Question

(a) Place the following substances in order of increasing volatility: \(\mathrm{CH}_{4}, \mathrm{CBr}_{4}, \mathrm{CH}_{2} \mathrm{Cl}_{2}, \mathrm{CH}_{3} \mathrm{Cl}, \mathrm{CHBr}_{3}\), and \(\mathrm{CH}_{2} \mathrm{Br}_{2}\) Explain. (b) How do the boiling points vary through this series?

Short answer

The substances in order of increasing volatility are: \(\mathrm{CH}_{4} < \mathrm{CH}_{3}\mathrm{Cl} < \mathrm{CH}_{2}\mathrm{Cl}_{2} < \mathrm{CH}_{2}\mathrm{Br}_{2} < \mathrm{CHBr}_{3} < \mathrm{CBr}_{4}\). Their volatility is directly related to the strength of their intermolecular forces, with weaker forces leading to higher volatility. Boiling points also depend on intermolecular forces, increasing in the same order as volatility decreases: \(\mathrm{CH}_{4} < \mathrm{CH}_{3}\mathrm{Cl} < \mathrm{CH}_{2}\mathrm{Cl}_{2} < \mathrm{CH}_{2}\mathrm{Br}_{2} < \mathrm{CHBr}_{3} < \mathrm{CBr}_{4}\).

Step-by-step solution

Identify the intermolecular forces in each substance

First, let's identify the types of intermolecular forces in each molecule: 1. \(\mathrm{CH}_{4}\) - nonpolar, dispersion forces 2. \(\mathrm{CBr}_{4}\) - nonpolar, dispersion forces 3. \(\mathrm{CH}_{2}\mathrm{Cl}_{2}\) - polar, dipole-dipole interactions 4. \(\mathrm{CH}_{3}\mathrm{Cl}\) - polar, dipole-dipole interactions 5. \(\mathrm{CHBr}_{3}\) - polar, dipole-dipole interactions 6. \(\mathrm{CH}_{2}\mathrm{Br}_{2}\) - polar, dipole-dipole interactions

Compare dispersion forces based on size and shape of molecules

Dispersion forces are present in all substances, but they are stronger in larger and less symmetrical molecules. Let's compare the dispersion forces for the nonpolar molecules (\(\mathrm{CH}_{4}\) and \(\mathrm{CBr}_{4}\)): 1. \(\mathrm{CH}_{4}\) has less mass and fewer electrons than \(\mathrm{CBr}_{4}\), so it has weaker dispersion forces. 2. \(\mathrm{CBr}_{4}\) has more mass and more electrons than \(\mathrm{CH}_{4}\), so it has stronger dispersion forces.

Compare dipole-dipole interactions between polar molecules

Dipole-dipole interactions are stronger in molecules with larger dipole moments. We can compare the dipole moments by examining the electronegativity difference between the atoms in the molecule. A larger electronegativity difference means a stronger dipole and stronger dipole-dipole interactions. Comparing electronegativity differences for polar molecules (\(\mathrm{CH}_{2}\mathrm{Cl}_{2}, \mathrm{CH}_{3}\mathrm{Cl}, \mathrm{CHBr}_{3}, \mathrm{CH}_{2}\mathrm{Br}_{2}\)): 1. \(\mathrm{CH}_{2}\mathrm{Cl}_{2}\) and \(\mathrm{CH}_{3}\mathrm{Cl}\) have similar electronegativity differences compared to \(\mathrm{CHBr}_{3}\) and \(\mathrm{CH}_{2}\mathrm{Br}_{2}\). 2. \(\mathrm{CH}_{3}\mathrm{Cl}\) has a smaller dipole moment than \(\mathrm{CH}_{2}\mathrm{Cl}_{2}\) due to its symmetrical structure. 3. \(\mathrm{CH}_{2}\mathrm{Br}_{2}\) and \(\mathrm{CHBr}_{3}\) have similar electronegativity differences but smaller than the previous pair.

Rank the substances in order of increasing volatility

Now, we can combine Steps 2 and 3 to rank the substances in order of increasing volatility. Since substances with weaker intermolecular forces are more volatile, the order will be as follows: \(\mathrm{CH}_{4} < \mathrm{CH}_{3}\mathrm{Cl} < \mathrm{CH}_{2}\mathrm{Cl}_{2} < \mathrm{CH}_{2}\mathrm{Br}_{2} < \mathrm{CHBr}_{3} < \mathrm{CBr}_{4}\) Explanation: - \(\mathrm{CH}_{4}\) has the weakest dispersion forces and is the most volatile. - \(\mathrm{CBr}_{4}\) has the strongest dispersion forces among the nonpolar molecules. - \(\mathrm{CH}_{3}\mathrm{Cl}\) and \(\mathrm{CH}_{2}\mathrm{Cl}_{2}\) have similar dipole moments but \(\mathrm{CH}_{3}\mathrm{Cl}\) has a smaller dipole moment due to its symmetry, making it slightly more volatile. - \(\mathrm{CH}_{2}\mathrm{Br}_{2}\) and \(\mathrm{CHBr}_{3}\) have the lowest electronegativity difference among the polar molecules, so they are more volatile than \(\mathrm{CBr}_{4}\).

Discuss boiling points

Boiling points depend on the strength of intermolecular forces, as stronger intermolecular forces require more energy to overcome. From the previous steps, we can infer the following trends in boiling points: - Nonpolar molecules with weaker dispersion forces will have the lowest boiling points (\(\mathrm{CH}_{4}\)) - Polar molecules with strong dipole-dipole interactions will have higher boiling points than nonpolar molecules with weaker dispersion forces. - Among the polar molecules, substances with larger dipoles will have higher boiling points (\(\mathrm{CH}_{2}\mathrm{Cl}_{2}, \mathrm{CH}_{2}\mathrm{Br}_{2}, \mathrm{CHBr}_{3}\)) - The substance with the strongest dispersion forces among the nonpolar molecules will have boiling points higher than the polar molecules with weaker dipoles (\(\mathrm{CBr}_{4}\)) As the intermolecular forces increase, the boiling points will also increase in the following order: \(\mathrm{CH}_{4} < \mathrm{CH}_{3}\mathrm{Cl} < \mathrm{CH}_{2}\mathrm{Cl}_{2} < \mathrm{CH}_{2}\mathrm{Br}_{2} < \mathrm{CHBr}_{3} < \mathrm{CBr}_{4}\)

Key concepts

Volatility in Chemistry

Volatility in chemistry refers to the tendency of a substance to vaporize, or turn into a gas, at a given temperature. Substances with high volatility vaporize readily at lower temperatures, while those with low volatility require higher temperatures to become gaseous. Volatility is largely determined by the strength of intermolecular forces within a substance—the weaker these forces, the more volatile the compound.

Considering simple organic compounds such as hydrocarbons and halogenated derivatives, such as methane (CH₄) and carbon tetrabromide (CBr₄), their volatility differences can be explained by their intermolecular forces. Methane, being a small nonpolar molecule with only dispersion forces, is highly volatile, meaning it easily vaporizes. In comparison, larger nonpolar molecules like carbon tetrabromide, which have greater dispersion forces due to their larger electron cloud and mass, will be less volatile and vaporize less readily.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules, which have regions of positive and negative charge due to an unequal distribution of electrons. These molecules have permanent dipoles that attract each other, affecting their physical properties. The strength of dipole-dipole interactions is influenced by the polarity of the molecule: the greater the difference in electronegativity between the bonded atoms, the larger the dipole moment and the stronger the interaction.

For molecules such as dichloromethane (CH₂Cl₂) and chloromethane (CH₃Cl), both are polar, but the symmetry of chloromethane reduces its dipole moment, making its dipole-dipole interactions weaker compared to dichloromethane. This difference in dipole moment leads to varying levels of volatility and boiling points among polar substances. Compounds with stronger dipole-dipole interactions will generally have lower volatility and higher boiling points.

Boiling Point Trends

The boiling point of a substance is the temperature at which it transitions from a liquid to a gas. It is a direct result of the intermolecular forces at play: stronger intermolecular forces yield higher boiling points because more energy is required to separate the molecules. For instance, as seen in the series of molecules in the exercise (CH₄, CBr₄, CH₂Cl₂, CH₃Cl, CHBr₃, CH₂Br₂), boiling points increase with stronger dipole-dipole interactions and larger dispersion forces.

Nonpolar molecules like methane (CH₄) exhibit the lowest boiling points due to weak dispersion forces. As we move to polar molecules, boiling points rise; however, among polar molecules, those with smaller dipole moments, like chloromethane (CH₃Cl), will have lower boiling points compared to those with larger dipole moments. In this sequence, carbon tetrabromide (CBr₄) with its strong dispersion forces, despite being nonpolar, has a higher boiling point than some of the polar molecules, illustrating the significant role of molecular size and mass in determining boiling point trends.