Question

Indicate the type of solute-solvent interaction (Section 11.2) that should be most important in each of the following solutions: \((\mathbf{a}) \mathrm{CCl}_{4}\) in benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right),(\mathbf{b})\) methanol \(\left(\mathrm{CH}_{3} \mathrm{OH}\right)\) in water, \((\mathbf{c}) \mathrm{KBr}\) in water, (d) HCl in acetonitrile \(\left(\mathrm{CH}_{3} \mathrm{CN}\right).\)

Short answer

In summary, the types of solute-solvent interactions in each solution are as follows: a) London dispersion forces for CCl₄ in benzene, b) hydrogen bonding for methanol in water, c) ion-dipole interactions for KBr in water, and d) dipole-dipole forces for HCl in acetonitrile.

Step-by-step solution

Analyze the polarity of solute and solvent in each solution

First, we will determine the polarity of the molecules involved in each solution. Polar molecules have electronegativity differences causing a dipole moment, whereas non-polar molecules do not have any dipole moment. a) CCl₄ (carbon tetrachloride) in benzene (C₆H₆) - CCl₄: Tetrahedral molecule with evenly distributed charges - Non-polar - Benzene: Evenly distributed electron cloud since the molecule is aromatic - Non-polar b) Methanol (CH₃OH) in water - Methanol: Hydroxyl group (OH) renders it polar molecule - Water: High polarity due to electronegativity differences between H and O and its bent structure c) KBr in water - KBr: Ionic compound - Water: Polar molecule d) HCl in acetonitrile (CH₃CN) - HCl: Polar molecule with high electronegativity difference between H and Cl atoms - Acetonitrile: Polar molecule with a triple bond in the cyano group

Identify the type of solute-solvent interaction in each solution

Now that we have analyzed the polarity of each solute and solvent, let's identify the type of interactions. a) CCl₄ in benzene - Both are non-polar molecules, hence the main type of interaction is London dispersion forces. b) Methanol in water - Both are polar molecules capable of forming hydrogen bonds. Hence, the main interaction type in this solution is hydrogen bonding. c) KBr in water - The ionic compound (KBr) will be solvated by the polar water molecules due to ion-dipole interactions that are most significant in this case. d) HCl in acetonitrile: - Both the solute and solvent are polar molecules. However, HCl has the ability to form hydrogen bonds, but acetonitrile does not. Hence in this case, the most dominant interaction is dipole-dipole forces.

Key concepts

Polarity

Understanding polarity is key to grasping the nature of solute-solvent interactions. Polarity refers to the distribution of electric charge around a molecule. When a molecule has an uneven distribution of electrons, it becomes polar. This occurs due to differences in electronegativity between atoms, causing a dipole moment. Polar molecules typically have regions of partial positive and negative charges.
For example, water is a classic polar molecule. Its bent structure and the high electronegativity of oxygen compared to hydrogen create a significant dipole moment. This polarity allows water to interact strongly with other polar molecules and ions.
In contrast, non-polar molecules have an even distribution of electrons, resulting in no net dipole moment. Molecules like carbon tetrachloride (CCl₄) and benzene are non-polar. Their electron clouds evenly surround the structure, canceling out any individual dipole moments.
Recognizing whether a solute and solvent are polar or non-polar helps predict the type of interactions they will exhibit. "Like dissolves like" is a helpful rule of thumb: polar solvents are excellent at dissolving polar solutes, and non-polar solvents dissolve non-polar solutes.

Hydrogen Bonding

Hydrogen bonding is a specialized type of dipole-dipole interaction that occurs in polar molecules containing hydrogen atoms bonded to highly electronegative atoms, like oxygen, nitrogen, or fluorine. It happens when a hydrogen atom is shared between two electronegative atoms, resulting in a strong attraction.
A classic example of hydrogen bonding can be observed in the interaction between water molecules. In methanol (CH₃OH) dissolving in water, the hydrogen bonds are significant because both water and methanol have polar O-H bonds. The hydrogen from one molecule can attract the lone pair electrons of the oxygen atom in another, creating a strong network of bonds.
This interaction is critical as it influences properties like boiling and melting points, viscosity, and solubility. For methanol and water, hydrogen bonding not only causes them to mix well but also affects their physical and chemical properties significantly.
It's important to note that hydrogen bonds are much stronger than typical dipole-dipole interactions, yet weaker than covalent or ionic bonds. This makes them versatile and crucial in many chemical and biological structures.

Ion-Dipole Interactions

Ion-dipole interactions are crucial in solutions where ionic compounds are dissolved in polar solvents. These interactions occur between charged ions and polar molecules, typically seen when salts like potassium bromide (KBr) dissolve in water.
In this interaction, the positive and negative charges of the ions attract the oppositely charged ends of the polar solvent molecules. For instance, when KBr dissolves in water, the positive potassium ions (K⁺) interact with the oxygen side (which is negatively charged) of the water molecules, while the bromide ions (Br⁻) attract the hydrogen side (positively charged). This strong attraction helps the ions become surrounded or solvated by water molecules.
Ion-dipole interactions are one of the strongest types of solute-solvent interactions, much stronger than hydrogen bonds. They play a vital role in the dissolution process of ionic compounds, making them vital in biological systems and industrial processes. Understanding ion-dipole interactions explains why salts dissolve readily in water and are crucial in conducting electricity in solution.