Suggest, giving reasons, which substance in each of the following pairs is likely to have the higher normal melting point (Lewis structures may help your arguments): (a) \(\mathrm{HCl}\) or \(\mathrm{NaCl}\); (b) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OC}_{2} \mathrm{H}_{5}\) (diethyl ether) or \(\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{OH}\) (butanol); (c) \(\mathrm{CH}_{3}\) or \(\mathrm{CHF}_{3} ;\) (d) \(\mathrm{C}_{2} \mathrm{H}_{4}\) or \(\mathrm{CH}_{3} \mathrm{OH}\).

Intermolecular forces are the forces of attraction or repulsion that act between neighboring particles (molecules, atoms, or ions). These forces vary in strength and play a critical role in determining the physical properties of substances, including melting points. Stronger intermolecular forces result in higher melting points, as more energy is required to break these forces apart for a substance to transition from solid to liquid.

There are several types of intermolecular forces, including hydrogen bonding, dipole-dipole interactions, and London dispersion forces. Understanding the differences between them helps us to predict the relative melting points of different substances.

Ionic and covalent bonds are the two main types of chemical bonds that hold compounds together. Ionic bonds form between metals and nonmetals when electrons are transferred from one atom to another, creating ions that attract each other due to opposite charges. Covalent bonds, on the other hand, involve the sharing of electrons between nonmetal atoms.

Generally, ionic compounds have higher melting points than covalent compounds because the electrostatic forces between ions in an ionic lattice are much stronger than the forces between molecules in a covalently-bonded substance. The strength of these bonds greatly affects the melting point of a substance.

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when a hydrogen atom is covalently bonded to an electronegative atom like nitrogen, oxygen, or fluorine. This bond is much stronger than other types of van der Waals forces due to the high electronegativity of the atoms involved.

This strong attraction significantly increases the melting point of a substance, explaining why water has a high melting point compared to other molecules of similar size. Substances capable of hydrogen bonding will typically have higher melting points than those that do not have this capability.

Van der Waals forces is a collective term that includes various types of weak intermolecular forces: dipole-dipole interactions, London dispersion forces, and hydrogen bonds (though hydrogen bonds are significantly stronger). Dipole-dipole interactions occur between polar molecules, London dispersion forces occur in all molecules (including noble gases), and hydrogen bonds occur when hydrogen is attached to a highly electronegative atom. Among these, London dispersion forces are the weakest intermolecular force.

While van der Waals forces are weaker than chemical bonds, they significantly influence the physical properties of molecules, including melting points, boiling points, and vapor pressures.

Molecules with permanent dipoles can interact with one another through dipole-dipole forces. These forces arise from the attraction of the positive end of one polar molecule to the negative end of another. The strength of the dipole-dipole interaction is directly proportional to the polarity of the molecule.

In the case of melting point comparison, compounds with stronger dipole-dipole interactions usually have higher melting points than those with weaker interactions or those that only have London dispersion forces. This is because more thermal energy is required to overcome the attractive forces between the molecules.

London dispersion forces, also known as induced dipole-induced dipole forces, are the weakest type of van der Waals force and are present in all molecules, whether polar or nonpolar. They result from the temporary fluctuations in electron distribution within molecules, which creates temporary dipoles that attract one another.

The strength of these forces increases with the size or molar mass of the molecule and with the surface area. Therefore, larger and more elongated molecules will have stronger London dispersion forces and consequently higher melting points than smaller or more compact molecules of similar composition.

Electronegativity refers to the ability of an atom to attract electrons in a chemical bond. The greater an atom's electronegativity, the more it pulls on bonding electrons. In molecules where one atom is significantly more electronegative than the others, polar bonds are formed, leading to a separation of charge and the establishment of a molecule with a permanent dipole.

The concept of electronegativity is essential for understanding the types of intermolecular forces present in a substance. Substances composed of atoms with high electronegativity differences are more likely to have strong intermolecular forces, such as ionic bonds or hydrogen bonds, which can result in higher melting points.