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 \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\), (c) methanol \(\left(\mathrm{CH}_{3} \mathrm{OH}\right)\) in water.

Step-by-Step Solution

Answer

The principal type of solute-solvent interaction in each solution is as follows: (a) ion-dipole interaction in KCl and water, (b) dipole-induced dipole (dispersion forces) in CH2Cl2 and benzene, and (c) dipole-dipole interaction in methanol and water. The solutions are ranked from weakest to strongest solute-solvent interaction: (1) CH2Cl2 in benzene, (2) methanol in water, and (3) KCl in water.

Step 1: Solution (a) - KCl in water

In this solution, potassium chloride (KCl) is an ionic compound and water is a polar solvent. The predominant type of solute-solvent interaction here is ion-dipole interaction. In this case, the positive potassium ions (K+) are attracted to the negative oxygen end of the water dipole, and the negative chloride ions (Cl-) are attracted to the positive hydrogen end of the water dipole.

Step 2: Solution (b) - CH2Cl2 in benzene

In this solution, dichloromethane (CH2Cl2) is a polar molecule and benzene (C6H6) is a nonpolar molecule. The predominant type of solute-solvent interaction in this case is dipole-induced dipole or dispersion forces. Although CH2Cl2 is a polar molecule, the interacting forces with nonpolar benzene are relatively weak.

Step 3: Solution (c) - Methanol in water

In this solution, methanol (CH3OH) is a polar molecule and water is also a polar solvent. The predominant type of solute-solvent interaction in this case is dipole-dipole interaction. In this case, the positive hydrogen end of the methanol is attracted to the negative oxygen end of the water molecule, and the negative oxygen end of the methanol interacts with the positive hydrogen end of the water molecule.

Step 4: Ranking the solutions

Based on the predominant types of interactions in each solution, we can rank the solutions from weakest to strongest solute-solvent interaction: 1. Weakest interaction: Solution (b) - CH2Cl2 in benzene (dipole-induced dipole / dispersion forces) 2. Intermediate interaction: Solution (c) - Methanol in water (dipole-dipole interaction) 3. Strongest interaction: Solution (a) - KCl in water (ion-dipole interaction)

Key Concepts

Ion-Dipole Interaction

Ion-dipole interactions are common when an ionic compound dissolves in a polar solvent. This type of interaction occurs between an ion (a charged particle) and a polar molecule, like water. In the example of potassium chloride (KCl) dissolving in water, the positive ions (potassium, K\(^+\)) are attracted to the slightly negative oxygen atoms in the water molecules. Conversely, the negative ions (chloride, Cl\(^-\)) are drawn to the somewhat positive hydrogen atoms in water.
This attractive force between the ions and the dipole of the water molecules helps to dissolve KCl in water. Ion-dipole interactions are particularly strong, often the strongest among different solute-solvent interactions. They play a crucial role in the dissolution of salts in water and other polar solvents, facilitating various biological processes and industrial applications.

Key characteristics of ion-dipole interactions include:

• Involvement of ions and polar molecules
• Strong forces due to attraction of opposite charges
• Common in solutions where ionic compounds are dissolved in polar solvents

Dipole-Induced Dipole Interaction

Dipole-induced dipole interactions occur when a polar molecule induces a temporary dipole in a nonpolar molecule. This happens because the presence of a polar molecule, with its uneven charge distribution, can cause a shift in the electron cloud of a nearby nonpolar molecule.
In the case of dichloromethane (CH\(_2\)Cl\(_2\)) dissolving in benzene (C\(_6\)H\(_6\)), the interaction is relatively weak compared to others. Dichloromethane is a polar molecule, but benzene is nonpolar. When mixed, the electron cloud of benzene can be distorted slightly by the dipole of dichloromethane, creating a temporary polarity.
This temporary dipole in benzene then interacts weakly with the CH\(_2\)Cl\(_2\), resulting in dipole-induced dipole interactions. Such interactions are generally weak and contribute to the solubility of some nonpolar substances in polar solvents.

Characteristics of dipole-induced dipole interactions include:

• Interaction between a polar molecule and a nonpolar molecule
• Temporary, weak dipoles formed
• Relatively weak compared to others, but still important in certain mixtures

Dipole-Dipole Interaction

Dipole-dipole interactions occur between molecules that have permanent dipoles. This type of interaction is evident in solutions composed entirely of polar molecules, such as methanol (CH\(_3\)OH) dissolving in water. Both methanol and water possess dipoles, thanks to the presence of electronegative atoms like oxygen.
The interaction is primarily due to the attraction between the positive end of one dipole and the negative end of another. For instance, in a methanol-water solution, the hydrogen of methanol's OH group (positive end) is attracted to the oxygen of water (negative end), and vice versa. Dipole-dipole interactions are stronger than dispersion forces but generally weaker than ion-dipole interactions.

Some defining features of dipole-dipole interactions include:

• Involvement of molecules with permanent dipoles
• Medium interaction strength
• Presence in solutions where polar molecules are dissolved in polar solvents

Dipole-dipole attractions are crucial for the physical properties of solutions, such as boiling and melting points, and solubility in various solvents.