5 Steps to a 5 AP Chemistry 2019

(^70) › STEP 4. Review the Knowledge You Need to Score High
Reactions questions will always appear in the free-response section of the AP exam.
AP Exam Format
Beginning with the 2007 AP exam, the treatment of chemical reactions was changed from
previous years. In the past, the free-response questions concerning chemical reactions
simply involved the formulas of the reactants and products chosen from a series of reac-
tions. You were not expected to write balanced chemical equations. However, under the
current AP Chemistry exam format you no longer are able to choose from a list of reactions.
You are expected to write a balanced chemical equation for every reaction given and answer
one or more questions about each reaction. If the reaction occurs in aqueous solution, you
will have to write the net ionic equation for the process. For the reactions question of the
AP exam, you will be expected not only to balance the equation, but also to have an under-
standing of why the reaction occurs. The reactions and concepts described may also appear
in other parts of the AP exam, such as the multiple-choice sections. Again, you will need to
have an understanding of why a particular reaction occurs. As you study this chapter, pay
particular attention to the explanations that accompany the reactions and equations. You
will be expected to demonstrate your understanding on the AP exam.
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 tempera-
ture, catalysts, etc. may be placed over or under the reaction arrow. And, very important,
the coefficients (the integers in front of the chemical species) indicate the number of each
reactant 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 Chapter 7, Stoichiometry, 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
balance the homonuclear (same nucleus) molecule last. Chemical species that fall into this
category 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.
TIP