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OVERVIEW

We live in a chemical world that has been partly manufactured through controlled chemical change. Consider all of
the synthetic fibers and plastics that are used in clothing, housing, and cars. Consider all the synthetic flavors and
additives in foods, how these foods are packaged, and how they are preserved. Consider also the synthetic drugs and
vitamins that keep you healthy. There are millions of such familiar products that are the direct result of chemical
research. Most of these products simply did not exist 60 years ago.
Many of the products of chemical research have remarkably improved the human condition. For example,
synthetic fertilizers have made it possible to supply food in quantities that would not otherwise be possible.
Chemists learned how to take nitrogen from the air and convert it into fertilizers on an enormous scale. Other
chemical research resulted in products such as weed killers, insecticides, and mold and fungus inhibitors.
Fertilizers and these products have made it possible to supply food for millions of people who would otherwise
have starved (Figure 10.1).
Yet we also live in a world with concerns about chemical pollutants, the greenhouse effect, acid rain, and a
disappearing ozone shield. The very nitrogen fertilizers that have increased food supplies also wash into rivers,
polluting the waterways and bays. Such dilemmas require an understanding of chemical products and the benefits
and hazards of possible alternatives. Understanding requires a knowledge of chemistry, since the benefits, and risks,
are chemical in nature.
Chapters 8 and 9 were about the modern atomic theory and how it explains elements and how compounds are
formed in chemical change. This chapter is concerned with describing chemical changes and the different kinds of
chemical reactions that occur. These reactions are explained with balanced chemical equations, which are concise
descriptions of reactions that produce the products used in our chemical world.

10.1 CHEMICAL FORMULAS
formulas can be written to show how the atoms are arranged in
In chapter 9, you learned how to name and write formulas for the molecule. Formulas that show the relative arrangements are
ionic and covalent compounds, including the ionic compound called structural formulas. Compare the structural formulas in
of table salt and the covalent compound of ordinary water. the illustration with the three-dimensional representations and
Recall that a formula is a shorthand way of describing the the molecular formulas.
elements or ions that make up a compound. There are basically How do you know if a formula is empirical or molecular?
three kinds of formulas that describe compounds: (1) empirical First, you need to know if the compound is ionic or cova-
formulas, (2) molecular formulas, and (3) structural formulas. lent. You know that ionic compounds are usually composed
Empirical and molecular formulas, and their use, will be con- of metal and nonmetal atoms with an electronegativity dif-
sidered in this chapter. Structural formulas will be considered ference greater than 1.7. Formulas for ionic compounds are
in chapter 12. always empirical formulas. Ionic compounds are composed
An empirical formula identifies the elements present in of many positive and negative ions arranged in an electrically
a compound and describes the simplest whole number ratio of neutral array. There is no discrete unit, or molecule, in an
atoms of these elements with subscripts. For example, the em- ionic compound, so it is only possible to identify ratios of
pirical formula for ordinary table salt is NaCl. This tells you that atoms with an empirical formula.
the elements sodium and chlorine make up this compound, and Covalent compounds are generally nonmetal atoms bonded
there is one atom of sodium for each chlorine atom. The empiri- to nonmetal atoms in a molecule. You could therefore assume
cal formula for water is H 2 O, meaning there are two atoms of that a formula for a covalent compound is a molecular formula
hydrogen for each atom of oxygen. unless it is specified otherwise. You can be certain it is a molecu-
Covalent compounds exist as molecules. A chemical for- lar formula if it is not the simplest whole number ratio. Glucose,
mula that identifies the actual numbers of atoms in a molecule is for example, is a simple sugar (also known as dextrose) with
known as a molecular formula. Figure 10.2 shows the structure the formula C 6 H 12 O 6 . This formula is divisible by 6, yielding a
of some common molecules and their molecular formulas. Note formula with the simplest whole number ratio of CH 2 O. There-
that each formula identifies the elements and numbers of at- fore, CH 2 O is the empirical formula for glucose, and C 6 H 12 O 6 is
oms in each molecule. The figure also indicates how molecular the molecular formula.

252 CHAPTER 10 Chemical Reactions 10-2