absorbing energy. If more energy is released than is needed to break the old
bonds, then the reaction is said to be exothermic, releasing energy. The
chemical reactions that provide the energy for our world are exothermic
reactions. In Chapter 9 Thermodynamics, you can read in more depth about
the energy changes that occur during reactions.
Reactions occur because of collisions. One chemical species collides with
another at the right place, transfers enough energy, and a chemical reaction
occurs. Such reactions can be very fast or very slow. In the chapter on
Kinetics, you can study how reactions occur and the factors that affect the
speed of reactions. But in this chapter we will review the balancing of
chemical equations, discuss the general types of chemical reactions,
and describe why these reactions occur.Keywords and Equations
There are no keywords or equations listed on the AP Exam that are specific
to this chapter.General Aspects of Chemical Reactions and Equations
Balancing Chemical Equations
The authors hope that, because you are preparing to take the AP exam, you have already
been exposed to the balancing of chemical equations. We will quickly review this topic and
point out some specific aspects of balancing equations as the different types of chemical
reactions are discussed.
A balanced chemical equation provides many types of information. It shows which
chemical species are the reactants and which species are the products. It may also indicate
in which state of matter the reactants and products exist. Special conditions of temperature,
catalysts, etc., may be placed over or under the reaction arrow. And, very importantly, the
coefficients (the integers in front of the chemical species) indicate the number of each reac-
tant that is used and the number of each product that is formed. These coefficients may
stand for individual atoms/molecules or they may represent large numbers of them called
moles (see the Stoichiometry chapter for a discussion of moles). The basic idea behind the
balancing of equations is the Law of Conservation of Matter, which says that in ordinary
chemical reactions matter is neither created nor destroyed. The number of each type of
reactant atom has to equal the number of each type of product atom. This requires adjust-
ing the reactant and product coefficients—balancing the equation. When finished, the
coefficients should be in the lowest possible whole-number ratio.
Most equations are balanced by inspection. This means basically a trial-and-error,
methodical approach to adjusting the coefficients. One procedure that works well is to bal-
ance the homonuclear (same nucleus) molecule last. Chemical species that fall into this cate-
gory include the diatomic elements, which you should know: H 2 , O 2 , N 2 , F 2 , Cl 2 , Br 2 , and I 2.
This is especially useful when balancing combustion reactions. If a problem states that
oxygen gas was used, then knowing that oxygen exists as the diatomic element is absolutely
necessary in balancing the equation correctly.Periodic Relationships
The periodic table can give us many clues as to the type of reaction that is taking place. One
general rule, covered in more detail in the Bonding chapter, is that nonmetals react with
other nonmetals to form covalent compounds, and that metals react with nonmetals to68 STEP 4. Review the Knowledge You Need to Score High
KEY IDEA