Problem 21 - Chemistry: The Central Science in SI Units, Global Edition

Question

Which member in each pair has the greater dispersion forces? (a) \(\mathrm{CH}_{3} \mathrm{OH}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH},\) (b) \(\mathrm{NH}_{3}\) or \(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3}\), (c) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) or \(\mathrm{CH}_{2} \mathrm{Br}_{2}\)

Step-by-Step Solution

Answer

In each pair, the molecule with greater dispersion forces is: (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (Ethanol), (b) \(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3}\) (Trimethylamine), and (c) \(\mathrm{CH}_{2} \mathrm{Br}_{2}\) (Dibromomethane). This is because these molecules are larger and have more electrons, resulting in a more polarizable electron cloud and stronger dispersion forces.

Step 1: Identify the molecular structures

First, let's draw the molecular structures for both \(\mathrm{CH}_{3} \mathrm{OH}\) (Methanol) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (Ethanol). Methanol: H3C-O-H Ethanol: H3C-CH2-O-H

Step 2: Compare the molecular sizes

It can be observed that ethanol has an extra CH2 group compared to methanol, which makes it a larger molecule with more electrons.

Step 3: Determine the greater dispersion forces

Since ethanol has a larger molecular size and more electrons compared to methanol, it has greater dispersion forces. Therefore, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (Ethanol) has the greater dispersion forces in Pair A. #Pair B: \(\mathrm{NH}_{3}\) and \(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3}\)#

Step 4: Identify the molecular structures

First, let's draw the molecular structures for both Ammonia (\(\mathrm{NH}_{3}\)) and Trimethylamine (\(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3}\)). Ammonia: H3N Trimethylamine: (H3C)3N

Step 5: Compare the molecular sizes

It can be observed that trimethylamine has three CH3 groups attached to the central nitrogen atom, making it a larger molecule with more electrons compared to ammonia.

Step 6: Determine the greater dispersion forces

Since trimethylamine is a larger molecule and has more electrons compared to ammonia, it has greater dispersion forces. Therefore, \(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3}\) (Trimethylamine) has the greater dispersion forces in Pair B. #Pair C: \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) and \(\mathrm{CH}_{2} \mathrm{Br}_{2}\)#

Step 7: Identify the molecular structures

First, let's draw the molecular structures for both Dichloromethane (\(\mathrm{CH}_{2} \mathrm{Cl}_{2}\)) and Dibromomethane (\(\mathrm{CH}_{2} \mathrm{Br}_{2}\)). Dichloromethane: H2C-Cl2 Dibromomethane: H2C-Br2

Step 8: Compare the molecular sizes

It can be observed that the only difference between dichloromethane and dibromomethane is the halogen substituent; one has chlorine, and the other has bromine. Bromine atoms are larger and have more electrons compared to chlorine atoms.

Step 9: Determine the greater dispersion forces

Since dibromomethane has larger halogen substituents (bromine atoms) with more electrons compared to dichloromethane, it has greater dispersion forces. Therefore, \(\mathrm{CH}_{2} \mathrm{Br}_{2}\) (Dibromomethane) has the greater dispersion forces in Pair C.

Key Concepts

Intermolecular Forces

When studying molecules, we come across different interactions between them. Intermolecular forces are the attractions that occur between molecules. They play a crucial role in determining the physical properties of substances such as boiling points, melting points, and solubilities. In this context, dispersion forces are a type of intermolecular force, also known as London dispersion forces.

Dispersion forces arise due to temporary fluctuations in the electron distribution within molecules. These fluctuations create temporary dipoles, inducing dipoles in neighboring molecules, resulting in attractive forces. Even though they are the weakest among intermolecular forces, they are significant in non-polar molecules and contribute to the overall intermolecular attractions. The strength of these forces increases with larger molecules due to more electrons and a larger surface area for interactions.

• Dispersion forces occur in all molecular interactions.
• They are crucial in non-polar molecules.
• Larger molecules have stronger dispersion forces.

Molecular Size

The size of a molecule significantly impacts the strength of dispersion forces. Molecular size is essentially tied to its structure and the number of atoms and electrons it contains. In comparing molecules, the one with larger molecular size often exhibits more substantial dispersion forces because it has more electrons and a larger surface area for potential interactions.

For instance, in comparing ethanol (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\)) and methanol (\(\mathrm{CH}_{3} \mathrm{OH}\)), ethanol has an additional \(\mathrm{CH}_{2}\) group, making it larger with more electrons. Similarly, trimethylamine (\(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3}\)) is larger than ammonia (\(\mathrm{NH}_{3}\)) due to its three \(\mathrm{CH}_{3}\) groups.

• Larger molecules have increased electron count.
• More electrons lead to more prominent dispersion forces.
• Structures with more atoms generally have bigger sizes.

Electron Cloud

The electron cloud of a molecule refers to the area around the nucleus where electrons are likely to be found. This cloud is not static but fluctuates, leading to temporary dipoles in molecules. The size and shape of the electron cloud contribute to the molecule’s polarizability, which affects dispersion forces.

A more extensive electron cloud signifies a higher polarizability, meaning the electrons can be more easily distorted. This distortion leads to more significant temporary dipoles, enhancing dispersion forces. For example, in the case of dichloromethane (\(\mathrm{CH}_{2} \mathrm{Cl}_{2}\)) and dibromomethane (\(\mathrm{CH}_{2} \mathrm{Br}_{2}\)), dibromomethane has a larger electron cloud due to heavier bromine atoms, resulting in stronger dispersion forces.

• The electron cloud's size affects polarizability.
• Larger clouds mean more easily distorted electrons.
• Enhanced distortion increases dispersion forces.