Question

You have three covalent compounds with three very different boiling points. All of the compounds have similar molar mass and relative shape. Explain how these three compounds could have very different boiling points.

Short answer

The three covalent compounds could have very different boiling points due to variations in the strength of intermolecular forces present, despite having similar molar mass and shape. One compound might have weak dispersion forces, resulting in a low boiling point, while another might have strong dipole-dipole forces or hydrogen bonding, leading to a higher boiling point. It is the differences in the type and strength of intermolecular forces that cause the variation in boiling points.

Step-by-step solution

Understand intermolecular forces

Intermolecular forces are the forces between molecules. They play a significant role in determining the physical properties of compounds like boiling points. There are three main types of intermolecular forces: dispersion forces (also known as London dispersion forces), dipole-dipole forces, and hydrogen bonding.

Dispersion Forces

Dispersion forces are the weakest intermolecular forces and occur in all compounds, regardless of their polarity. These forces arise from the temporary fluctuations in electron distribution, leading to an instantaneous dipole. The strength of dispersion forces increases with increasing molar mass and size of the molecule. However, in this case, all three compounds have similar molar mass, so the dispersion forces should be roughly the same in all compounds.

Dipole-Dipole Forces

Dipole-dipole forces occur in polar molecules (molecules with a net dipole moment), where the positive end of one molecule is attracted to the negative end of another molecule. Dipole-dipole forces are stronger than dispersion forces, so a compound with dipole-dipole forces present would generally have a higher boiling point compared to a nonpolar compound with similar molecular mass.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative elements such as nitrogen (N), oxygen (O), or fluorine (F). Hydrogen bonds are much stronger than regular dipole-dipole forces and dispersion forces, resulting in compounds with higher boiling points.

Explain the variation in boiling points

Considering the intermolecular forces, the three covalent compounds could have different boiling points due to their differences in the strength of the intermolecular forces present. For example, one compound might have only weak dispersion forces, resulting in a low boiling point, while another compound might have strong dipole-dipole forces or hydrogen bonding, resulting in a higher boiling point. Even though their molar mass and shape are similar, it is the differences in the type and strength of intermolecular forces that could lead to their different boiling points.