The table shows the bond enthalpies for hydrogen bonds and covalent bonds between pairs of atoms. Which column corresponds to the bond enthalpies of hydrogen bonds?
The bond enthalpy is the energy required to break a particular bond in one mole of gaseous particles. We can see the bond enthalpies in column B are much larger than the bond enthalpies in column A. Since the bond enthalpies in column B are much larger, column B must correspond to a much stronger bond. So which type of bond is stronger, hydrogen bonds or covalent bonds?
Hydrogen bonds occur between a hydrogen atom of one molecule and a lone pair on a highly electronegative atom on another molecule. This highly electronegative atom is usually nitrogen, oxygen, or fluorine. The electronegative atom attracts the electron density in the molecule, resulting in a partial negative charge. This negative charge is particularly strong because of the presence of the lone pair. In the other molecule, the hydrogen has a partial positive charge. That makes hydrogen bonding a special kind of dipole–dipole interaction, and a particularly strong one at that. As hydrogen bonding occurs between two molecules, we can more generally call it an intermolecular force.
Covalent bonds, on the other hand, form between two atoms in a molecule. Covalent bonds are formed when two nonmetal atoms share a pair of electrons. Though hydrogen bonding is a very strong intermolecular force, intermolecular forces are all weaker than intramolecular forces. This means that hydrogen bonds must correspond to column A in the table and covalent bonds must correspond to column B in the table. So the column that corresponds to the bond enthalpies of hydrogen bonds is column A.