Problem 63 - Introductory Chemistry Atoms First

Question

(a) Name the strongest intermolecular force in \(\mathrm{CH}_{3} \mathrm{OH}, \mathrm{CH}_{3} \mathrm{Cl}, \mathrm{CH}_{3} \mathrm{CH}_{3}\), and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (b) Rank these molecules from lowest to highest boiling point.

Step-by-Step Solution

Answer

The strongest intermolecular forces in each molecule are: CH3OH - Hydrogen bonding, CH3Cl - Dipole-dipole interactions, CH3CH3 and CH3CH2CH3 - London dispersion forces. The molecules ranked from lowest to highest boiling point are: CH3CH3, CH3CH2CH3, CH3Cl, and CH3OH.

Step 1: Identify the types of intermolecular forces in each molecule

Start by looking at each molecule and determining the type of intermolecular forces present. 1. CH3OH: In this molecule, there is a hydrogen atom bonded to an oxygen atom, which makes it capable of forming hydrogen bonds. 2. CH3Cl: In this molecule, there is a polar C-Cl bond, resulting in dipole-dipole interactions. 3. CH3CH3: This molecule is nonpolar and only experiences London dispersion forces. 4. CH3CH2CH3: This molecule is also nonpolar and only experiences London dispersion forces.

Step 2: Determine the strongest intermolecular force in each molecule

The strongest intermolecular force for each molecule is the following: 1. CH3OH: Hydrogen bonding 2. CH3Cl: Dipole-dipole interactions 3. CH3CH3: London dispersion forces 4. CH3CH2CH3: London dispersion forces

Step 3: Rank the molecules based on the strength of their intermolecular forces

Hydrogen bonding is the strongest type of intermolecular force, followed by dipole-dipole interactions and London dispersion forces. So, we can rank the molecules as follows: 1. CH3OH (strongest intermolecular force) 2. CH3Cl 3. CH3CH3 and CH3CH2CH3 (weakest intermolecular forces)

Step 4: Rank the molecules based on their boiling points

The boiling point of a substance is directly related to the strength of its intermolecular forces. Substances with stronger intermolecular forces have higher boiling points. Based on this information, we can rank the molecules from lowest to highest boiling point as follows: 1. CH3CH3 (lowest boiling point) 2. CH3CH2CH3 3. CH3Cl 4. CH3OH (highest boiling point)

Key Concepts

Hydrogen Bonding

Understanding hydrogen bonding is crucial when studying the properties of molecules. Hydrogen bonds are a type of strong dipole-dipole attraction, but they occur only between a hydrogen atom and a highly electronegative atom, such as nitrogen, oxygen, or fluorine. In methanol (\( \text{CH}_3\text{OH} \) for instance, the hydrogen attached to the oxygen is capable of forming hydrogen bonds with other electronegative atoms.

This intermolecular force significantly raises the boiling point of a substance, making methanol boil at a higher temperature compared to molecules where hydrogen bonding is absent. This is because the energy required to break the hydrogen bonds during the phase change from liquid to gas is considerably greater.

Hydrogen bonding also plays a fundamental role in the unique properties of water and the structure of DNA, confirming its significance in both chemical and biological contexts.

Dipole-Dipole Interactions

Dipole-dipole interactions occur in molecules where there is an uneven distribution of electrons, creating a permanent dipole. For example, in methyl chloride (\( \text{CH}_3\text{Cl} \) the chlorine atom is more electronegative than the carbon atom, resulting in a polar molecule.

The positive end of one polar molecule is attracted to the negative end of another, causing these molecules to align themselves in a specific manner. This interaction is generally weaker than hydrogen bonding but stronger than London dispersion forces. Substances with dipole-dipole interactions tend to have higher boiling points than nonpolar molecules because more energy is required to disrupt these intermolecular attractions.

London Dispersion Forces

London dispersion forces are the weakest type of intermolecular force, yet they are universal, affecting all molecules. These forces are a result of the temporary dipoles that occur when electrons move about within molecules. Nonpolar molecules like ethane (\( \text{CH}_3\text{CH}_3 \) and propane (\( \text{CH}_3\text{CH}_2\text{CH}_3 \) exhibit only these forces.

The strength of London dispersion forces increases with the size of the molecule and the number of electrons. Therefore, larger nonpolar molecules tend to have higher boiling points compared to smaller ones due to stronger dispersion forces. It's noteworthy that these forces can also influence the boiling points of polar molecules but are usually overseen due to the presence of stronger intermolecular attractions.

Boiling Point

The boiling point of a substance is a valuable indicator of its intermolecular forces. It is the temperature at which a liquid transitions to a gas, and this occurs when the molecules have enough energy to overcome their intermolecular attractions. As seen in the example with methanol (\( \text{CH}_3\text{OH} \) having the highest boiling point due to strong hydrogen bonds, and ethane (\( \text{CH}_3\text{CH}_3 \) with the lowest due to weak London dispersion forces, boiling points correlate directly with the strength of intermolecular forces.

To summarize, the ranking from lowest to highest boiling point of the given molecules reflects the hierarchy of intermolecular forces at play: London dispersion forces present in nonpolar molecules contribute to the lower boiling points and dipole-dipole interactions, and especially hydrogen bonding in polar molecules, lead to higher boiling points. This knowledge elevates our comprehension of molecular behavior and is pivotal in fields ranging from material science to pharmacology.