CHM2046 Exam 1 Review: Intermolecular Forces, Solutions, and Phase Changes

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Intermolecular Forces

Types of Intermolecular Forces

Intermolecular forces are the attractions between molecules that determine many physical properties of substances, such as boiling and melting points.

London Dispersion Forces: Present in all molecules, but are the only forces in nonpolar molecules (e.g., Xe).
Dipole-Dipole Forces: Occur between polar molecules (e.g., NH3).
Hydrogen Bonding: A special, strong type of dipole-dipole interaction found in molecules where H is bonded to N, O, or F (e.g., H2O).
Ionic-Dipole Forces: Occur between ionic compounds and polar solvents (e.g., NaCl in H2O solution).

Example: In H2O, hydrogen bonding is the strongest intermolecular force present.

Solutions and Solubility

Types of Solutions

Saturated Solution: Contains the maximum amount of solute that can dissolve at a given temperature.
Unsaturated Solution: Contains less solute than can dissolve at a given temperature.
Supersaturated Solution: Contains more solute than is normally possible at a given temperature; unstable.

Solubility and Intermolecular Forces

"Like dissolves like": Polar solutes dissolve in polar solvents; nonpolar solutes dissolve in nonpolar solvents.
Example: NaCl is soluble in water due to strong ion-dipole interactions.

Colligative Properties

Depend on the number of solute particles, not their identity.
Include boiling point elevation, freezing point depression, and vapor pressure lowering.
Van't Hoff Factor (i): Number of particles a compound dissociates into in solution.

Phase Changes and Heating Curves

Phase Changes

Fusion (Melting): Solid to liquid.
Vaporization: Liquid to gas.
Sublimation: Solid to gas.
Condensation: Gas to liquid.
Deposition: Gas to solid.
Freezing: Liquid to solid.

Example: On a heating curve, the flat segments represent phase changes where temperature remains constant as energy is used to break intermolecular forces.

Properties of Solids

Classification of Solids

Molecular Solids: Composed of molecules held together by intermolecular forces (e.g., I2).
Ionic Solids: Composed of ions held together by ionic bonds (e.g., K2O).
Atomic Solids: Composed of atoms held together by covalent or metallic bonds (e.g., Fe).

Band Gaps

Band Gap: The energy difference between the valence band and conduction band in solids.
Order of increasing band gap: Conductor < Semiconductor < Insulator.

Thermochemistry

Endothermic vs. Exothermic Processes

Endothermic: Absorbs heat from surroundings (e.g., melting ice).
Exothermic: Releases heat to surroundings (e.g., combustion).

Example: Dissolving NaOH in water is exothermic if the enthalpy of solution is negative.

Mathematical Operations and Calculations

Gas Law Calculations

Use the ideal gas law to relate mass, volume, temperature, and pressure:

Where P = pressure, V = volume, n = moles, R = gas constant, T = temperature (K).

Calculating Vapor Pressure of Solutions

Raoult's Law: The vapor pressure of a solution is proportional to the mole fraction of the solvent.

Where is the mole fraction of the solvent and is the vapor pressure of the pure solvent.

Calculating the Radius of a Cube

Given the volume of a cube, the length of one side is:

The radius is half the length of a side:

Tables

Type of Solid	Constituent Particles	Forces Holding Them Together	Examples
Molecular	Molecules	Intermolecular forces	I2
Ionic	Ions	Ionic bonds	K2O
Atomic	Atoms	Covalent/metallic bonds	Fe

Additional info:

Some questions reference diagrams (e.g., heating curves) and require interpretation of phase changes and temperature plateaus.
Students should be familiar with identifying exothermic and endothermic processes based on enthalpy values.
Understanding the relationship between molecular structure and physical properties (e.g., viscosity, boiling point) is essential for these topics.