Problem 45 - Chemistry: The Central Science

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?

Step-by-Step Solution

Answer

The substances in order of increasing volatility are CH4 < CH3Cl ≈ CH2Cl2 < CBr4 < CH2Br2 < CHBr3. This order is based on the strength and types of intermolecular forces present in each substance, such as van der Waals forces and dipole-dipole interactions. The boiling points of these substances increase in the same order, with CH4 having the lowest boiling point and CHBr3 having the highest boiling point.

Step 1: Analyze substance structures

We need to understand the structures of the given substances and determine which intermolecular forces are present. CH4: Methane, a nonpolar molecule with van der Waals forces. CBr4: Carbon tetrabromide, a nonpolar molecule with van der Waals forces. CH2Cl2: Dichloromethane, a polar molecule with dipole-dipole forces and van der Waals forces. CH3Cl: Chloromethane, a polar molecule with dipole-dipole forces and van der Waals forces. CHBr3: Bromoform, a polar molecule with dipole-dipole forces and van der Waals forces. CH2Br2: Dibromomethane, a polar molecule with dipole-dipole forces and van der Waals forces.

Step 2: Compare intermolecular forces specific for each substance

To understand their volatility, we need to compare their specific intermolecular forces and their strengths. In general, van der Waals forces are the weakest intermolecular forces, followed by dipole-dipole interactions and finally hydrogen bonding (not relevant to this exercise). Keep in mind that the charge of the atoms affects the strength of the intermolecular forces. Comparing these forces: - CH4 has the weakest van der Waals forces due to the small size of the hydrogen atoms and carbon atom. - CBr4 has stronger van der Waals forces compared to CH4 due to the larger and heavier bromine atoms. - CH2Cl2 and CH3Cl are polar but contain only weak van der Waals forces, so they have similar strength interactions. - CHBr3 and CH2Br2 are polar and have van der Waals forces, hence they exhibit the strongest interactions.

Step 3: Arrange substances in order

Now that the intermolecular forces have been compared, we can arrange them in order of increasing volatility: CH4 < CH3Cl ≈ CH2Cl2 < CBr4 < CH2Br2 < CHBr3

Step 4: Analyze boiling points

Boiling points are correlated to the strength of intermolecular forces in a substance. Stronger intermolecular forces results in higher boiling points. Based on the arranged order of increasing volatility, the boiling points would vary in the following manner: CH4 - Lowest boiling point CH3Cl - Slightly higher boiling point CH2Cl2 - Slightly higher boiling point CBr4 - Even higher boiling point CH2Br2 - Even higher boiling point CHBr3 - Highest boiling point In summary, ordering the given substances in increasing volatility and analyzing their boiling points gives the following order: CH4 < CH3Cl ≈ CH2Cl2 < CBr4 < CH2Br2 < CHBr3, with boiling points increasing in this order.

Key Concepts

Intermolecular Forces

Intermolecular forces are the attractions that occur between molecules, playing a crucial role in determining the physical properties of substances. They dictate how molecules interact with each other, influencing properties like boiling points and volatility. There are several types of intermolecular forces, with the most common being:

• Van der Waals forces (also known as London dispersion forces) - These are the weakest and arise due to temporary shifts in electron density in atoms or nonpolar molecules.
• Dipole-dipole interactions - These occur between polar molecules, where positive and negative ends of different molecules attract each other.
• Hydrogen bonding - A special case of dipole-dipole interactions, but stronger, occurs when hydrogen is bonded to highly electronegative elements like N, O, or F. Not relevant in this particular exercise.

Understanding these forces is key as they determine how easily a substance can transition into a gaseous state, which is directly related to its volatility.

Boiling Points

Boiling points are a representation of the temperature at which a substance transitions from the liquid phase to the gas phase. They are indicative of the strength of intermolecular forces present in a substance. When we analyze boiling points effectively, we find that:

• Stronger intermolecular forces lead to higher boiling points because more energy is needed to separate the molecules.
• Weak intermolecular forces yield lower boiling points, requiring less energy for the phase transition.

The exercise lists substances such as methane (\(\mathrm{CH}_4\)), dichloromethane (\(\mathrm{CH}_2\mathrm{Cl}_2\)), and chloromethane (\(\mathrm{CH}_3\mathrm{Cl}\)) as examples. Methane has the lowest boiling point due to its weak van der Waals forces, while bromoform (\(\mathrm{CHBr}_3\)) with stronger dipole-dipole interactions has a much higher boiling point.

Polarity

Polarity in molecules is due to the distribution of electrons and their arrangement around atoms. It causes some molecules to have partial positive charges on one side and partial negative charges on the other, making them polar. For example, in the molecule dichloromethane (\(\mathrm{CH}_2\mathrm{Cl}_2\)), the chlorine atoms attract electrons more than the hydrogen, creating a dipole.Factors affecting polarity include:

• The difference in electronegativity between atoms: Greater differences lead to more polar bonds.
• The molecular geometry: Symmetrical arrangements can lead to nonpolar molecules, even if they contain polar bonds.

Polarity plays a key role in determining which type of intermolecular forces will be present, as polar molecules engage in dipole-dipole interactions.

Dipole-Dipole Interactions

Dipole-dipole interactions are specific types of intermolecular attractions that occur between polar molecules. Each molecule has a positive pole and a negative pole, akin to tiny magnets.Here is how they function:

• Molecules align such that the positive area of one is near the negative area of another. This alignment enhances the overall attraction between molecules.
• The strength of these interactions depends on the magnitude of the dipole moment. Larger dipoles lead to stronger dipole-dipole interactions.

For example, chloromethane (\(\mathrm{CH}_3\mathrm{Cl}\)) and bromoform (\(\mathrm{CHBr}_3\)) are polar and exhibit dipole-dipole interactions, making them less volatile compared to nonpolar substances like methane. Dipole-dipole interactions, while stronger than van der Waals forces, are weaker than hydrogen bonds, highlighting their middle-ground status in intermolecular forces.