Question

Which substance in each pair would be expected to show the largest vapor pressure at a given temperature? Explain your reasoning. a. \(\mathrm{H}_{2} \mathrm{O}(l)\) or \(\mathrm{HF}(l)\) b. \(\mathrm{CH}_{3} \mathrm{OCH}_{3}(l)\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(l)\) c. \(\mathrm{CH}_{3} \mathrm{OH}(l)\) or \(\mathrm{CH}_{3} \mathrm{SH}(l)\)

Short answer

In summary, at a given temperature: a. \(\mathrm{HF}(l)\) will have a higher vapor pressure than \(\mathrm{H}_{2} \mathrm{O}(l)\) due to weaker hydrogen bonding. b. \(\mathrm{CH}_{3} \mathrm{OCH}_{3}(l)\) will have a higher vapor pressure than \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(l)\) due to weaker dipole-dipole interactions as opposed to hydrogen bonding. c. \(\mathrm{CH}_{3} \mathrm{SH}(l)\) will have a higher vapor pressure than \(\mathrm{CH}_{3} \mathrm{OH}(l)\) due to weaker dipole-dipole interactions compared to hydrogen bonding.

Step-by-step solution

Pair a: \(\mathrm{H}_{2} \mathrm{O}(l)\) vs \(\mathrm{HF}(l)\)

Step 1: Identify the intermolecular forces - \(\mathrm{H}_{2} \mathrm{O}(l)\) has hydrogen bonding and London dispersion forces. - \(\mathrm{HF}(l)\) also has hydrogen bonding and London dispersion forces. Step 2: Compare the strengths of intermolecular forces - Hydrogen bonding is stronger in \(\mathrm{H}_2 \mathrm{O}\) because there are more hydrogen bonding sites available per molecule than in \(\mathrm{HF}\). - London dispersion forces are stronger in \(\mathrm{H}_2 \mathrm{O}\) than in \(\mathrm{HF}\) due to similar molar masses. Step 3: Determine the substance with higher vapor pressure - \(\mathrm{HF}\) has weaker intermolecular forces because it has fewer hydrogen bonding sites, so it will have the highest vapor pressure in comparison to \(\mathrm{H}_2 \mathrm{O}\).

Pair b: \(\mathrm{CH}_{3} \mathrm{OCH}_{3}(l)\) vs \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(l)\)

Step 1: Identify the intermolecular forces - \(\mathrm{CH}_{3} \mathrm{OCH}_{3}(l)\) has dipole-dipole interactions and London dispersion forces. - \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) has hydrogen bonding and London dispersion forces. Step 2: Compare the strengths of intermolecular forces - Hydrogen bonding is stronger than dipole-dipole interactions; hence, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) has stronger intermolecular forces. - London dispersion forces are similar in both since they have similar molar masses. Step 3: Determine the substance with higher vapor pressure - \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) has weaker intermolecular forces (dipole-dipole) compared to hydrogen bonding in \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\), so it will have the highest vapor pressure.

Pair c: \(\mathrm{CH}_{3} \mathrm{OH}(l)\) vs \(\mathrm{CH}_{3} \mathrm{SH}(l)\)

Step 1: Identify the intermolecular forces - \(\mathrm{CH}_{3} \mathrm{OH}(l)\) has hydrogen bonding and London dispersion forces. - \(\mathrm{CH}_{3} \mathrm{SH}(l)\) has dipole-dipole interactions and London dispersion forces. Step 2: Compare the strengths of intermolecular forces - Hydrogen bonding in \(\mathrm{CH}_{3} \mathrm{OH}(l)\) is stronger than the dipole-dipole interaction in \(\mathrm{CH}_{3} \mathrm{SH}(l)\). - London dispersion forces are similar in both since they have similar molar masses. Step 3: Determine the substance with higher vapor pressure - \(\mathrm{CH}_{3} \mathrm{SH}\) has weaker intermolecular forces (dipole-dipole) compared to hydrogen bonding in \(\mathrm{CH}_{3} \mathrm{OH}\), so it will have the highest vapor pressure.

Key concepts

Intermolecular Forces

Intermolecular forces (IMFs) are the bonds or attractions that act between molecules or discrete particles. Understanding these forces helps to explain many physical properties of substances, including why materials have different states of matter, melting points, boiling points, and vapor pressures.

IMFs are categorized into different types depending on the nature of the interactions: hydrogen bonding, London dispersion forces, and dipole-dipole interactions are some of the most important types to know. The strength of these forces has a direct impact on a substance's vapor pressure: generally speaking, the stronger the intermolecular forces, the lower the vapor pressure, because molecules are held together more tightly and are less likely to escape into the vapor phase.

Hydrogen Bonding

Hydrogen bonding is an especially strong type of dipole-dipole interaction that occurs in molecules containing hydrogen atoms directly bonded to highly electronegative atoms, such as oxygen, nitrogen, or fluorine.

The key feature of hydrogen bonding is that the hydrogen atom shares its electron with a greedy electronegative partner, leaving the hydrogen with a significant positive charge that can strongly attract the negative regions of nearby molecules. This interaction is particularly strong because of its highly polar nature.

For instance, water (H₂O) can form multiple hydrogen bonds because of its bent shape and the presence of two hydrogen atoms that can bond to the oxygen atoms of other water molecules. This is why water has a relatively low vapor pressure compared to other similar size molecules without hydrogen bonding capability.

London Dispersion Forces

London dispersion forces are the weakest intermolecular force and are present in all molecules, regardless of their polarity. They arise due to the temporary shifts in electron density within molecules, leading to the formation of transient dipoles.

Larger molecules with more electrons typically have stronger London dispersion forces because there is a greater likelihood of temporary dipoles forming. This factor explains why heavier noble gases, such as xenon and radon, are liquids or solids at room temperature, despite being nonpolar and only experiencing London dispersion forces in their interactions.

London dispersion forces play a significant role in determining vapor pressure, especially for nonpolar substances or those with similar molar masses, where differences in their dispersion forces will affect their volatility.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules, where there is a permanent separation of charges within the molecule due to uneven electron distribution. Each molecule has a positive end and a negative end, allowing for attractive forces between the oppositely charged areas of two adjacent molecules.

These interactions are stronger than London dispersion forces but weaker than hydrogen bonding. They significantly influence a molecule's physical properties, notably its boiling and melting points, and its vapor pressure. For example, for molecules of comparable size and shape, those with stronger dipole-dipole interactions will typically have a lower vapor pressure, as the molecules are held together more tightly.

Substances with similar molar masses but different polarities demonstrate how dipole-dipole interactions affect volatility. Polar molecules will have lower vapor pressures than nonpolar ones because the polar molecules can align and attract each other more effectively.