Lewis Theory

`H_2CO_3 (aq) ? H_2O(l) + CO_2(g)`

Since it can be made by removing H₂O from H₂CO₃, CO₂ is called the acid anhydride of H₂CO₃. (The term anhydride is derived from anhydrous, 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 concentration when dissolved in water; for example,

`P4O_(10)(s) + 6H_2O(l) -> 4H_3 PO_4 (aq)`

`H_3 PO_4 (aq) + H_2 O(l) ? H_3 O+(aq) + H_2 PO_4^(-) (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 pair 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 bond.

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 proton (H⁺) 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 8 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) `BeCl_2(g)`

(b) `CH_3 OH`

(c) `SO_2`

(d) `CF_4`

Worked Problem