AP Chemistry 3.9 Separation of Solubility and Mixtures Chromatography Study Notes - New Syllabus 2024-2025

Separation of Mixtures Based on Intermolecular Interactions

The components of a liquid solution cannot be separated by simple filtration because the solute particles are molecular or ionic in scale and uniformly dispersed. However, they can be separated using techniques that exploit differences in intermolecular interactions among the components of the mixture and between those components and their surrounding media.

Chromatography

Chromatography (including paper, thin-layer, or column chromatography) separates components of a mixture based on their differing attractions (intermolecular forces) for two competing phases:

• Mobile phase: The solvent or gas that carries the components through the system.
• Stationary phase: The solid or liquid surface over which the components move.

Principle: Each component interacts with both the mobile and stationary phases via intermolecular forces. The relative strength of these interactions determines the component’s rate of movement and, therefore, its position in the separation pattern (chromatogram).

• Strong attraction to stationary phase: Moves more slowly → remains near the origin.
• Weak attraction to stationary phase (stronger IMF with mobile phase): Moves faster → travels farther up or through the medium.

Intermolecular Forces Involved:

• Hydrogen bonding (e.g., between polar solutes and polar stationary phase)
• Dipole–dipole interactions
• London dispersion forces (dominant in nonpolar systems)

Key Idea: The differential strength of intermolecular attractions determines the migration rate of each substance, allowing components to separate based on polarity or IMF type.

Distillation

Distillation separates the components of a liquid mixture based on differences in boiling points, which arise from differences in intermolecular force strength and the resulting vapor pressures of each component.

• Substances with weaker IMFs have higher vapor pressures and lower boiling points → evaporate first.
• Substances with stronger IMFs have lower vapor pressures and higher boiling points → remain in the liquid phase longer.

Principle: As the mixture is heated, the more volatile component (with weaker IMF) vaporizes preferentially, and the vapor can be condensed and collected separately. This process effectively separates the mixture based on molecular interactions.

Key Relationships:

• \( \mathrm{Boiling\,Point \propto IMF\,Strength} \)
• \( \mathrm{Vapor\,Pressure \propto \dfrac{1}{IMF\,Strength}} \)

Key Idea: Distillation exploits IMF differences — weaker attractions correspond to greater volatility and lower boiling points, enabling physical separation by phase change.