Video Transcript
Which of the following is the
correct order of boiling points of carboxylic acids from lowest to highest? (A) Ethanoic acid, methanoic acid,
propanoic acid, butanoic acid. (B) Butanoic acid, propanoic acid,
ethanoic acid, methanoic acid. (C) Methanoic acid, propanoic acid,
butanoic acid, ethanoic acid. (D) Methanoic acid, ethanoic acid,
butanoic acid, propanoic acid. Or (E) methanoic acid, ethanoic
acid, propanoic acid, butanoic acid.
First off, the two factors that
affect the boiling point of a liquid are the pressure being exerted on the liquid,
also known as the external pressure, and the types of intermolecular forces, which
are defined as attractive or repulsive forces between molecules. Because the information in the
question and answer choices provided are not focused on the external pressure, but
rather on the chemical names of carboxylic acids and therefore their chemical
structures, we will focus on analyzing the intermolecular forces of the carboxylic
acid molecules listed in the answer choices.
Recall that carboxylic acids are
organic molecules with a carboxyl group as the signature functional group present in
the molecule and an R group that varies depending on the carboxylic acid being
discussed. Taking a closer look at the
carboxylic acid chemical names listed in the answer choices, the ending “-oic acid”
indicates each of the four compounds contains one carboxyl group. If we reference the prefix list
used in naming organic compounds, we will notice the first four prefixes, meth-,
eth-, prop-, and but-, indicate chain lengths of one, two, three, and four carbon
atoms long. Lastly, the -an- in all four
compounds indicates the hydrocarbon portion of the molecule contains only single
bonds.
Upon analyzing the chemical
structures of these carboxylic acid molecules, the intermolecular forces present are
hydrogen bonding and dispersion forces. The question we should ask
ourselves is, which intermolecular force mentioned, if not both, should we focus on
to answer this question? One might think we should focus on
hydrogen bonding because it is more notably the stronger intermolecular force and
should have more of an effect on a liquid’s ability to vaporize. However, each of the four different
carboxylic acids listed contain one carboxyl group. And the carboxyl group is
responsible for the hydrogen bonding experienced in carboxylic acids.
Since the four carboxylic acids
each have the same number of carboxyl groups in their chemical structures, it means
the overall effects of this intermolecular force on the boiling points will be
similar for all molecules mentioned and will not assist in determining the order of
their boiling points for this question. The remaining intermolecular force
that these carboxylic acid molecules exhibit is known as dispersion forces. If we take a look at the overall
structural formulas for our four carboxylic acid molecules, we will notice the main
difference between them is their R group or alkyl chain.
In regards to dispersion forces,
the larger the molecule, the more electrons available to create temporary dipoles
that attract one another, the stronger the dispersion forces between molecules. Analyzing the skeletal formulas for
methanoic acid, the carboxylic acid with the shortest chain length or smallest R
group, and comparing it with the carboxylic acid with the longest chain length or
the largest R group, butanoic acid, we can see when another identical carboxylic
acid molecule comes in close proximity to each, the molecule with the longer chain
length and larger molecular mass has more electrons and will exhibit stronger
dispersion forces than the molecule with the shorter chain length.
This is because with more electrons
and more surface area contact between molecules with longer chain lengths, the
partial positive and partial negative charges will be larger and will generate a
stronger attraction or stronger dispersion forces between molecules. This will require more energy to
separate these molecules and will result in a higher boiling point. Based on this reasoning, the
correct answer choice should list the carboxylic acids in order of increasing chain
length to depict them in order of increasing boiling points.
If we were to compare the chain
length of the R groups, the chain length increases gradually going from methanoic
acid to ethanoic acid, so we can rule out answer choices (A), (B), and (C), and
gradually increases from ethanoic acid to propanoic acid and finally propanoic acid
to butanoic acid. This means the correct answer
choice is (E).
