 5 PoO 3 At 2 O 7

BeO B 2 O 3 CO 2 N 2 O 5 OF 2