Cracking the AP Chemistry Exam
Part IV
Content Review for the AP Chemistry Exam
Chapter 8
Big Idea #6: Equilibrium, Acids and Bases, Titrations, and Solubility
ACIDS AND BASES DEFINITIONS
Arrhenius
S. A. Arrhenius defined an acid as a substance that ionizes in water and produces hydrogen ions (H+ ions). For instance, HCl is an acid.
HCl→H++Cl−
He defined a base as a substance that ionizes in water and produces hydroxide ions (OH− ions). For instance, NaOH is a base.
NaOH → Na++OH−
Brønsted-Lowry
J. N. Brønsted and T. M. Lowry defined an acid as a substance that is capable of donating a proton, which is the same as donating an H+ ion, and they defined a base as a substance that is capable of accepting a proton. This definition is the one that will be used most frequently on the exam.
Look at the reversible reaction below.
HC2H3O2+H2O↔C2H3O2−+H3O+
According to Brønsted-Lowry
HC2H3O2 and H3O+ are acids.
C2H3O2− and H2O are bases.
Now look at this reversible reaction.
NH3+H2O↔NH4++OH−
According to Brønsted-Lowry
NH3 and OH− are bases.
H2O and NH4+ are acids.
So in each case, the species with the H+ ion is the acid, and the same species without the H+ ion is the base; the two species are called a conjugate pair. The following are the acid-base conjugate pairs in the reactions above:
HC2H3O2 and C2H3O2−
NH4+ and NH3
H3O+ and H2O
H2O and OH−
Notice that water can act either as an acid or base. Any substance which has that ability is called amphoteric.