Video Transcript
Some substances can be classified as Lewis acids, some can be classified as Arrhenius
acids, and some can even be classified as Brønsted–Lowry acids. Which of the following figures describes the relationship between these different
classification terms?
Let’s start by clearing some space to discuss these three different classification
terms for acids. According to the Arrhenius classification, a substance is an acid if it produces H+
ions in an aqueous medium. For example, when hydrogen chloride gas dissolves in water, it ionizes into H+ and
Cl− ions. However, the hydrogen ions quickly combine with water molecules to form hydronium, or
H3O+, ions. We can recognize the chemical formula HCl aqueous as representing hydrochloric
acid.
An Arrhenius base is a substance that produces OH− ions in an aqueous medium. For example, when solid sodium hydroxide dissolves in water, it ionizes into Na+ and
OH− ions. However, there are substances, like ammonia, which act as bases, but their molecules
do not directly produce OH− ions. Instead, ammonia accepts an H+ ion from another molecule. This led to the Brønsted–Lowry classification, which defines an acid as an H+ ion
donor and a base as an H+ ion acceptor. So, in the reaction with ammonia, water is acting as a Brønsted–Lowry acid.
The Brønsted–Lowry concept, however, fails to classify substances like borane, or
BH3. When borane reacts with ammonia, it acts as an acid, but it doesn’t produce or donate
H+ ions. Substances like borane are electron deficient and classified as Lewis acids. During the reaction, borane accepts an electron pair from ammonia. A new covalent bond forms between the nitrogen and boron atoms, producing a molecule
called an adduct. In the reaction, ammonia acts as a Lewis base and donates a pair of electrons because
it is electron rich.
So, we can define a Lewis acid as a substance that can accept a lone pair of
electrons, forming a bond. And a Lewis base is a substance that can donate a lone pair of electrons, forming a
bond. So, even though the Brønsted–Lowry classification encompasses more acids and bases
than the Arrhenius definition, the Lewis classification of acids and bases is even
broader.
Now, let’s bring the answer choices back up on the screen to select the correct
answer. Since the Lewis definition of an acid is the broadest, the box representing the Lewis
classification in the figure should be the largest. This means we can eliminate answer choices (B), (C), and (E) because the Lewis box is
not the largest in these figures. Since we also know that the Arrhenius definition is the least broad, it should have
the smallest box. Therefore, we can eliminate answer choice (A) and select answer choice (D) as the
correct answer.
In conclusion, the figure that correctly describes the relationship between the
Lewis, Arrhenius, and Brønsted–Lowry classifications of acids is the figure in
answer choice (D).
