Question

Based on the type or types of intermolecular forces, predict the substance in each pair that has the higher boiling point:(a) propane \(\left(\mathrm{C}_{3} \mathrm{H}_{8}\right)\) or \(n\) -butane \(\left(\mathrm{C}_{4} \mathrm{H}_{10}\right),(\mathbf{b})\) diethyl ether \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}\right)\) or 1 -butanol \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\right)\) (c) sulfur dioxide \(\left(\mathrm{SO}_{2}\right)\) or sulfur trioxide \(\left(\mathrm{SO}_{3}\right),(\mathbf{d})\) phosgene \(\left(\mathrm{Cl}_{2} \mathrm{CO}\right)\) or formaldehyde \(\left(\mathrm{H}_{2} \mathrm{CO}\right)\)

Short answer

The substance with higher boiling points in each pair are: (a) n-butane, (b) 1-butanol, (c) sulfur trioxide, and (d) phosgene. This is due to stronger intermolecular forces such as London Dispersion Forces, Hydrogen Bonding, and Dipole-Dipole Forces present in these substances compared to their counterparts in each pair.

Step-by-step solution

Compare molecule size and London Dispersion forces

Both molecules are nonpolar hydrocarbons, so London Dispersion Forces are the dominant intermolecular forces. Since n-butane is larger, London Dispersion Forces are stronger in n-butane than in propane.

Predict the substance with higher boiling point

As n-butane has stronger London Dispersion Forces, it will have a higher boiling point than propane. (b) Diethyl ether (CH3CH2OCH2CH3) and 1-butanol (CH3CH2CH2CH2OH)

Identify the intermolecular forces

Both molecules have London Dispersion Forces. However, diethyl ether has Dipole-Dipole Forces due to the presence of an oxygen atom, while 1-butanol has more potent Hydrogen Bonding due to the presence of an OH group.

Predict the substance with higher boiling point

As 1-butanol has stronger Hydrogen Bonding, it will have a higher boiling point than diethyl ether. (c) Sulfur dioxide (SO2) and sulfur trioxide (SO3)

Identify the intermolecular forces

Both molecules are polar due to their molecular geometries, leading to Dipole-Dipole Forces as the dominant intermolecular forces.

Compare Dipole-Dipole Forces

SO3 has a larger dipole moment than SO2, meaning the Dipole-Dipole Forces are stronger in SO3.

Predict the substance with higher boiling point

As SO3 has stronger Dipole-Dipole Forces, it will have a higher boiling point than SO2. (d) Phosgene (Cl2CO) and formaldehyde (H2CO)

Identify the intermolecular forces

Both molecules are polar, with Dipole-Dipole Forces as the dominant intermolecular forces, as well as London Dispersion Forces.

Compare Dipole-Dipole Forces and London Dispersion Forces

Phosgene has more significant dipole moment due to the presence of highly electronegative Cl atoms, which will increase the strength of Dipole-Dipole Forces. Additionally, phosgene has a larger surface area due to the presence of Cl atoms, which will increase the London Dispersion Forces.

Predict the substance with higher boiling point

As phosgene has stronger intermolecular forces (both Dipole-Dipole and London Dispersion Forces), it will have a higher boiling point than formaldehyde.

Key concepts

Boiling Point Prediction

Understanding the boiling point of a substance is essential in many scientific and industrial applications. Predicting the boiling point involves evaluating the strength of intermolecular forces present within a substance. The general rule is that stronger intermolecular forces lead to a higher boiling point, as more energy is required to overcome these forces during the phase transition from liquid to gas.

Intuitively, larger molecules with greater surface areas tend to have higher boiling points due to the increased presence of London Dispersion Forces. In situations where molecules have different types of intermolecular forces, such as Hydrogen Bonding or Dipole-Dipole Forces, higher boiling points are expected for substances with stronger or more of these interactions. For instance, substances with Hydrogen Bonding typically have much higher boiling points than those with just London Dispersion Forces.

London Dispersion Forces

Among the various intermolecular forces, London Dispersion Forces are considered the weakest and are present in all molecules, whether they are polar or non-polar. These forces are induced when the movement of electrons within atoms or molecules creates a temporary dipole, leading to an instantaneous and induced dipole in adjacent molecules.

The strength of the London Dispersion Forces is affected primarily by the size and shape of the molecules. Larger molecules with more electrons have stronger dispersion forces because there's a bigger chance for temporary dipoles to occur, and the force impacts more electrons. This explains why n-butane, with a larger molecular structure than propane, exhibits stronger London Dispersion Forces and, as a result, has a higher boiling point.

Hydrogen Bonding

Hydrogen Bonding is a particularly strong type of Dipole-Dipole Force and arises when a hydrogen atom is covalently bonded to a highly electronegative element, such as nitrogen, oxygen, or fluorine. This configuration leads to an uneven distribution of electron density that creates a permanent dipole.

When this hydrogen atom comes into close proximity with the electronegative part of another molecule, a strong intermolecular bond forms. This special interaction requires a significant amount of energy to break, contributing to notably higher boiling points for compounds capable of Hydrogen Bonding. For example, the presence of an OH group allows 1-butanol to form Hydrogen Bonds, resulting in its higher boiling point compared to diethyl ether, which only exhibits Dipole-Dipole Forces and London Dispersion Forces.

Dipole-Dipole Forces

Dipole-Dipole Forces are a type of intermolecular force present in polar molecules, where there is an uneven distribution of electrons within the molecule. This uneven distribution leads to the formation of a permanent dipole—a positive end and a negative end within the molecule.

These dipoles attract each other, with the positive end of one molecule being attracted to the negative end of another. The strength of the Dipole-Dipole Forces is generally associated with the polarity of the molecule—the greater the difference in electronegativity between the atoms, the stronger the dipole moment, and the stronger the intermolecular forces. Such is the case for sulfur trioxide (SO3), which with a larger dipole moment compared to sulfur dioxide (SO2), exhibits stronger Dipole-Dipole Forces and hence a higher boiling point.