Problem 11 - Chemistry The Molecular Science

Question

Rank these substances in order of increasing noncovalent intermolecular attractions. For each substance, name the types of intermolecular attractions that occur. (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{3}\)

Step-by-Step Solution

Answer

(b) < (a) < (d) < (c)

Step 1: Identify Intermolecular Forces

First, identify the types of intermolecular forces present in each substance. For (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\), a nonpolar alkanes, it exhibits London dispersion forces.For (b) \(\mathrm{CH}_{3} \mathrm{CH}_{3}\), another nonpolar alkane, it also exhibits London dispersion forces.For (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\), an alcohol, it has hydrogen bonding due to \(-\mathrm{OH}\) group, as well as London dispersion and dipole-dipole interactions.For (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{3}\), an ether, it displays dipole-dipole interactions and London dispersion forces.

Step 2: Determine Strength of Intermolecular Forces

Compare the strength of the intermolecular forces across the substances. London dispersion forces have the weakest strength. Dipole-dipole forces are stronger than London forces but weaker than hydrogen bonds. Hydrogen bonding is the strongest of these forces.Therefore, substance (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) with hydrogen bonding has the strongest interactions. Substance (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{3}\), with dipole-dipole attractions, has stronger forces than substances (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) and (b) \(\mathrm{CH}_{3} \mathrm{CH}_{3}\), which only have London dispersion forces.

Step 3: Rank the Substances

Based on the strength of the intermolecular forces:1. (b) \(\mathrm{CH}_{3} \mathrm{CH}_{3}\) - only London dispersion forces, lowest interactions.2. (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) - only London dispersion forces, higher than (b) due to size.3. (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{3}\) - dipole-dipole forces and London dispersion.4. (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) - hydrogen bonding, dipole-dipole, and London dispersion interactions.

Key Concepts

London Dispersion Forces

London Dispersion Forces, also known as van der Waals forces, are one of the fundamental types of intermolecular attractions. They are particularly significant in nonpolar molecules. These forces arise due to the momentary fluctuations in electron density around a molecule, which creates temporary dipoles.

Since these forces are temporary and rely on the movement of electrons, they tend to be the weakest among intermolecular forces. However, they become stronger with increasing molecular size or mass. This means that larger and heavier molecules will have stronger London dispersion forces.

• Commonly found in nonpolar molecules.
• Temporary and weakest intermolecular forces.
• Strength increases with molecular size or weight.

Understanding these forces is crucial because even though they are weak, they are present in all molecules and can dictate physical properties such as boiling points and solubility.

Hydrogen Bonding

Hydrogen Bonding is a strong type of intermolecular attraction that occurs when a hydrogen atom is covalently bonded to a highly electronegative atom, such as oxygen, nitrogen, or fluorine. The presence of these bonds creates a significant dipole, as the hydrogen atom carries a partial positive charge, and the electronegative atom bears a partial negative charge.

This kind of bonding is notably stronger than other dipole interactions due to the high polarity of the bond. Substances like water and alcohols, where this type of bond commonly occurs, exhibit higher boiling points and increased solubility with other polar substances.

• Occurs with hydrogen and electronegative atoms (O, N, F).
• Produces significant dipoles due to polarity.
• Contributes to higher boiling points and solubility.

Its presence is pivotal in numerous biological processes and contributes to the unique characteristics of water.

Dipole-Dipole Interactions

Dipole-Dipole Interactions are found in polar molecules where permanent dipoles exist. Unlike London Dispersion Forces, which are temporary, dipole-dipole interactions are permanent and occur due to the electrostatic attraction between the positive end of one polar molecule and the negative end of another.

These forces are stronger than London Dispersion Forces but generally weaker than hydrogen bonds. They play a crucial role in determining the physical properties of substances, such as melting and boiling points.

• Exists in molecules with permanent dipoles.
• Stronger than London forces but weaker than hydrogen bonds.
• Affects physical properties like boiling and melting points.

Polar molecules, such as ethers and many organic compounds, rely on these interactions to align themselves properly in the liquid and solid states, affecting how substances mix and interact.