Video Transcript
Which of the following statements
best defines a Lewis acid? (A) A substance that can donate a
pair of electrons, (B) a substance that can accept a pair of electrons, (C) a
substance that can donate an H+ ion, (D) a substance that can accept an H+ ion, or
(E) a substance that produces OH− ions.
The first thing that’s important to
identify is that we’re not looking for a simply correct answer. We’re looking for the best
statement out of the five. Lewis described acidity and
basicity in terms of the acceptance or donation of lone pairs of electrons. He describes substances like
ammonia with reactive lone pairs as bases and electron-deficient substances, like
BH3, as acids. When the two react, a bond is
formed between the area with the lone pair and the electron-deficient area.
The first statement suggests that a
Lewis acid is a substance that can donate a pair of electrons. This corresponds to the description
of a Lewis base, not a Lewis acid. The second statement suggests that
a Lewis acid accepts a pair of electrons. This fits nicely with what we’ve
already seen. So, let’s hold on to this answer
and check the other three.
Our third option is that a Lewis
acid is a substance that can donate a hydrogen ion. These would be substances like
hydrochloric acid and nitric acid. Here’s the reaction of a common
base sodium hydroxide with hydrochloric acid, forming sodium chloride and water. This reaction does conform to the
description of Lewis acids and bases because a lone pair from the hydroxide ion is
attacking the hydrogen ion.
Statement (C) is not our correct
answer because even though a substance that can donate a hydrogen ion is a Lewis
acid, not all Lewis acids donate hydrogen ions. So, this statement is not the best
description. Instead, this type of acid is
commonly referred to as a Brønsted–Lowry acid. The next statement changes the game
by talking about accepting hydrogen ions. This is a good definition of a
Brønsted–Lowry base and not a Lewis acid.
Finally, the last statement
suggests that a Lewis acid is a substance that produces OH− ions, hydroxide
ions. A good example of this is the
addition of solid sodium hydroxide to water, producing hydroxide ions in
solution. Sodium hydroxide is an Arrhenius
base. It dissociates in water to produce
hydroxide ions. The general term that covers
statement (E) is alkali because there are some substances that are not Arrhenius
bases that still react with water to produce hydroxide ions. Either way, this is definitely not
an acid and definitely not a Lewis acid, meaning of the five statements we’ve been
given, the one that best defines a Lewis acid is a substance that can accept a pair
of electrons.
