Question

True or false: (a) \(\mathrm{CBr}_{4}\) is more volatile than \(\mathrm{CCl}_{4} .(\mathbf{b}) \mathrm{CBr}_{4}\) has a higher boiling point than \(\mathrm{CCl}_{4}\) . (c) CBr. has weaker intermolecular forces than \(\mathrm{CCl}_{4}\) . (d) \(\mathrm{CBr}_{4}\) has a higher vapor pressure at the same temperature than \(\mathrm{CCl}_{4}\) .

Short answer

(a) False, (b) True, (c) False, (d) False

Step-by-step solution

Statement (a): \(\mathrm{CBr}_{4}\) is more volatile than \(\mathrm{CCl}_{4}\).

Comparing the molecular weights of these two compounds, we find that \(\mathrm{CBr}_{4}\) has a higher molecular weight due to the heavier mass of the Br atoms. This leads to stronger London dispersion forces between the molecules, which are the primary intermolecular forces in these two compounds. Because \(\mathrm{CBr}_{4}\) has stronger intermolecular forces, it should be less volatile than \(\mathrm{CCl}_{4}\). Therefore, statement (a) is false. #b) Determine the boiling point#

Statement (b): \(\mathrm{CBr}_{4}\) has a higher boiling point than \(\mathrm{CCl}_{4}\).

As mentioned before, the heavier mass of the Br atoms in \(\mathrm{CBr}_{4}\) creates stronger London dispersion forces between the molecules. These stronger intermolecular forces require more energy to overcome, leading to a higher boiling point for \(\mathrm{CBr}_{4}\). Thus, statement (b) is true. #c) Determine the intermolecular forces#

Statement (c): \(\mathrm{CBr}_{4}\) has weaker intermolecular forces than \(\mathrm{CCl}_{4}\).

As discussed earlier, the higher molecular weight of \(\mathrm{CBr}_{4}\) causes it to have stronger London dispersion forces between the molecules compared to \(\mathrm{CCl}_{4}\). So, statement (c) is false. #d) Determine the vapor pressure#

Statement (d): \(\mathrm{CBr}_{4}\) has a higher vapor pressure at the same temperature than \(\mathrm{CCl}_{4}\).

Since we established earlier that \(\mathrm{CBr}_{4}\) is less volatile than \(\mathrm{CCl}_{4}\), it will have a lower vapor pressure at the same temperature. Thus, statement (d) is false. In summary: - Statement (a) is false - Statement (b) is true - Statement (c) is false - Statement (d) is false

Key concepts

Volatility

Volatility refers to a substance's tendency to vaporize. It is closely related to how easily particles can escape from the surface of a liquid.

• Volatile substances have weaker intermolecular forces, making it easier for their molecules to enter the gas phase.
• In contrast, substances with strong intermolecular forces require more energy for molecules to break free from the liquid and become gas, resulting in lower volatility.

Given this, \( \mathrm{CBr}_{4} \) has stronger intermolecular forces compared to \( \mathrm{CCl}_{4} \), due to its higher molecular weight and resulting stronger London dispersion forces.
Hence, \( \mathrm{CBr}_{4} \) is less volatile, meaning it's less prone to vaporizing than \( \mathrm{CCl}_{4} \). This explains why the initial statement regarding the comparative volatility of these two substances is false.

Boiling Point Comparison

The boiling point of a substance is the temperature at which it transitions from a liquid to a gas. This is directly influenced by the strength of the intermolecular forces present in the substance.

• Substances with strong intermolecular forces have higher boiling points because more energy is needed to overcome these forces and convert the liquid into vapor.
• On the other hand, substances with weaker intermolecular forces boil at lower temperatures.

In the case of \( \mathrm{CBr}_{4} \) vs. \( \mathrm{CCl}_{4} \), the heavier bromine atoms in \( \mathrm{CBr}_{4} \) lead to stronger London dispersion forces compared to the chlorine atoms in \( \mathrm{CCl}_{4} \).
As a result, \( \mathrm{CBr}_{4} \) has a higher boiling point than \( \mathrm{CCl}_{4} \), a fact that supports the statement that \( \mathrm{CBr}_{4} \) does indeed have a higher boiling point.

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid form at a given temperature. It's an indicator of a liquid’s evaporation rate.

• Higher vapor pressure means a liquid evaporates more readily, indicating weaker intermolecular forces.
• Conversely, a lower vapor pressure signifies stronger intermolecular forces and less evaporation.

In the context of \( \mathrm{CBr}_{4} \) and \( \mathrm{CCl}_{4} \), \( \mathrm{CBr}_{4} \) has stronger London dispersion forces, making it less volatile.
Therefore, \( \mathrm{CBr}_{4} \), with its stronger intermolecular forces, has a lower vapor pressure compared to \( \mathrm{CCl}_{4} \) at the same temperature.
This explains why \( \mathrm{CBr}_{4} \) does not have a higher vapor pressure, marking the original statement as false.