The Foundations of Chemistry

CaO(s)
CO 2 (g)88nCaCO 3 (s)

Pyrosulfuric acid is produced by dissolving sulfur trioxide in concentrated sulfuric acid:

SO 3 (g)
H 2 SO 4 (
)88nH 2 S 2 O 7 (
)

Pyrosulfuric acid, H 2 S 2 O 7 , is then diluted with water to make H 2 SO 4 :

H 2 S 2 O 7 (
)
H 2 O(
)88n2H 2 SO 4 (
)

Oxides of the Group IA and IIA metals react with water to form metal hydroxides, e.g.:

CaO(s)
H 2 O(
)88nCa(OH) 2 (aq)

DECOMPOSITION REACTIONS

Decomposition reactionsare those in which a compound decomposes to produce

(1) two elements, (2) one or more elements andone or more compounds, or (3) two

or more compounds.

Examples of each type follow.

1 Compound nElement
Element

The electrolysis of water produces two elements by the decomposition of a compound. A

compound that ionizes, such as H 2 SO 4 , is added to increase the conductivity of water and

the rate of the reaction (Figure 1-8), but it does not participate in the reaction:

electrolysis

2H 2 O(
) 8888888888n2H 2 (g)
O 2 (g)

Small amounts of oxygen can be prepared by the thermal decomposition of certain

oxygen-containing compounds. Some metal oxides, such as mercury(II) oxide, HgO,

decompose on heating to produce oxygen:

heat

2HgO(s) 8888n2Hg(
)
O 2 (g)

mercury(II) oxide

2 Compound nCompound
Element

The alkali metal chlorates, such as KClO 3 , decompose when heated to produce the corre-

sponding chlorides and liberate oxygen. Potassium chlorate is a common laboratory source

of small amounts of oxygen:

heat

2KClO 3 (s) 8888n 2KCl(s) 
3O 2 (g)

potassium chlorate MnO 2 potassium chloride

Nitrate salts of alkali metals or alkaline earth metals decompose to form metal nitrites

and oxygen gas.

2NaNO 3 (s)88n2NaNO 2 (s)
O 2 (g)

Hydrogen peroxide decomposes to form water and oxygen.

H 2 O 2 (
)88n2H 2 O(
)
O 2 (g)

4-7

Decomposition reactions can be
considered as the opposite of
combination reactions.

146 CHAPTER 4: Some Types of Chemical Reactions

Mercury(II) oxide, a red compound,
decomposes when heated into the
two elements: mercury (a metal) and
oxygen (a nonmetal). Mercury vapor
condenses on the cooler upper
portion of the test tube.

Manganese dioxide, MnO 2 , is used as
a catalyst, a substance that speeds up a
chemical reaction but is not consumed.
Here it allows the decomposition to
occur at a lower temperature.