s8pe-30901-ca
12/6/05
4:29 PM
MAZER
Page 275
Chemical reactions can be classified.
Scientists classify chemical reactions in several ways. All reactions
form new products. The ways these products are made, however,
can differ.
In a synthesis reaction, a new compound is formed by the
combination of simpler reactants. For example, nitrogen dioxide (NO2),
a part of smog, forms when nitrogen and oxygen combine in the air.
Synthesis
N2
N
+
N
2O2
O
O
O
O
Synthesis means “making
a substance from simpler
substances.”
2NO2
N
O
O
N
O
O
In a decomposition reaction, a reactant breaks down
into simpler products. The products could be elements or other compounds. Decomposition reactions can be thought of as being the
reverse of synthesis reactions. For example, water can be decomposed
into its elements—hydrogen and oxygen.
Decomposition
2H2O
reading tip
+
2H2
reading tip
Decomposition means
“separation into parts.”
O2
O
H
H
H H
O
O
H
O
H H
H
In a combustion reaction, one reactant is always oxygen.
Another reactant often contains carbon and hydrogen. The carbon
and hydrogen atoms combine with oxygen, producing carbon dioxide
and water. The burning of methane is a combustion reaction.
Combustion
CH4
+
H
C
H
H
2O2
O
H
2H2O
H
C
H
O
O
O
H
Check Your Reading
Combustion is the process
of burning with oxygen.
O
O
O
O
+
CO2
reading tip
H
How are synthesis reactions different from
decomposition reactions?
Chapter 9: Chemical Reactions 275
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