Video Transcript
Which of the following alkanes has
the highest melting point? (A) Ethane, (B) butane, (C)
pentane, (D) propane, or (E) hexane.
Alkanes are saturated
hydrocarbons. This means that the molecule only
contains single-bonded carbon and hydrogen atoms. Straight-chain alkanes are named by
placing a stem which indicates the number of carbon atoms in the molecule in front
of the suffix A-N-E. From this, we can see that ethane
contains two carbon atoms, butane contains four carbon atoms, pentane contains five
carbon atoms, propane contains three carbon atoms, and hexane contains six carbon
atoms. We now need to determine how the
number of carbon atoms in a straight-chain alkane affects its melting point.
To do this, let’s take a look at
the structures of propane and pentane. Shown here is both the displayed
formula and skeletal formula of propane, C3H8. Propane, like all other alkanes, is
nonpolar. So the only force of attraction
that exists between propane molecules is dispersion force. Shown here is the displayed formula
and skeletal formula of pentane, C5H12. Like propane, the only force of
attraction between pentane molecules is dispersion force. But pentane molecules have a larger
surface area and more electrons than propane molecules. This means that there will be
stronger dispersion forces between pentane molecules than propane molecules. Stronger intermolecular forces mean
that more energy is required to separate the molecules during melting. Thus, pentane will have a higher
melting point than propane.
In general, for straight-chain
alkanes, the melting point will increase as the length of the carbon chain
increases. We want to know which of the
alkanes has the highest melting point. This means we are looking for the
alkane with the longest carbon chain, in other words, the straight-chain alkane with
the most carbon atoms. Of the answer choices, hexane has
the most carbon atoms. So the alkane that has the highest
melting point is answer choice (E) hexane.
