Video Transcript
Which of the following is not correct about compounds that can form hydrogen bonds? (A) They are polar compounds. (B) They have high boiling points. (C) They dissolve in polar solvents. (D) They have high surface tensions. Or (E) they have high vapor pressures.
Hydrogen bonds are dipole–dipole interactions that exist between covalently bonded hydrogen atoms and electron lone pairs on strongly electronegative elements, which are usually fluorine, oxygen, or nitrogen. The highly electronegative element, in this example, the oxygen in water, has a partial negative charge, which we can show by using the symbol 𝛿− because it withdraws most electron density from its bond with the hydrogen atom. The hydrogen atoms have much lower electron densities and carry a partial positive charge, which we can represent using 𝛿+.
Hydrogen bonds form between these oppositely charged partial charges in adjacent molecules. Hydrogen bonds are the strongest intermolecular forces of attraction. And they cause unique physical and chemical properties to molecules that engage in them. Because it requires a lot of thermal energy to overcome these intermolecular forces, molecules that form hydrogen bonds have unusually high boiling points. They also typically have low vapor pressure as they are not prone to spontaneously change from the liquid state to the gaseous state.
The surface tension of a substance is the force per unit area that is needed to expand the surface of a liquid. Substances will have low surface tension if their molecules experience weak intermolecular forces of attraction. A substance with a high surface tension contain molecules that experience strong intermolecular forces of attraction. A liquid has a particularly high surface tension if it has hydrogen–bond forming molecules. This is because these strong hydrogen bonds hold the molecules together tightly, which creates the high tension on the surface.
Using this information, let’s have a look at our answer choices. We can see that answer choices (A), (B), (C), and (D) are all true, as compounds that form hydrogen bonds are polar with partially negative and partially positive portions of their molecules. Because they are polar compounds, they would also dissolve in other polar compounds. Due to their strong intermolecular forces of attractions, they have high boiling points and high surface tensions. Because hydrogen–bond forming molecules would generally have low vapor pressure, the statement that is not correct about compounds that can form hydrogen bonds is answer choice (E): they have high vapor pressures.
