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Combustion is a rapid reaction with O 2 that releases A reducing agent supplies electrons to the substance being
energy, usually with a flame. A very similar, although much slower reduced. Hydrogen and carbon are commonly used reducing
reaction takes place in plant and animal respiration. In respira- agents. Carbon is commonly used as a reducing agent to extract
tion, carbohydrates combine with O 2 and release energy used metals from their ores. For example, carbon (from coke, which is
for biological activities. This reaction is slow compared to com- coal that has been baked) reduces Fe 2 O 3 , an iron ore, in the reaction
bustion and requires enzymes to proceed at body temperature.
2 Fe 2 O 3 (s) + 3 C(s) → 4 Fe(s) + 3 CO 2 ↑
Nonetheless, CO 2 and H 2 O are the products.
The Fe in the ore gained electrons from the carbon, the reduc-
ing agent in this reaction.
10.3 TYPES OF CHEMICAL REACTIONS
The reactions involving hydrocarbons and carbohydrates with
oxygen are examples of an important group of chemical reac- CONCEPTS Applied
tions called oxidation-reduction reactions. Historically, when
the term oxidation was first used, it specifically meant reactions Silver Polish
involving the combination of oxygen with other atoms. But Silverware and silver-plated objects often become tarnished
fluorine, chlorine, and other nonmetals were soon understood when the silver is oxidized by sulfur, forming Ag 2 S.
to have similar reactions to those of oxygen, so the definition was Commercial silver polishes often act by removing the
changed to one concerning the shifts of electrons in the reaction. oxidized layer with an abrasive. The silver can also be
An oxidation-reduction reaction (or redox reaction) is polished by reducing the Ag 2 S back to metallic silver without
broadly defined as a reaction in which electrons are transferred removing a layer. Place the tarnished silver in a clean
from one atom to another. As is implied by the name, such a re- aluminum pan with about 80 g sodium bicarbonate
(NaHCO 3 ) and 80 g NaCl dissolved in each liter of near-
action has two parts and each part tells you what happens to the
boiling water. A sufficient amount should be prepared to
electrons. Oxidation is the part of a redox reaction in which there
cover the silver object or objects. The salts provide ions to
is a loss of electrons by an atom. Reduction is the part of a redox
help transfer electrons and facilitate the reaction. The
reaction in which there is a gain of electrons by an atom. The name reaction is
also implies that in any reaction in which oxidation occurs, reduc-
3 Ag 2 S + 2 Al + 6 H 2 O → 6 Ag + 2 Al(OH) 3 + 3 H 2 S
tion must take place, too. One cannot take place without the other.
Substances that take electrons from other substances are (Note: H 2 S has a rotten egg odor.)
called oxidizing agents. Oxidizing agents take electrons from
the substances being oxidized. Oxygen is the most common
oxidizing agent, and several examples have already been given Many chemical reactions can be classified as redox or non-
about how it oxidizes foods and fuels. Chlorine is another com- redox reactions. Another way to classify chemical reactions is to
monly used oxidizing agent, often for the purposes of bleaching consider what is happening to the reactants and products. This
or killing bacteria (Figure 10.9). type of classification scheme leads to four basic categories of
chemical reactions, which are (1) combination, (2) decomposi-
tion, (3) replacement, and (4) ion exchange reactions. The first
three categories are subclasses of redox reactions. It is in the
ion exchange reactions that you will find the first example of a
reaction that is not a redox reaction.

COMBINATION REACTIONS
A combination reaction is a synthesis reaction in which two
or more substances combine to form a single compound. The
combining substances can be (1) elements, (2) compounds, or
(3) combinations of elements and compounds. In generalized
form, a combination reaction is
X + Y → XY
Many redox reactions are combination reactions. For ex-
ample, metals are oxidized when they burn in air, forming
a metal oxide. Consider magnesium, which gives off a bright
white light as it burns:

2 Mg(s) + O 2 ( g) → 2 MgO(s)
FIGURE 10.9 Oxidizing agents take electrons from other sub-
stances that are being oxidized. Oxygen and chlorine are commonly Note how the magnesium-oxygen reaction follows the general-
used, strong oxidizing agents. ized form of X + Y → XY.

260 CHAPTER 10 Chemical Reactions 10-10