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FIGURE 10.11 Mercury(II) oxide is decomposed by heat,
leaving the silver-colored element mercury behind as oxygen
is driven off. This is an example of a decomposition reaction,
2 HgO → 2 Hg + O 2 ↑. Compare this equation to the general form
FIGURE 10.10 Rusting iron is a common example of a com- of a decomposition reaction.
bination reaction, where two or more substances combine to form a
new compound. Rust is iron(III) oxide formed on this old car from
the combination of iron and oxygen under moist conditions.

Plaster is a building material made from a mixture of
The rusting of metals is oxidation that takes place at a slower calcium hydroxide, Ca(OH) 2 , and plaster of Paris, CaSO 4 . The
pace than burning, but metals are nonetheless oxidized in the calcium hydroxide is prepared by adding water to calcium oxide
process (Figure 10.10). Again noting the generalized form of a (CaO), which is commonly called quicklime. Calcium oxide is
combination reaction, consider the rusting of iron: made by heating limestone or chalk (CaCO 3 ), and
Δ

CaCO 3 (s) → CaO(s) + CO 2 ↑
4 Fe(s) + 3 O 2 ( g) → 2 Fe 2 O 3 (s)
Nonmetals are also oxidized by burning in air, for example, Note that all the decomposition reactions follow the gener-
when carbon burns with a sufficient supply of O 2 : alized form of XY → X + Y.
C(s) + O 2 ( g) → CO 2 ( g)
REPLACEMENT REACTIONS
Note that all the combination reactions follow the general-
In a replacement reaction, an atom or polyatomic ion is
ized form of X + Y → XY.
replaced in a compound by a different atom or polyatomic
ion. The replaced part can be either the negative or the posi-
DECOMPOSITION REACTIONS tive part of the compound. In generalized form, a replacement
A decomposition reaction, as the term implies, is the opposite of reaction is
a combination reaction. In decomposition reactions, a compound
XY + Z → XZ + Y

is broken down into (1) the elements that make up the compound, (negative part replaced)
(2) simpler compounds, or (3) elements and simpler compounds.
or
Decomposition reactions have a generalized form of
XY + A → AY + X

XY → X + Y

(positive part replaced)
Decomposition reactions generally require some sort of
Replacement reactions occur because some elements have a
energy, which is usually supplied in the form of heat or electrical
stronger electron-holding ability than other elements. Elements
energy. An electric current, for example, decomposes water into
that have the least ability to hold on to their electrons are the
hydrogen and oxygen:
most chemically active. Figure 10.12 shows a list of chemical
electricity

2 H 2 O(1) → 2 H 2 ( g) + O 2 ( g) activity of some metals, with the most chemically active at
the top. Hydrogen is included because of its role in acids (see
Mercury(II) oxide is decomposed by heat, an observation
chapter 11). Take a few minutes to look over the generalizations
that led to the discovery of oxygen (Figure 10.11):
listed in Figure 10.12. The generalizations apply to combina-
Δ

2 HgO(s) → 2 Hg(s) + O 2 ↑ tion, decomposition, and replacement reactions.
10-11 CHAPTER 10 Chemical Reactions 261