Video Transcript
Which of the following statements
correctly describes the difference between a strong and a weak acid? (A) In an aqueous solution, a
strong acid ionizes partially, whilst a weak acid ionizes completely. (B) A strong acid is more
concentrated than a weak acid. (C) In an aqueous solution, a
strong acid will contain fewer ions than a weak acid. (D) In an aqueous solution, a
strong acid ionizes completely, whilst a weak acid only partially ionizes. (E) A weak acid is more
concentrated than a strong acid.
In this question, we are being
asked to identify the statement that is a correct description of the difference
between a strong and a weak acid. When reading through the answer
choices, we notice that choices (A), (C), and (D) describe ionization or the number
of ions in an aqueous solution, while choices (B) and (E) compare the concentration
of strong and weak acids. To solve this problem, we’ll need
to have a better understanding of both of these properties.
According to the Bronsted–Lowry
theory of acids and bases, an acid is a substance that donates a hydrogen ion during
a reaction. In general, when an acid dissolves
in water, hydrogen ions are released from the acid molecules and are accepted by the
water molecules, forming H3O+ ions. What is left behind of the acid
molecule is a negatively charged ion. The process by which the acid
molecules dissociate to form separate ions is called ionization. And the strength of an acid is
directly related to an acid’s ability to ionize.
On the other hand, solution
concentration is a measure of the amount of solute that is dissolved in a solvent or
a solution. The strength of an acid does not
depend on its concentration. Knowing this allows us to eliminate
answer choices (B) and (E) because in these answer choices, strong and weak acids
are being compared based on their concentration.
To help us select the correct
answer from the remaining answer choices, we’ll need to get a better understanding
of how strong and weak acids ionize. The beakers shown in the cartoon
contain acidic solutions that have the same concentration. When the acid in the top beaker
dissolved in water, all of the acid molecules dissociated into ions. We would say that the acid ionized
completely. In contrast, in the bottom beaker,
when the acid dissolved in water, only some of the acid molecules dissociated into
ions. We would say that the acid ionized
partially.
When acids ionize completely, the
ions produced cannot react to produce more acid molecules. However, if an acid ionizes
partially, like the acid in the bottom beaker, the reaction is reversible. The chemical equation for a
partially ionized acid will use the equilibrium arrow. And the ions produced can react to
reform some of the acid molecules. Acids that only partially ionize
when dissolved in water are weak acids, and acids that completely ionize when
dissolved in water are strong acids.
Let’s take a look at answer choice
(A). Answer choice (A) states that a
strong acid ionizes partially and a weak acid ionizes completely. These descriptions are the opposite
of what is true. Strong acids ionize completely, and
weak acids ionize partially. So we can eliminate answer choice
(A).
Now let’s look at answer choice
(C). Answer choice (C) states that a
strong acid contains fewer ions than a weak acid. If two acidic solutions have the
same concentration, such as what we see in the cartoon, we would expect that the
strong acid would contain more ions. For this reason, we can eliminate
answer choice (C).
The statement that correctly
describes the difference between a strong and a weak acid is answer choice (D); in
an aqueous solution, a strong acid ionizes completely, whilst a weak acid only
partially ionizes.
