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) KCl in water, (b) ${\mathrm{CH}}_2{\mathrm{Cl}}_2$ in benzene $\left({\mathrm{C}}_6{\mathrm{H}}_6\right),(\mathbf{c})$ methanol $\left({\mathrm{CH}}_3\mathrm{OH}\right)$ in water.

Short answer

The principal types of solute-solvent interactions for the given solutions are: (a) Ion-dipole interaction for KCl in water, (b) Dispersion forces for ${\mathrm{CH}}_2{\mathrm{Cl}}_2$ in benzene, and (c) Dipole-dipole interaction for methanol in water. The ranking of the solutions based on the strength of solute-solvent interactions is: Weakest - (b) ${\mathrm{CH}}_2{\mathrm{Cl}}_2$ in benzene, Medium - (c) Methanol in water, and Strongest - (a) KCl in water.

Step-by-step solution

Identify Types of Solute-Solvent Interactions

There are mainly three types of solute-solvent interactions: 1. Ion-dipole interactions: Occur between ions and polar molecules 2. Dipole-dipole interactions: Occur between polar molecules 3. Dispersion forces: Occur between non-polar molecules

Analyze the Given Solutions

We will analyze each of the given solutions to determine the type of solute-solvent interaction: (a) KCl in water: KCl is an ionic compound and water (H2O) is a polar molecule. Therefore, the interaction is an ion-dipole interaction. (b) ${\mathrm{CH}}_2{\mathrm{Cl}}_2$ in benzene $\left({\mathrm{C}}_6{\mathrm{H}}_6\right)$: Both solute (${\mathrm{CH}}_2{\mathrm{Cl}}_2$) and solvent (benzene) are non-polar molecules. Therefore, the interaction is due to dispersion forces. (c) Methanol $\left({\mathrm{CH}}_3\mathrm{OH}\right)$ in water: Both methanol and water are polar molecules. Therefore, the interaction is a dipole-dipole interaction.

Rank the Solutions Based on Strength of Interactions

Ion-dipole interactions are generally stronger than dipole-dipole interactions, which in turn are stronger than dispersion forces. Therefore, the order of strength of solute-solvent interactions is: Weakest: (b) ${\mathrm{CH}}_2{\mathrm{Cl}}_2$ in benzene (Dispersion forces) Medium: (c) Methanol in water (Dipole-dipole interactions) Strongest: (a) KCl in water (Ion-dipole interaction)

Key concepts

Ion-Dipole Interactions

Understanding ion-dipole interactions is crucial for grasping the behavior of ionic compounds when they dissolve in polar solvents. These occur when ions from a compound interact with the dipoles in a polar solvent.
For example, when potassium chloride (KCl) dissolves in water, the positive potassium (K⁺) ions and the negative chloride (Cl^-) ions are surrounded by water molecules.
The dipole nature of water means it has a partially positive charge on its hydrogen atoms and a partially negative charge on its oxygen atom.

• The positive K⁺ ions are attracted to the oxygen side of the water molecules.
• The negative Cl^- ions are attracted to the hydrogen side of the water molecules.

These attractions hold the ions tightly in solution, making ion-dipole interactions very strong, surpassing other types of solute-solvent interactions like dipole-dipole or dispersion forces.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between molecules that have permanent dipoles, meaning they always have a positive and a negative side.
These molecules are typically polar and attract each other based on their respective charges, much like magnets.
An excellent example is methanol dissolving in water.
Both methanol (CH₃OH) and water are polar molecules.

• In methanol, the oxygen atom is more electronegative, causing the oxygen-hydrogen bond to be polar.
• In water, the O-H bonds create an overall dipole moment.

As a result, the positive end of one molecule aligns with the negative end of another, leading to substantial attraction.
While dipole-dipole interactions are significant, they tend to be weaker than ion-dipole interactions, as they only involve neutral molecules rather than charged ions.

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are the weakest type of solute-solvent interactions and occur between non-polar molecules.
These forces arise due to temporary fluctuations in electron density within a molecule that lead to temporary dipoles.
These dipoles can induce others in neighboring molecules, leading to attraction between them.
An example of dispersion forces in action is when dichloromethane (${\text{CH}}_2{\text{Cl}}_2$) dissolves in benzene (${\text{C}}_6{\text{H}}_6$).

• Both substances lack a permanent dipole moment and are non-polar.
• While normally distributed electrons become momentarily unsymmetrical, creating temporary dipoles.

While these momentary attractions are weaker than dipole-dipole interactions and far weaker than ion-dipole interactions, they are essential in understanding the behavior of non-polar substances.
Despite being the least potent, dispersion forces are universal, present in all molecular interactions, and crucial for the liquid state of noble gases and non-polar compounds.