Lewis Acids and Bases
Many oxyacids are rather unstable and cannot be isolated in pure form. An example is carbonic acid, H2CO3, which decomposes to water and carbon dioxide:
H2CO3(aq) H2O(l) + CO2(g)
Since it can be made by removing H2O from H2CO3, CO2 is called the acid anhydrideA substance that reacts with water to form an acid or a base. A substance from which water has been removed. of H2CO3. (The term anhydride is derived from anhydrousFree of water. Often used to describe a solid having no water of crystallization or a solvent from which traces of water have been removed., meaning “not containing water.”) Acid anhydrides are usually oxides of nonmetallic elements. Some common examples and their corresponding oxyacids are SO2—H2SO3; SO3—H2SO4; P4O10—H3PO4; N2O5—HNO3. Any of these anhydrides increases the hydronium-ion concentrationA measure of the ratio of the quantity of a substance to the quantity of solvent, solution, or ore. Also, the process of making something more concentrated. when dissolved in water; for example,
P4O10(s) + 6H2O(l) → 4H3PO4(aq)
H3PO4(aq) + H2O(l) H3O+(aq) + H2PO4–(aq)
In the Arrhenius sense, then, acid anhydrides are acids, but according to the Brönsted-Lowry definition, they are not acids because they contain no hydrogen.
In 1923, at the same time that the Brönsted-Lowry definition was proposed, G. N. Lewis suggested another definition which includes the acid anhydrides and a number of other substances as acids. According to the Lewis definition, an acid is any species which can accept a lone pairIn a covalently bonded molecule or ion, a pair of electrons not shared between two atoms and hence not involved in a bond. of electrons, and a base is any species which can donate a lone pair of electrons. An acid-base reaction in the Lewis sense involves formation of a coordinate covalent bondA bond between two atoms in which the shared electrons are considered to be contributed by only one of the atoms..
The Lewis definition has little effect on the types of molecules we expect to be basic. All the Brönsted-Lowry bases, for example, NH3, O2–, H–, contain at least one lone pair. Lewis’ idea does expand the number of acids, though. The protonThe positively charged particle in an atomic nucleus; its mass is similar to the mass of a hydrogen atom. is not the only species which can form a coordinate covalent bond with a lone pair. Cations of the transition metals, which are strongly hydrated, do the same thing:
So can electron deficient compounds such as boron trifluoride:
EXAMPLE 1 Identify the Lewis acids and bases in the following list. Write an equation for the combination of each acid with the Lewis base H2O.
(a) BeCl2(g); (b) CH3OH; (c) SO2; (d) CF4.
a) The Lewis diagram
shows that Be is electron deficient. Therefore BeCl2(g) is a Lewis acid. Because of the lone pairs on the Cl atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume., BeCl2 can also act as a Lewis base, but Cl is rather electronegative and reluctant to donate electrons, so the Lewis base strength of BeCl2 is less than the Lewis acid strength.
b) There are lone pairs on O in CH3OH, and so it can serve as a Lewis base.
c) The S atom in SO2 can accept an extra pair of electrons, and so SO2 is a Lewis acid. The O atoms have lone pairs but are only weakly basic for the same reason as the Cl atoms in part (a).
d) Although there are lone pairs on the F atoms, the high electronegativityThe tendency of an atom (nucleus and core electrons) within a molecule to attract electrons in bonds. of F prevents them from being donated to form coordinate covalent bonds. Consequently CF4 has essentially no Lewis-base character.
Many Lewis acid-base reactions occur in media other than aqueous solution. The Brönsted-Lowry theory accounts for almost all aqueous acid-base chemistry. Therefore the Brönsted-Lowry concept is most often intended when the words acid or base are used. The Lewis definition is useful when discussing transition-metal ions, however, and is discussed again in the sections on Metals.