Problem 69 - General Chemistry

Question

Predict the order of increasing vapor pressure at a given temperature for the following compounds: a. \(\mathrm{FCH}_{2} \mathrm{CH}_{2} \mathrm{~F}\) b. \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) c. \(\mathrm{FCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) Explain why you chose this order.

Step-by-Step Solution

Answer

Order of increasing vapor pressure: b < c < a.

Step 1: Understand Vapor Pressure

Vapor pressure is determined by the ease with which molecules escape from the liquid to the gas phase. Substances with stronger intermolecular forces generally have lower vapor pressures because molecules have more difficulty escaping into the gas phase.

Step 2: Consider the Types of Intermolecular Forces

Examine the compounds for their intermolecular forces: hydrogen bonding, dipole-dipole, and London dispersion forces. - \(\mathrm{FCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) can form hydrogen bonds and has dipole-dipole interactions and dispersion forces. - \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) has multiple sites for hydrogen bonding. - \(\mathrm{FCH}_{2} \mathrm{CH}_{2} \mathrm{~F}\) primarily exhibits dipole-dipole and dispersion forces.

Step 3: Rank According to Intermolecular Forces

The strongest intermolecular forces will result in the lowest vapor pressure. - \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) has the strongest hydrogen bonding due to two hydroxyl groups, resulting in the lowest vapor pressure. - \(\mathrm{FCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) has one hydroxyl group for hydrogen bonding, which is less than \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) but more than \(\mathrm{FCH}_{2} \mathrm{CH}_{2} \mathrm{~F}\). - \(\mathrm{FCH}_{2} \mathrm{CH}_{2} \mathrm{~F}\) will have the highest vapor pressure due to the absence of hydrogen bonding.

Step 4: Determine the Correct Order

Based on the evaluation: 1. \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) has the lowest vapor pressure,2. Followed by \(\mathrm{FCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\), 3. \(\mathrm{FCH}_{2} \mathrm{CH}_{2} \mathrm{~F}\) has the highest vapor pressure.

Key Concepts

Intermolecular Forces

Intermolecular forces (IMFs) are forces of attraction or repulsion between neighboring molecules. These forces are crucial for determining the physical properties of a substance, such as boiling and melting points, and vapor pressure. The strength of intermolecular forces affects how easily molecules can separate from each other to enter the vapor phase.
Different types of intermolecular forces include:

• Hydrogen bonding
• Dipole-dipole interactions
• London dispersion forces

These forces can vary significantly in strength. Generally, the stronger the intermolecular forces, the lower the vapor pressure of a substance. This means that molecules are less likely to escape into the vapor phase if they are held together strongly by these forces. When comparing substances, knowing which intermolecular forces are present helps predict the behavior of each compound.

Hydrogen Bonding

Hydrogen bonding is a very strong type of dipole-dipole interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom, such as oxygen, nitrogen, or fluorine. This creates a significant attraction not only within the molecule but also between neighboring molecules. Hydrogen bonds are responsible for many unique properties of substances.
Characteristics of Hydrogen Bonding:

• Strongest form of dipole-dipole interactions
• Occurs in molecules where H is directly bonded to O, N, or F
• Significantly lowers vapor pressure due to strong cohesion

In the context of the given compounds, \( ext{HOCH}_2 ext{CH}_2 ext{OH} \) displays hydrogen bonding at two sites due to the presence of two hydroxyl (OH) groups. This leads to very strong intermolecular attractions and, therefore, a lower vapor pressure. The compound \( ext{FCH}_2 ext{CH}_2 ext{OH} \) also demonstrates hydrogen bonding, but since it has only one OH group, the effect is slightly weaker.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules where there is a separation of positive and negative charges. These interactions are stronger than London dispersion forces but weaker than hydrogen bonds.
Characteristics of Dipole-Dipole Interactions:

• Exist in polar molecules
• Strength depends on the polarity of the molecules
• Stronger than dispersion forces but weaker than hydrogen bonds

In \( ext{FCH}_2 ext{CH}_2 ext{F} \), dipole-dipole forces arise due to the electronegative fluorine atoms. However, because there's no hydrogen bonding, these interactions are not as strong as those found in \( ext{HOCH}_2 ext{CH}_2 ext{OH} \). The presence of dipole-dipole interactions helps explain the moderate vapor pressure of \( ext{FCH}_2 ext{CH}_2 ext{F} \) compared to its counterparts with hydrogen bonding.

London Dispersion Forces

London dispersion forces are a universal type of intermolecular force present in all molecules, whether polar or non-polar. These forces are the weakest type of intermolecular attractions and arise from temporary dipoles formed when electron distributions within molecules temporarily shift.
Characteristics of London Dispersion Forces:

• Present in all molecular interactions
• Weaker than hydrogen bonds and dipole-dipole forces
• Increases with molecular size and mass

In the compounds discussed, London dispersion forces play a role in \( ext{FCH}_2 ext{CH}_2 ext{F} \) since it lacks hydrogen bonding. These forces, while weak, contribute to the total intermolecular attractions in the absence of stronger forces. Although not the primary force in determining vapor pressure, these interactions support the overall retention of molecules in the liquid phase.