Kaplan MCAT General Chemistry Review

A more general definition of acids and bases was proposed independently by Johannes Brønsted and Thomas Lowry in 1923. A Brønsted-Lowry acid is a species that donates hydrogen ions, while a Brønsted-Lowry base is a species that accepts hydrogen ions. The advantage of this definition over Arrhenius’s is that it is not limited to aqueous solutions. For example, OH^-, NH₃, and F^- are all Brønsted-Lowry bases because each has the ability to accept hydrogen protons. However, neither NH₃ nor F^- can be classified as Arrhenius bases because they do not dissociate to produce OH^- ions in aqueous solutions. You’ll notice, however, being the perceptive student that you are, that according to both of these definitions, there’s only one way for a species to be an acid, and that is to produce a hydrogen ion. The only difference between the two definitions for acidic compounds is the requirement (or lack thereof) of an aqueous medium in the Arrhenius definition. Most acid-base chemistry on the Physical Sciences section of the MCAT will involve the transfer of hydrogen ions in accordance with the Brønsted-Lowry definitions.

MCAT Expertise

The Brønsted-Lowry definition is a more general description and much more useful and common on the MCAT than the other two. It is all about the proton (H⁺).

Brønsted-Lowry acids and bases always occur in pairs because the definitions require the transfer of a proton from the acid to the base. These are conjugate acidbase pairs (see below). For example, H₃O⁺ is the conjugate acid of the base H₂O, and NO₂^- is the conjugate base of HNO₂.

H₃O⁺ (aq) H₂O (aq) + H⁺ (aq)

HNO₂ (aq) NO ₂^- (aq) + H⁺ (aq)

LEWIS

At approximately the same time as Brønsted and Lowry, Gilbert Lewis also proposed definitions for acids and bases. Lewis defined an acid as an electron-pair acceptor (acid = acceptor) and a base as an electron-pair donor. Lewis’s are the most inclusive definitions: Every Arrhenius acid is also a Brønsted-Lowry acid, and every Brønsted-Lowry acid is also a Lewis acid (and likewise for the bases). However, the converse is not true: The Lewis definition encompasses some species not included within the Brønsted-Lowry definition. For example, BCl₃ and AlCl₃ are species that can each accept an electron pair, which qualifies them as Lewis acids, but they will not donate a hydrogen ion, which disqualifies them as Brønsted-Lowry acids (or Arrhenius acids, for that matter). On the MCAT, you may encounter Lewis acids more often in the Biological Sciences section, specifically in the organic chemistry reactions for which Lewis acids act as catalysts, such as in the anti-addition of diatomic halogens to alkenes.

MCAT Expertise

This is the most general and inclusive description. Lewis acids tend to show up as catalysts in organic chemistry reactions.

NOMENCLATURE OF ARRHENIUS ACIDS

The name of an Arrhenius acid is related to the name of the parent anion (the anion that combines with H⁺ to form the acid). Acids formed from anions whose names end in –ide have the prefix hydro- and the ending –ic.

Acids formed from oxyanions are called oxyacids. If the anion ends in -ite (less oxygen), then the acid will end with –ous acid. If the anion ends in –ate (more oxygen), then the acid will end with –ic acid. Prefixes in the names of the anions are retained. Some examples include the following:

MCAT Expertise

There are always some exceptions to the rules. For instance, MnO₄^- is called permanganate even though there are no “manganate” or “manganite” ions, but this will not be important to memorize for Test Day.

Properties of Acids and Bases

Acids and bases are usually characterized according to their relative tendencies either to donate or to accept hydrogen ions. Furthermore, aqueous acid and base solutions can be characterized according to their concentrations of hydrogen and hydroxide ions.

AUTO-IONIZATION OF WATER AND HYDROGEN ION EQUILIBRIA

Since many acid-base reactions take place in water—and on the MCAT this is almost exclusively the case—it is very important that you understand the behavior of acidic and basic compounds vis-à-vis the acid-base behavior of water. Only then can you fully appreciate the meaning and significance of such terms as strong acid or weak base or measurements of pH or pOH.

The Acid-Base Behavior of Water