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.