Question

In each pair of compounds, pick the one with the higher vapor pressure at a given temperature. Explain your reasoning. a. \(\mathrm{CH}_{4}\) or \(\mathrm{CH}_{3} \mathrm{Cl}\) b. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) or \(\mathrm{CH}_{3} \mathrm{OH}\) c. \(\mathrm{CH}_{3} \mathrm{OH}\) or \(\mathrm{H}_{2} \mathrm{CO}\)

Short answer

a. CH4, b. CH3OH, c. H2CO. Lighter molecules with weaker intermolecular forces tend to have higher vapor pressures at a given temperature.

Step-by-step solution

Understanding Vapor Pressure and Molecular Forces

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase. It increases with temperature and depends on the intermolecular forces present within a substance. Substances with weaker intermolecular forces have higher vapor pressures because their molecules can escape to the gas phase more easily.

Comparing CH4 and CH3Cl

To compare the vapor pressure of CH4 (methane) and CH3Cl (chloromethane), we need to consider their molecular weights and the type of intermolecular forces. Both molecules have similar shapes and dispersion forces, but CH3Cl has a greater molecular weight and could potentially have dipole-dipole interactions due to the presence of chlorine, which is more electronegative than carbon. Therefore, methane (CH4), which is lighter and has only dispersion forces, will have the higher vapor pressure at a given temperature.

Comparing CH3CH2CH2OH and CH3OH

CH3CH2CH2OH (1-propanol) and CH3OH (methanol) differ in size and the potential for hydrogen bonding. Methanol is smaller and has less surface area for dispersion forces than 1-propanol. Also, both can form hydrogen bonds, but the larger structure of 1-propanol means its molecules are better able to interact with each other. Methanol, being smaller and having less extensive hydrogen bonding, will have a higher vapor pressure than 1-propanol.

Comparing CH3OH and H2CO

Both CH3OH (methanol) and H2CO (formaldehyde) can form hydrogen bonds. However, methanol can form stronger hydrogen bonds due to its OH group, which leads to higher attraction between molecules. Formaldehyde is smaller and has less extensive hydrogen bonding capabilities than methanol. As a result, formaldehyde (H2CO), with its weaker hydrogen bonds, will exhibit a higher vapor pressure when compared to methanol.

Key concepts

Intermolecular Forces

When studying vapor pressure in substances, it's crucial to understand the role of intermolecular forces, which are the forces of attraction between molecules. There are several types, including dispersion forces, dipole-dipole interactions, and hydrogen bonds. Dispersion forces, also known as London forces, are the weakest type and exist between all molecules, regardless of their polarity. Molecules with stronger intermolecular forces have a harder time escaping into the vapor phase, therefore exhibiting a lower vapor pressure.

Dipole-dipole interactions occur in polar molecules where there is a significant difference in electronegativity between atoms, leading to a positive and negative end. The higher the polarity, the stronger the dipole-dipole interactions, which can decrease vapor pressure. Understanding these concepts sheds light on why methane (CH₄), with its weaker dispersion forces, would have a higher vapor pressure compared to chloromethane (CH₃Cl), which contains both dispersion and dipole-dipole interactions.

Molecular Weight and Vapor Pressure

Molecular weight is another essential factor affecting vapor pressure. Generally, heavier molecules have lower vapor pressures because their greater mass results in less tendency to escape into the gas phase. Factors like molecular structure and the presence of polar bonds also come into play, but, fundamentally, lighter molecules often have a higher vapor pressure due to their ability to disperse more easily.

This is evident when comparing methane (CH₄) and chloromethane (CH₃Cl); methane is lighter and, therefore, has a higher vapor pressure. The difference in vapor pressure between methanol (CH₃OH) and 1-propanol (CH₃CH₂CH₂OH) highlights the influence of both molecular weight and molecular structure on their escape into the vapor phase.

Hydrogen Bonding

Hydrogen bonding is a specific type of dipole-dipole interaction that occurs when hydrogen is attached to highly electronegative atoms like oxygen, nitrogen, or fluorine. These bonds are particularly strong due to the high polarity and can significantly reduce a substance's vapor pressure. Molecules capable of hydrogen bonding will generally have lower vapor pressures because the strong attraction between molecules makes it more difficult for individual molecules to escape to the vapor phase.

For instance, methanol (CH₃OH) and formaldehyde (H₂CO) can both form hydrogen bonds, but methanol has a higher tendency for this type of bonding due to its hydrogen attached to oxygen. This leads to methanol having lower vapor pressure compared to formaldehyde, which, while capable of hydrogen bonding, has weaker and less extensive capabilities for this interaction. The comparison helps explain why formaldehyde would exhibit a higher vapor pressure than methanol.