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) 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 interactions in the given solutions are: ion-dipole interaction in KCl in water, dipole-induced dipole interaction in \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) in benzene, and hydrogen bonding in methanol in water. Ranking the solutions from weakest to strongest interactions are: \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) in benzene, methanol in water, and KCl in water.

Step 1: Identify the principal type of solute-solvent interaction in each solution.

Let's look at the three given solutions and analyze the solute-solvent interactions taking place. (a) KCl in water: In this case, the solute (KCl) is an ionic compound and the solvent (water) is polar. Therefore, the principal type of solute-solvent interaction is ion-dipole interaction between the K+ and Cl- ions and the polar water molecules. (b) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) (\(\mathrm{CH}_{3} \mathrm{OH}\right)\) in benzene: The solute here is dichloromethane (\(\mathrm{CH}_{2} \mathrm{Cl}_{2}\)), which is a polar molecule because of the presence of polar C-Cl bonds. The solvent, benzene, is a nonpolar molecule. As a result, there will be dipole-induced dipole interaction between polar \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) and the nonpolar benzene. (c) Methanol (\(\mathrm{CH}_{3} \mathrm{OH}\right)\) in water: In this case, both the solute (methanol) and the solvent (water) are polar molecules with hydrogen bonding capacity. Therefore, the principal type of solute-solvent interaction is hydrogen bonding between the oxygen atom in methanol and the hydrogen atoms in the water molecules.

Step 2: Rank the solutions by strength of solute-solvent interactions.

Now that we have identified the principal types of solute-solvent interactions in the three solutions, let's rank them from the weakest to the strongest interaction. 1. Weakest Interaction: Solution (b) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) in benzene (dipole-induced dipole) 2. Intermediate Interaction: Solution (c) Methanol in water (hydrogen bonding) 3. Strongest Interaction: Solution (a) KCl in water (ion-dipole) Thus, the solutions are ranked from weakest to strongest interactions as follows: \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) in benzene, methanol in water, and KCl in water.

Key Concepts

Ion-Dipole Interactions

Ion-dipole interactions are a key type of attraction that occurs when an ionic compound dissolves in a polar solvent. This happens when there is an electrostatic attraction between the charged ions of the solute and the partial charges within the solvent molecules. A classic example is potassium chloride, or KCl, dissolving in water.
In this situation, the positively charged potassium ions (\(K^{+}\)) are attracted to the partially negative oxygen atoms of water (\(H_2O\)), while the negatively charged chloride ions (\(Cl^{-}\)) are drawn toward the partially positive hydrogen atoms of water. This can be visualized like tiny magnets, each seeking out its complementary charge. Ion-dipole interactions are generally quite strong because they involve the full ionic charge compared to the weaker interactions between partial charges.

• They are crucial in enabling ionic substances to dissolve in water or other polar solvents, forming solutions.
• Typically, the strength of the interaction correlates with the charge on the ion and polarity of the solvent.

The result is a homogeneous, stable solution thanks to the balance and strength of these interactions.

Hydrogen Bonding

Hydrogen bonding is a special and particularly strong kind of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. This interaction is especially important in alcohols, such as methanol (\(\mathrm{CH}_{3} \mathrm{OH}\)), dissolving in water.
Here, the hydrogen atom of methanol forms a bond with the oxygen atom of another methanol molecule or with the water molecule, resulting in a network of hydrogen bonds. This network of hydrogen bonds leads to high boiling points and significant liquid phase stability.

• Hydrogen bonds greatly affect the physical properties of substances, such as boiling and melting points.
• These bonds are crucial for many biological processes, including DNA structure and protein folding.

Though weaker than covalent bonds, hydrogen bonds significantly impact the behavior of molecules in solution, playing a critical role in the solubility and miscibility of compounds like methanol and water.

Dipole-Induced Dipole Interactions

Dipole-induced dipole interactions are a fascinating process that occurs when a polar molecule, like dichloromethane (\(\mathrm{CH}_{2} \mathrm{Cl}_{2}\)), disturbs the electron cloud of a nonpolar molecule, such as benzene (\(\mathrm{C}_{6} \mathrm{H}_{6}\)). This results in a temporary dipole within the nonpolar molecule, enabling an attraction.
The polar molecule’s charge induces a temporary dipole in the nonpolar molecule by slightly shifting its electrons, thereby encouraging an interaction in the otherwise nonpolar substance. These interactions are typically weaker than both hydrogen bonds and ion-dipole interactions because the induced dipole is temporary.

• These interactions are crucial for the solubility of nonpolar substances in polar solvents.
• They can influence the properties of mixtures, particularly in solutions involving gases and nonpolar liquids.

While they are not as strong as ion-dipole or hydrogen bonding interactions, dipole-induced dipole interactions are essential in understanding complex mixtures of polar and nonpolar substances.