21.1. Acid-Base Definitions http://www.ck12.org
the sulfate ion. Aconjugate baseis the particle produced when an acid donates a proton. The sulfate ion is the
conjugate base of the hydrogen sulfate ion.
A typical Brønsted-Lowry acid-base reaction contains two conjugate acid-base pairs as shown below.
HNO 2 (aq)+PO^34 −(aq)⇀↽NO− 2 (aq)+HPO^24 −(aq)One conjugate acid-base pair is HNO 2 / NO 2 −, while the other pair is HPO 42 −/ PO 43 −.
Lewis Acids and Bases
Gilbert Lewis (1875-1946) proposed a third theory of acids and bases that is even more general than the Arrhenius
and Brønsted-Lowry theories. ALewis acidis a substance that accepts a pair of electrons to form a covalent bond.
ALewis baseis a substance that donates a pair of electrons to form a covalent bond. In a Lewis acid-base reaction,
a new covalent bond is formed, but both of the electrons in that bond were originally present on the base. A hydrogen
ion, which lacks any electrons, can readily accept a pair of electrons to form a covalent bond. It is an acid under
both the Brønsted-Lowry and Lewis definitions. Ammonia has a lone pair on its central nitrogen atom. The reaction
between ammonia and the hydrogen ion can be depicted as shown below.
The lone pair on the nitrogen atom provides both electrons for the new covalent N-H bond, so the NH 3 is a Lewis
base and the H+is a Lewis acid.
Some reactions that do not qualify as acid-base reactions under the other definitions do so under only the Lewis
definition. An example is the reaction of ammonia with boron trifluoride.
Boron trifluoride is the Lewis acid, while ammonia is again the Lewis base. As there is no hydrogen ion involved in
this reaction, it qualifies as an acid-base reaction only under the Lewis definition. TheTable21.3 summarizes the
three acid-base theories.