LEWIS ACIDS AND BASES (15.12)
STUDY OBJECTIVES
- Define acids and bases according to the Lewis system.
- Identify Lewis acids and bases in given chemical equations.
Electron-Pair Donors and Acceptors.
A more general approach to acids and bases was proposed in 1923 by
G. N. Lewis who also developed the use of electron dot structures. Lewis
noticed that for a H+ ion to be accepted by a base, the
base must possess at least one unshared electron-pair. The electron pair forms
the bond to the proton.
A Lewis base is a molecule or ion that can donate a pair of electrons
to form a bond. A Lewis acid is a molecule or ion that can accept a pair
of electrons and form a bond.
The formation of ammonium ion from H+ and ammonia is a
simple example. The ammonia molecule is the Lewis base because it donates a
pair of electrons to H+ to form a bond. By accepting the
pair of electrons the proton is a Lewis acid. Note that a new covalent bond
is formed by the donation of the electron pair. Recall
when both electrons of the shared pair are donated by the same atom the bond
is called a coordinate covalent bond. An acid-base reaction in
the Lewis system is donation of an electron-pair to form a new covalent bond
between the acid and the base.
Metal cations can be Lewis acids because they have lost their valence electrons
and so have at least one vacant orbital in which to accept an electron-pair
from a base. Consider the beryllium ion which can form four coordinate covalent
bonds.
Be2+(aq) + 4H2O(l) 
[Be(H2O)4]2+
Water molecules with unshared electron-pairs on their oxygen atoms are Lewis
bases and beryllium is a Lewis acid because it accepts electron-pairs to form
Be—OH2 bonds. Beryllium can form four Be—OH2 bonds.
EXAMPLE Lewis Acids and Bases
Identify the Lewis acids and bases in each of the following reactions.
| Ag+(aq) |
+ |
Cl–(aq) |
AgCl(s) |
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Correct!
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| Hg2+(aq) |
+ |
4I– |
HgI42-(aq) |
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Correct!
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| BF3(g) |
+ |
NF3(g) |
F3N—BF3(s) |
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Correct!
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| SO2(g) |
+ |
H2O(l) |
H2SO3(aq) |
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Correct!
Click a Hint button for help.
Lewis acids are species that accept electron-pairs. In each of the above reactions
the first reactant species is the Lewis acid, and the second reactant is the
Lewis base (electron-pair donor). The Lewis acids in (a) and (b) are capable
of accepting electron-pairs because they are positive ions and have previously
lost electrons. In (c) the boron atom has an incomplete octet. And in (d) the
S atom in SO2 is "oxidized," that is, its valence electrons are shifted
toward the more electronegative oxygen atoms and away from the S atom. In all
four reactions new coordinate covalent bonds are formed.
OBJECTIVE CHECK
Complete the following questions to check your understanding of the material.
Select the check button to see if you answered correctly.
Why are nitrogen compounds and ions such as NH3, CH3NH2,
and good examples of Lewis bases?
Why can H+ ion, Mg2+ ion, and Al3+
ion act as Lewis acids?
