Problem 16 - Chemistry The Central Science

Question

Indicate the principal type of solute-solvent interaction in each of the following solutions and rank the solutions from weakest to strongest solute- solvent interaction: (a) \(\mathrm{KCl}\) in water, (b) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) in benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right),(\mathrm{c})\) methanol \(\left(\mathrm{CH}_{3} \mathrm{OH}\right)\) in water.

Step-by-Step Solution

Answer

The primary solute-solvent interactions in the given solutions are as follows: (a) ion-dipole interaction for KCl in water, (b) dispersion interaction for CH2Cl2 in benzene, and (c) hydrogen bonding interaction for methanol in water. The solutions can be ranked from weakest to strongest interaction as follows: CH2Cl2 in benzene, methanol in water, and KCl in water.

Step 1: Identify solutes and solvents

In the given solutions, we have the following solutes and solvents: a) KCl in water b) CH2Cl2 in benzene (C6H6) c) Methanol (CH3OH) in water

Step 2: Determine the type of interaction

For each solution, let's assess the interaction between the solute and the solvent: a) KCl is an ionic compound, and water is a polar solvent. Ionic compounds dissolved in polar solvents generally undergo ion-dipole interactions. b) CH2Cl2 is a polar molecule due to electronegativity difference between carbon and chlorine atoms, while benzene is a non-polar solvent. The type of interaction for this case is a dispersion interaction (weakest of the three types of van der Waals forces), between the polar CH2Cl2 molecules and the non-polar benzene molecules. c) Methanol is a polar molecule due to the presence of the hydroxyl group (OH-) and water is also a polar solvent. The interaction between a polar solute and a polar solvent is a hydrogen bonding interaction, specifically between the hydroxyl groups in both methanol and water.

Step 3: Rank the solutions based on interaction strength

Based on the type of solute-solvent interactions, we can order the solutions from weakest to strongest interaction as follows: - Dispersion interaction: CH2Cl2 in benzene (weakest) - Hydrogen bonding interaction: Methanol in water (medium strength) - Ion-dipole interaction: KCl in water (strongest) So, the final ranking of the solutions from weakest to strongest solute-solvent interaction is: (b) CH2Cl2 in benzene, (c) methanol in water, and (a) KCl in water.

Key Concepts

Ionic Compounds

Ionic compounds are made up of charged particles called ions. These ions form when atoms either lose or gain electrons, resulting in positively charged cations and negatively charged anions. In a solution, ionic compounds dissociate into their individual ions. A classic example is potassium chloride, or \( \text{KCl} \), which separates into \( \text{K}^+ \) and \( \text{Cl}^- \) ions when dissolved in water. Solute-Solvent Interactions
Ionic compounds often dissolve well in polar solvents like water. The interaction between the ionic compound and the water is known as ion-dipole interaction.

• Ion-Dipole Interaction: The polar water molecules attract the ions in the ionic compound. The positively charged part of the water molecule (hydrogen) interacts with negative ions, while the negatively charged part (oxygen) interacts with positive ions.

This interaction is quite strong due to the charge of the ions and is generally stronger than many other types of solute-solvent interactions.

Polar Molecules

Polar molecules have an uneven distribution of electron charge, resulting in areas of partial positive and partial negative charges within the molecule. This is usually due to differences in electronegativity between the atoms involved. One common example of a polar molecule is dichloromethane (\( \text{CH}_2\text{Cl}_2 \)). This molecule has a net dipole moment due to the difference in electronegativity between carbon and chlorine. Interactions with Non-Polar Solvents
When polar molecules like \( \text{CH}_2\text{Cl}_2 \) are mixed with non-polar solvents like benzene, they primarily experience dispersion forces, a type of van der Waals force.

• Dispersion Interactions: These are the weakest types of intermolecular forces and arise from temporary shifts in the density of electrons in electron clouds. While relatively weak, they can still lead to solute-solvent interactions in mixtures of polar and non-polar substances.

These weak interactions make it generally more challenging for polar molecules to dissolve effectively in non-polar solvents.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction. It occurs when a hydrogen atom, which is covalently bonded to an electronegative atom like oxygen, nitrogen, or fluorine, interacts with another electronegative atom containing a lone pair of electrons. Methanol (\( \text{CH}_3\text{OH} \)) provides a good example of hydrogen bonding. The hydrogen atom in its hydroxyl group (\(-\text{OH}\)) can form hydrogen bonds with other polar molecules like water. Hydrogen Bonding in Solutions
Methanol can easily dissolve in water due to hydrogen bonding.

• Hydrogen Bonding with Water: The hydrogen bonds form between the hydroxyl group of methanol and the polar water molecules. The partial positive charge of the hydrogen atom in methanol attracts the partial negative charge on the oxygen atom in water, and vice versa, creating a tight interaction.

This leads to a medium-strength interaction that is stronger than dispersion forces but generally weaker than ion-dipole interactions.