Fundamentals of Acids and Bases

Acid  -  A substance that has the potential to donate a proton or accept an electron pair.

Acidic  -  Having a pH less than 7 or a pOH greater than 7.

Amphiprotic  -  A species that can either donate or accept a proton, e. g. water.

Amphoteric  -  A species that can either donate or accept a hydroxide ion, such as Al(OH)₃. Many chemistry texts incorrectly use this term to mean that a substance can act as either an acid or a base.

Arrhenius Model  -  Arrhenius proposed that acids are substances that produce protons (H⁺) in aqueous solution, whereas bases produce hydroxide ions (OH^-) in aqueous solution. Compare his model with the Bronsted-Lowry definition and the Lewis definition.

Base  -  A substance that can accept a proton, release OH^-, or donate an electron pair.

Basic  -  Having a pH greater than 7 or a pOH less than 7.

Bronsted-Lowry Definition  -  Bronsted and Lowry define an acid as a proton (H⁺) donor and a base as a proton acceptor. Compare this model with the Arrhenius Model and the Lewis definition.

Conjugate Acid  -  A molecule that can be described as a base that has gained one proton.

Conjugate Base  -  A molecule that can be described as an acid that has lost one proton.

Dissociate  -  Separate into its ion constituents.

Lewis Definition  -  Lewis defined an acid as an electron pair acceptor and a base as an electron pair donor. Compare his model with the Arrhenius model and the Bronsted-Lowry definition.

Oxyacid  -  A molecule of the form AO_n(OH)_m, where A is a non-metal.

Pauling-Ricci rule  -  For oxyacids, the more electron withdrawing (more electropositive) the non- metal center, the stronger the acid due to a weakening of the O-H bond. This trend is approximated by the equation: PK _a = 8 - 9f + 4n , where f is the formal charge on A when all oxygens are singly bound to A, and n represents the number of O atoms bound to A that are not bound to an H.

pH  -  A measure of the hydrogen ion concentration, it is equal to - log [H⁺], where [H⁺] is the concentration of protons.

pK _a  -  A measure of the strength of an acid, it is equal to – log K _a, where K _a is the acid dissociation constant in water.

pK _b  -  A measure of the strength of a base, it is equal to – log K _b, where K _b is the base dissociation constant in water.

pOH  -  A measure of the hydroxide ion concentration, it is equal to - log [OH^-], where [OH^-] is the concentration of hydroxide ions.

Strong Acid  -  An acid with a pK _a less than zero. Strong acids completely dissociate in water.

Strong Base  -  A base with a pK _b less than zero. Strong bases completely dissociate in water.

Weak Acid  -  An acid with a pK _a greater than zero. Weak acids do not completely dissociate in water.

Weak Base  -  A base with a pK _b greater than zero. Weak bases do not completely dissociate in water.