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, $(\mathbf{d})\mathrm{HCl}$ in acetonitrile $\left({\mathrm{CH}}_3\mathrm{CN}\right)$

Short answer

(a) In the solution of CCl4 in benzene, the main solute-solvent interaction is dispersion forces (London dispersion forces) as both molecules are nonpolar. (b) In the solution of methanol in water, the predominant solute-solvent interaction is hydrogen bonding due to the presence of polar O and H atoms in both solute and solvent. (c) In the solution of KBr in water, the main solute-solvent interaction is ion-dipole interaction because of the ionic nature of KBr and the polarity of water. (d) In the solution of HCl in acetonitrile, the predominant solute-solvent interaction is dipole-dipole interaction as both solute and solvent have significant dipole moments.

Step-by-step solution

(a) Solution: CCl4 in benzene

For this solution, both solute CCl4 and solvent benzene are nonpolar molecules. The predominant solute-solvent interaction in this solution should be dispersion forces (London dispersion forces). This is because neither molecule has a significant permanent dipole moment, leaving dispersion forces as the most significant interaction.

(b) Solution: Methanol in water

Methanol (CH3OH) and water (H2O) are both polar molecules, each having at least one polar bond and a significant dipole moment. For this solution, the most important solute-solvent interaction should be dipole-dipole interaction, specifically, hydrogen bonding. This is due to the presence of highly electronegative O and H atoms in both solute and solvent, which can interact strongly via the hydrogen bonding mechanism.

(c) Solution: KBr in water

For this solution, solute KBr is an ionic substance (metal bonded to a non-metal), and the solvent water (H2O) is a polar molecule. The predominant solute-solvent interaction in this solution should be ion-dipole interaction. This is because the ionic character of KBr leads to strong electrostatic interactions with the polar water molecules.

(d) Solution: HCl in acetonitrile

For this solution, solute HCl is a polar molecule (a strong acid with a polar bond), and the solvent, acetonitrile (CH3CN), is also a polar molecule. The predominant solute-solvent interaction in this solution should be dipole-dipole interaction. This is because both solute and solvent have significant dipole moments, which allow them to interact strongly via dipole-dipole interactions.

Key concepts

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are the weakest type of molecular interactions. These forces are present in all molecules, but they become particularly significant in nonpolar molecules. While nonpolar molecules do not have permanent dipole moments, transient dipoles can develop due to the random movement of electrons within the molecules. These shifting electrons can create an instantaneous dipole that induces a dipole in neighboring molecules. The result is a very weak, short-lived attraction between atoms or molecules.
Carrying out solute-solvent interactions with dispersion forces is common in solutions where both the solute and the solvent are nonpolar, such as in the case of carbon tetrachloride (CCl4) in benzene (C6H6). Because both molecules lack permanent dipoles, the London dispersion forces become the primary interaction type.

• Transient dipoles cause weak attractions.
• Present in all molecules, but more crucial in nonpolar ones.
• Short-lived and relatively weak compared to other forces.

Despite their weakness, dispersion forces are crucial in understanding the behaviors of gases, liquids, and solids containing nonpolar molecules.

Hydrogen Bonding

Hydrogen bonding is a special case of dipole-dipole interaction, recognized for its relative strength compared to other dipole-dipole forces. It occurs when a hydrogen atom is bonded to a highly electronegative atom, such as nitrogen, oxygen, or fluorine. The highly electronegative atom attracts the bonding electron pair towards itself, thus creating a significant dipole.
This interaction is especially relevant in solutions like methanol (CH3OH) in water. Both methanol and water can form hydrogen bonds because they contain hydroxyl (-OH) groups. These groups make them capable of strong intermolecular interactions through hydrogen bonds.

• Strong compared to other types of dipole-dipole interactions.
• Occurs with highly electronegative atoms like N, O, F.
• Important in biological molecules, such as DNA and proteins.

The presence of hydrogen bonds in a liquid influences its boiling and melting points and overall solubility characteristics.

Ion-Dipole Interaction

Ion-dipole interactions occur when ionic compounds interact with polar molecules. These interactions are stronger than regular dipole-dipole interactions and are crucial for dissolving ionic substances in solvents.
In an ion-dipole interaction, the ion (from the solute, such as KBr) will interact with the partial charges present in a polar molecule (like water). This is because the charged ion creates strong electrostatic attractive forces with the oppositely charged ends of the polar molecules. In the solution of potassium bromide (KBr) in water, these ion-dipole interactions are what help to solvate (dissolve) the ions by breaking the ionic bonds and surrounding them with water molecules.

• Stronger than regular dipole-dipole interactions due to ionic strength.
• Play a crucial role in the solubility of ionic compounds in water.
• Allow ionic substances to be solvated effectively by polar solvents.

This type of interaction is critical for understanding how salts dissolve in water, influencing both chemical processes and everyday life scenarios.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules, where the positive end of one molecule is attracted to the negative end of another. This type of interaction is stronger than dispersion forces but weaker than hydrogen bonding and ion-dipole interactions.
An example of dipole-dipole interaction is seen in the solution of hydrogen chloride (HCl) in acetonitrile (CH3CN). Both are polar molecules with net dipole moments. Hence, they align themselves so that the positive end of one molecule is close to the negative end of another, creating a stable intermolecular force.

• Occurs between permanent dipoles in polar molecules.
• Stronger than dispersion forces, but generally weaker than hydrogen bonds and ion-dipole interactions.
• Influence physical properties such as boiling and melting points.

Dipole-dipole interactions are vital in understanding the behaviors of substances with permanent dipoles, contributing to the material's overall physical and chemical properties.