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.

Short answer

The principal type of solute-solvent interactions in the given solutions are: (a) KCl in water - ion-dipole forces, (b) CH2Cl2 in benzene (C6H6) - dispersion forces, and (c) methanol (CH3OH) in water - hydrogen bonding. Ranking the solutions from weakest to strongest solute-solvent interaction, we have: (1) CH2Cl2 in benzene (C6H6) - dispersion forces, (2) methanol (CH3OH) in water - hydrogen bonding, and (3) KCl in water - ion-dipole forces.

Step-by-step solution

Identify solute-solvent interactions in each solution

The main types of solute-solvent interactions are ion-dipole, dipole-dipole, hydrogen bonding, and dispersion forces. For each solution, we will examine what type of interaction is present: (a) KCl in water: KCl is an ionic compound, and water is a polar solvent with a dipole moment. The solute-solvent interaction in this solution is dominated by ion-dipole forces. (b) CH2Cl2 in benzene (C6H6): Both CH2Cl2 and benzene are non-polar molecules. There are no ions or dipoles involved. In this case, the main solute-solvent interaction is dispersion forces (also known as London forces). (c) Methanol (CH3OH) in water: Both methanol and water are polar molecules with dipole moments. Moreover, both molecules can form hydrogen bonds. The solute-solvent interaction in this solution is dominated by hydrogen bonding.

Rank the solute-solvent interactions from weakest to strongest

Now we will rank the solute-solvent interactions based on their strength. Typically, ion-dipole forces are the strongest, followed by hydrogen bonding, dipole-dipole, and finally dispersion forces. Therefore, the ranking from weakest to strongest solute-solvent interaction is as follows: 1. CH2Cl2 in benzene (C6H6): dispersion forces 2. Methanol (CH3OH) in water: hydrogen bonding 3. KCl in water: ion-dipole forces

Key concepts

Ion-Dipole Forces

When we talk about ion-dipole forces, we're discussing one of the strongest types of solute-solvent interactions. Imagine a salt like potassium chloride (KCl) dissolving in water. Here, KCl breaks down into ions: potassium (K⁺) and chloride (Cl^-).
A water molecule has a slight positive and negative end—known as a dipole—due to its polar nature. So, the positive end of a water molecule attracts the negatively charged chloride ion, while the negative end attracts the positively charged potassium ion.
This attraction between ions and polar water molecules is what we refer to as ion-dipole forces. They are highly effective in dissolving ionic compounds like many salts in water, explaining why KCl dissolves very well in water.

• Strongest among solute-solvent interactions
• Occurs when an ionic compound mixes with a polar solvent

Ion-dipole forces play a crucial role in many biological and chemical processes, emphasizing their importance in understanding solute-solvent interactions.

Dispersion Forces

Dispersion forces, also known as London forces, are the weakest type of solute-solvent interactions. Even though they are the weakest, they are incredibly important because they occur in every molecule or atom, whether polar or nonpolar.
In the scenario involving CH₂Cl₂ in benzene, both are non-polar molecules, meaning there are no permanent dipoles present. Here, dispersion forces become the primary interaction, resulting from temporary fluctuations in electron densities within the molecules.
This creates a brief dipole in one molecule, inducing a similar temporary dipole in a neighboring one. The attraction between these instantaneous dipoles is what we call dispersion forces.

• Weakest solute-solvent interaction
• Present in all molecules, regardless of their polarity

Though weak, dispersion forces are essential for several physical phenomena, including the condensation of gases and the formation of liquids and solids.

Hydrogen Bonding

Among the stronger types of molecular interactions, hydrogen bonding is key to many properties of substances. When considering methanol (CH₃OH) dissolved in water, hydrogen bonds become the primary solute-solvent interaction.
Methanol and water are both polar molecules capable of forming these strong interactions. A hydrogen bond occurs when a hydrogen atom covalently bound to an electronegative atom like oxygen, nitrogen, or fluorine, forms an attraction to another nearby electronegative atom in a different molecule.
This results in strong attractions that can greatly influence the physical properties of a substance, such as its boiling and melting points.

• Second only to ion-dipole in strength
• Occurs in polar molecules with hydrogen attached to F, O, or N

Hydrogen bonds are responsible for holding together many biological structures, such as the DNA double helix, and play a crucial role in many chemical processes.