Question 11.2: One of the compounds shown here is a liquid at room temperat...
One of the compounds shown here is a liquid at room temperature. Which one and why?
\begin{matrix} O \\ \parallel \\ H – C – H \\ \\ \text{Formaldehyde}\end{matrix} \begin{matrix} \underset{|}{H} \\ H – C – F \\ \overset{|}{H} \\ \\ \text{Fluoromethane}\end{matrix} \begin{matrix} H – O – O – H \\ \\ \text{Hydrogen peroxide}\end{matrix}
The three compounds have similar molar masses:
Formaldehyde 30.03 g/mol
Fluoromethane 34.03 g/mol
Hydrogen peroxide 34.02 g/mol
so the strengths of their dispersion forces are similar. All three compounds are also polar, so they have dipole–dipole forces. Hydrogen peroxide, however, is the only compound of the three that also contains H bonded directly to F, O, or N. Therefore it also has hydrogen bonding and is likely to have the highest boiling point of the three. Since the example stated that only one of the compounds was a liquid, you can safely assume that hydrogen peroxide is the liquid. Note that, although fluoromethane contains both H and F, H is not directly bonded to F, so fluoromethane does not have hydrogen bonding as an intermolecular force. Similarly, although formaldehyde contains both H and O, H is not directly bonded to O, so formaldehyde does not have hydrogen bonding either.