Water can react with ammonia and
hydrogen chloride according to the following chemical equations. H2O plus NH3 produces NH41+ plus
OH1−. H2O plus HCl produces H3O1+ plus
Cl1−. According to these equations, how
can water be defined in terms of Lewis’s theory? (A) A Lewis acid, (B) a Lewis base,
or (C) both a Lewis acid and a Lewis base.
To answer this question, we must
consider how water is defined in regards to the Lewis acid–base theory. To do this, we must understand the
definitions for a Lewis acid and a Lewis base. A Lewis acid is a substance that
can accept a lone pair of electrons, forming a bond, while a Lewis base can donate
an electron pair. So we must identify whether water
in these chemical reactions is accepting or donating a lone pair of electrons or if
it is doing both.
Let’s have a closer look at these
reactions using Lewis dot structures to determine the behavior of water. In this first reaction, water
reacts with ammonia by accepting a lone pair of electrons to form the hydroxide ion
and the ammonium ion. Because water accepted a lone pair
of electrons in this reaction, it can be described as a Lewis acid.
Let’s have a look at the second
reaction between water and hydrogen chloride. In this reaction, water donates a
lone pair of electrons as it bonds with a hydrogen ion to form the hydronium ion and
the chloride ion. Because water has donated a lone
pair of electrons in this reaction, it is behaving as a Lewis base.
Therefore, water can be classified
as an amphoteric species, which means it can act as an acid in some circumstances
and as a base in others. Therefore, according to the
equations, water can be defined in terms of Lewis’s theory as answer choice (C),
both a Lewis acid and a Lewis base.