Question

True or false: (a) \(\mathrm{CBr}_{4}\) is more volatile than \(\mathrm{CCl}_{4}\). (b) \(\mathrm{CBr}_{4}\) has a higher boiling point than \(\mathrm{CCl}_{4}\). (c) \(\mathrm{CBr}_{4}\) 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

Both CBr4 and CCl4 are nonpolar molecules due to their symmetrical tetrahedral shapes. The primary intermolecular force present in both molecules is London dispersion forces (LDF). It is important to note that LDF strength increases with size and molecular weight of the atoms involved. #Step 2: Compare Molecular Weights#

Bromine atoms are larger and have a higher molecular weight than chlorine atoms. As a result, the overall molecular weight of CBr4 is greater than that of CCl4. Hence, the London dispersion forces in CBr4 are stronger than those in CCl4. #Step 3: Relate Intermolecular Forces to Volatility#

Volatility is a measure of how easily a substance evaporates. Molecules with weaker intermolecular forces evaporate more quickly than those with stronger forces. Because CCl4 has weaker LDF forces than CBr4, it would be more volatile. #Step 4: Evaluate Statements (a) and (c)#

: (a) Evaluate CBr4 Volatility

Comparing the volatility of CBr4 and CCl4, we determined that CCl4 is more volatile. So, the statement "CBr4 is more volatile than CCl4" is false.

(c) Evaluate CBr4 Intermolecular Forces

Comparing the intermolecular forces of CBr4 and CCl4, we determined that CBr4 has stronger LDF forces. So, the statement "CBr4 has weaker intermolecular forces than CCl4" is false. #Step 5: Relate Intermolecular Forces to Boiling Points and Vapor Pressure#

Substances with stronger intermolecular forces usually have higher boiling points and lower vapor pressures. Since CBr4 has stronger LDF forces than CCl4, it should have a higher boiling point and lower vapor pressure at the same temperature. #Step 6: Evaluate Statements (b) and (d)#

(b) Evaluate CBr4 Boiling Point

Comparing boiling points, we determined that CBr4 has a higher boiling point than CCl4 due to its stronger LDF forces. So, the statement "CBr4 has a higher boiling point than CCl4" is true.

(d) Evaluate CBr4 Vapor Pressure

Comparing vapor pressures, we determined that CBr4 has a lower vapor pressure at the same temperature due to its stronger LDF forces. So, the statement "CBr4 has a higher vapor pressure at the same temperature than CCl4" is false. In conclusion: (a) False (b) True (c) False (d) False

Key concepts

London Dispersion Forces

London dispersion forces (LDF) are a type of intermolecular force that occurs between nonpolar molecules. Despite being the weakest of the van der Waals forces, they are critically important for substances that lack other types of stronger intermolecular bonds, such as hydrogen bonding or ionic attractions.

LDF are caused by temporary fluctuations in the electron distribution within molecules, which create instantaneous dipoles that induce dipoles in adjacent molecules. This results in a temporary attractive force between the molecules. The strength of LDF is influenced by the size and shape of the molecules as well as their ability to polarize. Larger and more polarizable atoms or groups within a molecule will have stronger London dispersion forces.

To help students better understand London dispersion forces, one could highlight that these forces are like the brief handshakes between two people walking by each other. They are quick and weak, but the larger the person's hand (analogous to the size of the atom or molecule), the stronger the handshake will be. It is this transient interaction that, in large numbers, can significantly impact the properties of a substance.

Molecular Volatility

Molecular volatility refers to the tendency of a substance to vaporize, or convert from a liquid or solid state to a gas. Volatility is not an intrinsic property of a substance but can be influenced by ambient conditions such as temperature and pressure as well as the strength of intermolecular forces within the substance.

Molecules with weaker intermolecular forces will have higher volatility because less energy is required to overcome the attractions holding the molecules in the liquid or solid state. This concept is crucial when considering storage and handling of chemicals, as more volatile substances will evaporate more quickly and could potentially lead to hazards or loss of material.

For students, think of volatility like the eagerness of people in a crowded room to leave—the weaker their reasons for staying (analogous to weaker intermolecular forces), the quicker they will head for the exits (vaporize).

Boiling Point Comparison

The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure surrounding the liquid, resulting in the formation of vapor bubbles within the liquid mass. Boiling point is a key characteristic for identifying and utilizing substances and is greatly affected by intermolecular forces.

Substances with stronger intermolecular forces will have higher boiling points because more energy is needed to separate the molecules. In a comparison between two substances, like CBr4 and CCl4, the one with the stronger intermolecular forces, in this case CBr4, will exhibit a higher boiling point. This principle is essential when separating mixtures by distillation or when designing processes that involve heating or cooling of chemicals.

For a simpler understanding, students can imagine boiling point as the 'breaking point' of loyalty among friends (molecules) in a group—the stronger the loyalty, the more effort (heat) it takes to go their separate ways (boil).