Suggest, giving reasons, which substance in each of the following pairs is likely to have the higher normal boiling point: (a) \(\mathrm{H}_{2} \mathrm{~S}\) or \(\mathrm{H}_{2} \mathrm{O}\); (b) \(\mathrm{NH}_{3}\) or \(\mathrm{PH}_{3}\); (c) \(\mathrm{KBr}\) or \(\mathrm{CH}_{3} \mathrm{Br}\); (d) \(\mathrm{CH}_{4}\) or \(\mathrm{SiH}_{4}\).

Understanding intermolecular forces (IMFs) is crucial when we examine boiling points of different substances. Intermolecular forces are the attractions between molecules, which are weaker than the bonds within molecules, such as covalent bonds. The strength of these forces significantly influences the boiling point of a compound: stronger intermolecular forces require more energy—in the form of heat—to overcome, leading to a higher boiling point. There are three primary types of IMFs:

• London dispersion forces: These are temporary, fleeting attractions due to momentary dips in electron distribution, present in all molecules.
• Dipole-dipole interactions: These occur between polar molecules, where the positive end of one molecule is attracted to the negative end of another.
• Hydrogen bonding: A particularly strong type of dipole-dipole interaction present when hydrogen is bonded to a very electronegative atom like nitrogen, oxygen, or fluorine.

The ability to recognize these different IMFs and understand their relative strengths is key to predicting and comparing boiling points.

Among the various intermolecular forces, hydrogen bonding stands out due to its significant influence on physical properties like boiling point. Hydrogen bonds are not actual bonds but rather strong dipole-dipole attractions that occur when hydrogen is covalently bonded to a highly electronegative atom.

Criteria for Hydrogen Bonding

Hydrogen bonding can only occur when hydrogen is bonded to either nitrogen, oxygen, or fluorine—the three most electronegative elements. This strict criterion is because these atoms are small and highly electronegative, which makes them capable of pulling the electron density away from hydrogen, creating a substantial dipole. The positively charged hydrogen atom of one molecule is then strongly attracted to the lone pair of a neighboring electronegative atom.In the context of boiling points, hydrogen bonding leads to a much higher boiling point because the intermolecular attraction is significantly stronger, requiring more energy to disrupt during the transition from liquid to gas.

Ionic and covalent bonds are two major types of chemical bonds that hold atoms together within a molecule or compound. Ionic bonds occur between a metal and a nonmetal and involve the complete transfer of one or more electrons from one atom to another. Due to the resulting charged ions, ionic compounds tend to have high melting and boiling points.In contrast, covalent bonds involve the sharing of electrons between two nonmetals. The molecules formed via covalent bonding typically have lower melting and boiling points than ionic compounds. This is because the forces between molecules (intermolecular forces), which must be overcome to change phases, are generally weaker than the forces holding ions together in an ionic compound (ionic bonds).When comparing substances with ionic and covalent bonds (like in pair c of the exercise), the ionic compound usually has the higher normal boiling point due to the strong electrostatic attraction between oppositely charged ions.

London dispersion forces are the weakest of all intermolecular forces, yet they are universally present in all molecular substances. These forces are temporary and arise from fluctuations in the electron distribution in atoms or molecules. The momentary uneven electron distribution creates a temporary dipole, which induces a dipole in a neighboring atom or molecule.

Factors Influencing London Dispersion Forces

• Number of electrons: More electrons mean stronger dispersion forces because there are more electrons to polarize and create temporary dipoles.
• Size of the atom or molecule: Larger atoms or molecules have more diffuse electron clouds, which can be more easily polarized, again leading to stronger dispersion forces.

Substances with only London dispersion forces (like non-polar molecules) have lower boiling points as these forces are weaker and easier to overcome. However, the magnitude of London dispersion forces can vary, and in cases like pair d in the exercise, the larger size and greater electron count of \texttt{SiH}\(_4\) relative to \texttt{CH}\(_4\) results in stronger intermolecular attractions and thus a higher boiling point.