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.