Page 11 - E-LKM ACID BASE_Neat

Lembar Kerja Mahasiswa - Asam Basa

The ammonia molecule thus acts as a Lewis base. The boron atom
in BF , having only six electrons in its valence shell and needing two
3
more to achieve an octet, accepts the pair of electrons from the

ammonia molecule. Hence, BF is functioning as a Lewis acid.
3

As this example illustrates, Lewis bases are substances that have

completed valence shells and unshared pairs of electrons (e.g., NH ,
3
-
H O, and O ). A Lewis acid, on the other hand, can be a substance
2
2
with an incomplete valence shell, such as BF or H .
+
3
A substance can also be a Lewis acid even when it has a central
atom with a complete valence shell. This works when the central
atom has a double bond that, by the shifting of an electron pair to an
adjacent atom, can make room for an incoming pair of electrons
from a Lewis base. Carbon dioxide is an example. When carbon

dioxide is bubbled into aqueous sodium hydroxide, the gas is
instantly trapped as the bicarbonate ion.

- -
CO + OH HCO 3(aq)
(aq)
2(aq)
Lewis acid–base theory represents the movement of electrons in this

reaction as follows.

-
The donation of an electron pair from the oxygen of the OH ion
_
produces a bond, so the OH ion is the Lewis base. The carbon atom
of the CO accepts the electron pair, so CO is the Lewis acid.
2
2
(Jespersen, 2010)

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