Problem 15 - Chemistry The Central Science In Si Units

Question

Indicate the type of solute-solvent interaction (Section 11.2) that should be most important in each of the following solutions: (a) \(\mathrm{CCl}_{4}\) in benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right),(\mathbf{b})\) methanol \(\left(\mathrm{CH}_{3} \mathrm{OH}\right)\) in water, \((\mathbf{c}) \mathrm{KBr}\) in water, \((\mathbf{d}) \mathrm{HCl}\) in acetonitrile \(\left(\mathrm{CH}_{3} \mathrm{CN}\right)\)

Step-by-Step Solution

Answer

(a) London dispersion forces; (b) hydrogen bonding; (c) ion-dipole interactions; (d) ion-dipole interactions.

Step 1: Understand Solute-Solvent Interactions

Solute-solvent interactions are the forces that occur between the solute molecules and solvent molecules during dissolution. These interactions include London dispersion forces, dipole-dipole interactions, hydrogen bonding, and ionic interactions, depending on the chemical nature of the solute and solvent.

Step 2: Analyze Interaction in (a) \( \text{CCl}_4 \) and Benzene

Both \(\text{CCl}_4\) and benzene \((\text{C}_6\text{H}_6)\) are nonpolar molecules. The most important interaction between nonpolar molecules is London dispersion forces. Therefore, \(\text{CCl}_4\) in benzene involves London dispersion forces as the most significant interaction.

Step 3: Analyze Interaction in (b) Methanol and Water

Methanol \((\text{CH}_3\text{OH})\) and water are both polar molecules capable of hydrogen bonding due to the presence of -OH groups. As a result, hydrogen bonding is the most important type of interaction in the solution of methanol in water.

Step 4: Analyze Interaction in (c) \( \text{KBr} \) and Water

\(\text{KBr}\) is an ionic compound that dissociates into \(\text{K}^+\) and \(\text{Br}^-\) ions in water. The primary interaction here is the ion-dipole interaction between the ions and the polar water molecules, which stabilizes the ions in solution.

Step 5: Analyze Interaction in (d) \( \text{HCl} \) and Acetonitrile

\(\text{HCl}\) is a polar molecule that can dissociate into \(\text{H}^+\) and \(\text{Cl}^-\) ions. Acetonitrile \((\text{CH}_3\text{CN})\) is a polar solvent. Thus, the primary solute-solvent interaction is the ion-dipole interaction between the ions formed from \(\text{HCl}\) and the polar molecules of acetonitrile.

Key Concepts

London Dispersion Forces

London dispersion forces are a type of weak intermolecular force that can occur between all kinds of molecules, whether they are polar or nonpolar. However, they are most significant between nonpolar molecules. These forces are the result of temporary shifts in electron density within a molecule, which creates a temporary dipole moment. That temporary dipole in one molecule can induce a similar temporary dipole in a neighboring molecule, leading to a weak, transient attraction.

Here's why this interaction is important:

• London dispersion forces increase with the size and the electron cloud of the molecules involved. Larger molecules with more electrons have stronger dispersion forces because of the greater ability to polarize.
• In the example of \( \text{CCl}_4 \) dissolving in benzene \( \text{C}_6\text{H}_6 \), both substances are nonpolar. This means that London dispersion forces are the primary type of solute-solvent interaction because there are no permanent dipoles to establish other forms of interaction.
• These forces are crucial because they can dictate the solubility of nonpolar substances in nonpolar solvents, allowing substances like waxes and oils to dissolve in organic solvents.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that occurs between molecules when hydrogen is bonded to highly electronegative atoms, such as oxygen, nitrogen, or fluorine. This bond is stronger than other dipole-dipole interactions due to the high polarity of the bonds and the small size of hydrogen, allowing closer approach of the interacting molecules.

Consider the following details:

• Hydrogen bonds are not true chemical bonds, but they are strong enough to significantly affect the properties of substances, particularly boiling and melting points, solubility, and specific heat.
• In the case of methanol \( \text{CH}_3\text{OH} \) in water, both substances can engage in hydrogen bonding due to their \(-\text{OH} \) groups. This results in a strong interaction that greatly enhances their miscibility.
• Hydrogen bonding is critical in biology, playing a crucial role in the structure of DNA and proteins, where they help maintain the characteristic shapes of these molecules.

Ion-Dipole Interactions

Ion-dipole interactions are forces that occur between charged ions and polar molecules. These are among the strongest types of intermolecular forces, due to the presence of a fully charged ion interacting with the partial charge on a dipole molecule. This interaction plays a critical role in the dissolution of ionic compounds in polar solvents.

Here are the key points about ion-dipole interactions:

• Water, being a polar solvent with its polar \(-\text{OH} \) bonds, is adept at forming these interactions. This explains its ability to dissolve ionic substances like salts.
• For example, when \( \text{KBr} \) is dissolved in water, the \(\text{K}^+ \) and \(\text{Br}^- \) ions interact with the dipole in water molecules. The positive ion \(\text{K}^+ \) attracts the lone pairs on the oxygen end of the water, while the negative \(\text{Br}^- \) is attracted to the hydrogen side.
• The strength of these interactions contributes to the high solubility of many salts in water, making it an excellent solvent.
• In the case of \( \text{HCl} \) in acetonitrile \( \text{CH}_3\text{CN} \), similar ion-dipole interactions help stabilize the dissociated ions in the polar solvent.