Video Transcript
In which of the following phases
can alkanes exist at room temperature? (A) Solids and liquids; (B) solids,
liquids, and gases; (C) solids and gases; (D) gases; (E) liquids and gases.
In this question, we need to
identify all the physical states possible for alkanes at room temperature. First of all, alkanes are saturated
hydrocarbons with only single bonds between carbon atoms. Methane is the smallest alkane and
has the chemical formula CH4. Decane is a much larger alkane and
has the chemical formula C10H22. Both molecules are alkanes because
they contain only single bonds between the carbon atoms.
An important relationship that can
help us solve this problem is that alkanes with longer carbon chains tend to have
higher melting and boiling points than alkanes with shorter carbon chains. The melting and boiling point of
methane are well below room temperature. Therefore, methane is a gas at room
temperature. We can see that decane has a
significantly higher melting and boiling point than methane. Because the melting point is below
room temperature and the boiling point is above room temperature, decane is a liquid
at room temperature.
Alkane molecules with longer carbon
chains tend to have higher melting and boiling points because there are stronger
dispersion forces between molecules. This is because the longer the
carbon chain becomes, the greater the surface area for contact between
molecules.
Let’s complete the following table
for straight-chain alkanes. If the straight-chain alkane
contains from one to four carbon atoms, it will be a gas at room temperature. This is because these alkanes have
the weakest dispersion forces between molecules and very little thermal energy is
needed to disrupt these forces and change the physical state. Straight-chain alkanes with five to
16 carbon atoms will be liquids at room temperature because they have stronger
dispersion forces between molecules. Finally, if the straight-chain
alkane contains 17 or more carbon atoms, it will be a solid at room temperature. These alkanes have the strongest
dispersion forces, and a large amount of thermal energy is needed to disrupt the
dispersion forces and change the phase.
So, in which of the following
phases can alkanes exist at room temperature? The answer is solids, liquids, and
gases, or answer choice (B).
