Question

Rationalize the difference in boiling points for each of the following pairs of substances: a. \(n\) -pentane \(\quad \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) \(36.2^{\circ} \mathrm{C}\) neopentane CC(C)(C)C \(9.5^{\circ} \mathrm{C}\) b. HF \(20^{\circ} \mathrm{C}\) \(\mathrm{HCl} \quad-85^{\circ} \mathrm{C}\) c. HCl \(-85^{\circ} \mathrm{C}\) \(\mathrm{LiCl} \quad 1360^{\circ} \mathrm{C}\) d. \(n\) -pentane \(\quad \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) \(36.2^{\circ} \mathrm{C}\) \(\begin{array}{lll}n \text { -hexane } & \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} & 69^{\circ} \mathrm{C}\end{array}\)

Short answer

The differences in boiling points for the given pairs can be rationalized as follows: a. n-pentane has a higher boiling point than neopentane due to its linear shape, which allows for stronger London dispersion forces between its molecules compared to the more branched neopentane. b. HF has a higher boiling point than HCl due to the stronger hydrogen bonds present in HF as a result of fluorine's high electronegativity. c. LiCl has a much higher boiling point than HCl because of its strong ionic bonding, compared to HCl's weaker dipole-dipole interactions and London dispersion forces. d. n-hexane has a higher boiling point than n-pentane due to its larger molecular size and surface area, which leads to stronger London dispersion forces.

Step-by-step solution

a. n-pentane and neopentane

n-pentane and neopentane are both nonpolar hydrocarbons. The difference in boiling points can be explained by the molecular size and branching. n-pentane has a linear shape, while neopentane is more branched. The linear shape of n-pentane allows for a larger surface area and stronger London dispersion forces between molecules, which leads to a higher boiling point compared to branched neopentane.

b. HF and HCl

The boiling point difference between HF and HCl can be attributed to the strength of intermolecular forces between molecules. HF has stronger hydrogen bonds due to the high electronegativity of fluorine compared to chlorine. These strong hydrogen bonds result in a higher boiling point for HF compared to HCl, which solely relies on less strong dipole-dipole interactions and London dispersion forces.

c. HCl and LiCl

The difference in boiling points between HCl and LiCl is due to the differences in the types of bonding. HCl is a covalent molecule with dipole-dipole interactions and London dispersion forces. In contrast, LiCl is an ionic compound with strong electrostatic attractions between the positively charged Li+ ions and negatively charged Cl- ions. This strong ionic bonding causes LiCl to have a much higher boiling point than HCl.

d. n-pentane and n-hexane

Both n-pentane and n-hexane are nonpolar hydrocarbons with linear structures. Their difference in boiling points can be explained by molecular size. As n-hexane has a greater number of carbon atoms (6) compared to n-pentane (5), its molecular size and surface area are greater. Consequently, n-hexane exhibits stronger London dispersion forces, leading to a higher boiling point when compared to n-pentane.

Key concepts

Intermolecular Forces

Intermolecular forces are the attractive forces that act between molecules. They are crucial in determining the physical properties of substances, such as boiling points. There are several types of intermolecular forces, including hydrogen bonding, dipole-dipole interactions, and London dispersion forces.

Substances with stronger intermolecular forces generally have higher boiling points since more energy is required to break these forces during the phase transition from liquid to gas. For example, in the case of HF (hydrogen fluoride) and HCl (hydrogen chloride), HF exhibits stronger hydrogen bonds due to the high electronegativity of fluorine. This results in a higher boiling point for HF compared to HCl, which has weaker dipole-dipole interactions.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction which occurs when a hydrogen atom is bonded to a highly electronegative element, such as nitrogen, oxygen, or fluorine, and is attracted to another electronegative atom in a different molecule. Hydrogen bonds are particularly strong intermolecular forces.

In our exercise, the discrepancy in boiling points between HF and HCl is largely due to the existence of hydrogen bonding in HF. Fluorine's extreme electronegativity allows for strong hydrogen bonds to form, creating a higher boiling point compared to HCl which doesn't have hydrogen bonding.

London Dispersion Forces

London dispersion forces are weak intermolecular forces that arise from the momentary fluctuations in electron distribution within molecules which create temporary dipoles. These forces are present in all molecules, whether polar or nonpolar, but they are the only type of intermolecular force in nonpolar molecules.

As seen with n-pentane and neopentane, the linear shape of n-pentane allows for stronger London dispersion forces due to a larger surface area, whereas the compact, branched structure of neopentane results in weaker London dispersion forces and consequently, a lower boiling point.

Molecular Size and Branching

The size of a molecule and its branching can significantly affect its boiling point through the alteration of surface area and the strength of London dispersion forces. Larger molecules have a higher boiling point because their increased surface area allows for more significant dispersion forces.

Branching reduces the surface area in contact between molecules, resulting in weaker dispersion forces. Comparing n-pentane to neopentane, we see that the linear n-pentane has a higher boiling point because of its larger surface area which increases the strength of London dispersion forces.

Ionic vs Covalent Bonding

Ionic and covalent bonds represent two different chemical bonding mechanisms. Ionic bonds occur between metals and non-metals and involve the transfer of electrons, resulting in the formation of positively and negatively charged ions. Covalent bonds, in contrast, involve the sharing of electrons between non-metal atoms.

In the context of boiling points, ionic compounds typically have much higher boiling points than covalent compounds. This is due to the strong electrostatic forces that hold the ions together in a lattice structure. For example, LiCl, being ionic, has a very high boiling point compared to covalent HCl, reaffirming this principle.