In which of the following pure substances will there be hydrogen bonding between the molecules? A) HF, B) C₂H₆, C) O₂, D) CH₂O, E) HI.
Although this question is primarily about hydrogen bonding, firstly, we need to note that we’re dealing with pure substances. So we need only consider interactions between identical molecules from our list of possible answers. Hydrogen bonding occurs when a hydrogen atom is covalently bonded directly to a very electronegative element, namely nitrogen, oxygen, or fluorine. It’s a strong intermolecular force. It’s not to be confused with covalent bonding. The N–H, O–H, or H–F bonds are so polarized but a single proton or positive charge is left exposed at the hydrogen end of each bond. This exposed proton, usually indicated with a delta plus symbol, can form an intermolecular hydrogen bond with a lone pair of electrons on a fluorine, nitrogen, or oxygen atom on an adjacent molecule.
In the diagram shown here, we see an intermolecular hydrogen bond forming between two molecules of water. So to answer this question, we need to decide if the molecules in our list of possible answers meet the criteria for hydrogen bonding. Firstly, let’s look at their structures. All our simple molecules containing covalent bonds. All of the molecules, except for C₂H₆, contain some lone pairs located on some of the atoms. The only molecule that contains a hydrogen atom directly bonded to nitrogen, oxygen, or florine is HF. This is looking like a promising answer.
Oxygen, O₂, contains no hydrogen at all. So it cannot form hydrogen bonds. Oxygen is not a correct answer here. In C₂H₆ and CH₂O, we see hydrogen directly bonded to carbon atoms. The C–H bonds in these molecules are not considered polarized at all. These molecules cannot form hydrogen bonds. C₂H₆ and CH₂O are not the correct answers.
In the HI, hydrogen is bonded directly to iodine. Although iodine is more electronegative than hydrogen, the H–I bond is not as polarized as the H–F bond. And hydrogen bonding is not seen in HI molecules. Iodine is a much larger atom than fluorine due to it having more occupied shells. The lone pairs on the iodine atoms occupy larger orbitals and their charge density is much less than the lone pairs on the fluorine atom in HF. HI is not a correct answer here either.
So in HF, we have all the criteria required for intermolecular hydrogen bonding. HF does form intermolecular hydrogen bonds. And this accounts for its very high boiling point relative to the other hydrogen halides. HF is the correct answer here.