Question

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

Short answer

(a) Dispersion forces (both CCl4 and benzene are non-polar) (b) Hydrogen bonding (methanol and water are both polar and can form hydrogen bonds) (c) Ion-dipole interactions (KBr dissociates into K+ and Br- in water) (d) Ion-dipole interactions (HCl dissociates into H+ and Cl- in polar acetonitrile)

Step-by-step solution

Solution (a) - CCl4 in benzene (C6H6)

In the case of CCl4 (carbon tetrachloride) dissolved in benzene (C6H6), both molecules are non-polar, which means that only dispersion forces can be considered. Dispersion forces are the weakest of the intermolecular forces and result from temporary dipoles created by the movement of electrons. These forces will be the most important type of solute-solvent interaction in this solution.

Solution (b) - Methanol (CH3OH) in water

Methanol (CH3OH) and water are both polar molecules. Additionally, methanol has an -OH group, which can form hydrogen bonds with water. Hydrogen bonding is a type of dipole-dipole interaction and is stronger than ordinary dipole-dipole forces. So, in this case, hydrogen bonding is the most important type of solute-solvent interaction.

Solution (c) - KBr in water

When KBr (potassium bromide) is dissolved in water, it dissociates into K+ cations and Br- anions. Water is a polar molecule. In this case, ion-dipole interactions are the most important type of solute-solvent interaction. Ion-dipole interactions occur when an ion interacts with a polar molecule, and these interactions are stronger than dipole-dipole interactions or dispersion forces.

Solution (d) - HCl in acetonitrile (CH3CN)

HCl (hydrochloric acid) is a polar molecule that dissociates into H+ cations and Cl- anions in solution. Acetonitrile (CH3CN) is a polar molecule with a carbon-nitrogen triple bond that creates a significant difference in electronegativity between the two atoms. In this solution, the ion-dipole interactions between the H+ and Cl- ions and the polar acetonitrile molecules are the most important type of solute-solvent interaction due to the strong attraction between the ions and the polar solvent molecules.

Key concepts

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are ubiquitous in chemistry as they occur in all molecular interactions. Despite being the weakest of all intermolecular forces, they play a crucial role when it comes to non-polar molecules. These forces arise due to momentary fluctuations in electron density around a molecule, creating temporary dipoles. These temporary dipoles can then induce a dipole in a neighboring molecule, resulting in an attractive force.

This type of interaction is incredibly noticeable in non-polar molecules like carbon tetrachloride (\(\text{CCl}_4\)) and benzene (\(\text{C}_6\text{H}_6\)). Since both molecules are non-polar, no permanent dipole exists. Thus, dispersion forces are the primary forces at play. Though weak in comparison to other forces, when present in large quantities as in a liquid, they add up, influencing the properties like boiling and melting points.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole-dipole interaction that occurs when a hydrogen atom is covalently bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine. This interaction is significant because it results in molecules being more tightly bound.

A classic example of hydrogen bonding is seen in a solution of methanol (\(\text{CH}_3\text{OH}\)) in water. Both methanol and water are polar, and the presence of the -OH group in methanol allows it to engage in hydrogen bonding with water molecules. This interaction does not only increase the solubility but also affects the physical properties of the solution like boiling and melting points. This is why hydrogen bonds are considered stronger than typical dipole-dipole interactions.

Ion-Dipole Interactions

Ion-dipole interactions are among the strongest types of intermolecular forces you will encounter. These occur when charged ions interact with polar molecules. The strength of these interactions is rooted in the attraction between the charged ion and the partial charges of the polar molecule.

Consider potassium bromide (\(\text{KBr}\)) in water. Upon dissolving, \(\text{KBr}\) dissociates into \(\text{K}^+\) and \(\text{Br}^-\) ions. Water molecules, being polar, orient their partial negative side towards the \(\text{K}^+\) and the partial positive side towards the \(\text{Br}^-\). This strong attraction is what makes substances like salts highly soluble in water. Ion-dipole interactions are crucial in biological processes and are more potent than dipole-dipole or dispersion forces.

Polar and Non-Polar Molecules

Understanding the nature of polar and non-polar molecules is fundamental in predicting solute-solvent interactions. A molecule is polar if it has a significant difference in electronegativities between its atoms, leading to a net dipole moment. Non-polar molecules, in contrast, have equal sharing of electron density and no net dipole.

For example, hydrochloric acid (\(\text{HCl}\)) is polar because of the significant difference in electronegativity between hydrogen and chlorine, resulting in a dipole moment. On the other hand, carbon tetrachloride (\(\text{CCl}_4\)) is non-polar as its symmetrical shape cancels out any dipole moments. Understanding these properties helps predict the types and strengths of interactions with solvents, which influences solubility and the behavior of solutions.