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Ch.13 - Properties of Solutions
Chapter 13, Problem 15d

Indicate the type of solute–solvent interaction that should be most important in each of the following solutions: d. HCl in acetonitrile (CH3CN).

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Identify the nature of the solute and solvent in the solution. Here, the solute is hydrochloric acid (HCl), which is a polar molecule, and the solvent is acetonitrile (CH3CN), which is also a polar molecule.
Understand the types of intermolecular forces that can occur between solute and solvent. The primary forces include hydrogen bonding, dipole-dipole interactions, and dispersion forces.
Analyze the molecular structure of HCl and CH3CN to determine the possible interactions. HCl has a hydrogen atom bonded to a chlorine atom, creating a significant dipole. Acetonitrile has a triple bond between carbon and nitrogen, which also creates a dipole.
Determine the most likely interaction based on the molecular structures. Since both HCl and CH3CN are polar, the primary interaction is likely to be dipole-dipole interactions.
Conclude that the most important solute-solvent interaction in a solution of HCl in acetonitrile is dipole-dipole interaction, due to the polar nature of both the solute and the solvent.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Solute-Solvent Interactions

Solute-solvent interactions refer to the various forces that occur between solute particles and solvent molecules when a solution is formed. These interactions can include hydrogen bonding, dipole-dipole interactions, and ion-dipole interactions, which significantly influence the solubility and behavior of solutes in solvents.
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Acetonitrile as a Solvent

Acetonitrile (CH₃CN) is a polar aprotic solvent, meaning it has a significant dipole moment but lacks hydrogen atoms that can participate in hydrogen bonding. This property allows it to solvate cations effectively through dipole-dipole interactions, making it suitable for dissolving ionic and polar solutes like HCl.
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Ion-Dipole Interactions

Ion-dipole interactions occur when an ion interacts with the polar molecules of a solvent. In the case of HCl in acetonitrile, the H⁺ ion from HCl will interact strongly with the dipole of acetonitrile, leading to effective solvation. This interaction is crucial for understanding the behavior of strong acids in polar solvents.
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