Suggest, giving reasons, which substance in each pair is likely to have the higher melting point (Lewis structures may help your arguments): (a) HCl or NaCl; (b) \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{OC}_{2} \mathrm{H}_{3}\) (diethyl ether) or \(\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{OH}\) (butanol); (c) \(\mathrm{HF}\) or \(\mathrm{HCl}\); (d) \(\mathrm{H}_{2} \mathrm{O}\) or \(\mathrm{CH}_{3} \mathrm{OH}\).

Understanding the intermolecular forces is essential when comparing the melting points of different substances. These forces are the attractions between molecules, determining the substance's phase (solid, liquid, gas) at a given temperature. In substances with stronger intermolecular forces, more energy is required to disrupt these interactions, hence they generally have higher melting and boiling points.

There are several types of intermolecular forces, including London dispersion forces, dipole-dipole interactions, and hydrogen bonding. London dispersion forces, also known as van der Waals forces, are the weakest and are present in all molecules. They originate from the temporary fluctuations in electron distribution within molecules. Dipole-dipole interactions occur between polar molecules where there's an uneven distribution of electrons. These are stronger than London dispersion forces but weaker than hydrogen bonds.

• London Dispersion Forces: Present in all molecules, weakest force.
• Dipole-Dipole Interactions: Occur in polar molecules, moderate strength.
• Hydrogen Bonding: Strongest type of van der Waals force.

When we look at different substances, analyzing these forces gives us good insight into why some substances melt at higher temperatures than others.

Hydrogen bonding is a specific, highly directional type of dipole-dipole interaction and is particularly strong compared to other intermolecular forces. It occurs when a hydrogen atom is bonded to a highly electronegative atom, such as nitrogen, oxygen, or fluorine, and this arrangement leads to a significant dipole. The hydrogen atom has a partial positive charge and is attracted to the electronegative element of another molecule.

This interaction is much stronger than ordinary dipole-dipole forces because of the higher electronegativities involved and the often smaller size of H-bonded molecules, allowing them to get closer together. In the context of the provided exercise, butanol and water can form hydrogen bonds due to their hydroxyl groups, resulting in higher melting points compared to their counterparts, diethyl ether and methanol, respectively.

Ionic bonds are electrostatic forces of attraction between oppositely charged ions. These bonds are formed when electrons are transferred from one atom to another, resulting in a cation (positively charged) and an anion (negatively charged). Ionic compounds, such as NaCl mentioned in the exercise, consist of a lattice of these ions, bound together by this strong interaction.

The strength of ionic bonds gives ionic compounds their characteristically high melting and boiling points. As such, NaCl has a considerably higher melting point than HCl because the latter is a molecular compound with dipole-dipole interactions, which are much weaker than the electrostatic forces holding the ionic lattice together.

• Electrostatic Attraction: Key feature of ionic bonds.
• High Melting/Boiling Points: Characteristic of ionic compounds.
• Lattice Structure: Defines the solid form of ionic compounds.

Covalent bonds are formed when two atoms share one or more pairs of electrons. The strength of covalent bonds lies in the mutual attraction of the atoms' nuclei for the shared electrons. These bonds are the main forces holding together the atoms within a molecule and are typically much stronger than intermolecular forces. However, when comparing melting points, we don't look at covalent bonds within a molecule but instead at the intermolecular forces between covalently bonded molecules.

In the context of the homework problem, diethyl ether and butanol both have molecules held together by covalent bonds. Yet, their melting points differ because of the different strengths of the intermolecular forces they experience. Butanol can engage in hydrogen bonding due to its hydroxyl group, while diethyl ether doesn’t have this capability and thus has a lower melting point.

• Shared Electrons: Defining characteristic of covalent bonds.
• Intra-molecular Strength: Covalent bonds are strong within a molecule.
• Inter-molecular Impact: Determines phase changes like melting and boiling.