Intermolecular Forces and Properties: Complete Course Breakdown & Practice

Intermolecular forces (IMFs) act between molecules. Distinguish the 4 major classes:

• London Dispersion Forces (LDF): Present in all species. Induced temporary dipoles whose strength scales with electron cloud polarizability (more electrons = stronger LDF).
• Dipole-Dipole Attractions: Electrostatic attraction between permanent molecular dipoles in polar substances.
• Hydrogen Bonding: Unusually strong dipole-dipole force occurring ONLY when H is covalently bonded to N, O, or F and attracted to a lone pair on an adjacent N, O, or F.
• Ion-Dipole Forces: Attraction between an ion and polar solvent molecules during dissolution (hydration spheres).

Compare the 4 fundamental types of crystalline solids by particle structure and bond nature:

• Ionic Solids (e.g. NaCl, MgO): Alternating ions; high melting points; brittle; conduct electricity only when molten or aqueous.
• Metallic Solids (e.g. Cu, Fe): Metal cations in mobile electron sea; malleable, ductile, thermal/electrical conductors.
• Covalent Network Solids (e.g. Diamond, SiO₂, Graphite): Continuous 3D covalent networks; extreme hardness and very high melting points.
• Molecular Solids (e.g. Ice, Dry Ice, I₂): Discrete molecules bound by weak IMFs; low melting points; non-conductive.

Phase states represent the competition between kinetic energy (thermal motion) and intermolecular attractive forces. On heating curves, temperature remains constant during phase transitions because heat added overcomes intermolecular attractions rather than increasing average kinetic energy. Vaporization requires vastly more energy than fusion because vaporization completely severs all intermolecular attractions.

Quantify gas systems using the Ideal Gas Equation: Apply gas density (ρ = PM / RT) and molar mass (M = mRT / PV). Master Dalton's Law of Partial Pressures (P_A = X_A · P_total) and account for water vapor pressure when collecting gases over water:

Kinetic Molecular Theory (KMT) models gases as point particles in constant random motion with elastic collisions and negligible particle volume. Maxwell-Boltzmann Distributions: As temperature rises, the distribution flattens and shifts right. At identical temperatures, lighter gas molecules move faster on average than heavier ones, but all gases share the exact same average kinetic energy!

Real gases depart significantly from ideal behavior under two key conditions:

• Low Temperature: Gas particles move slowly, allowing intermolecular attractions to pull particles together, causing fewer and softer wall collisions (P_real < P_ideal).
• High Pressure: Molecules are crowded closely; real particle volume is no longer negligible relative to total container volume (V_real > V_ideal).

Molarity measures concentration: M = \text{moles solute} / \text{liters solution}. During dilution (M₁V₁ = M₂V₂), total solute moles remain constant while volume increases. When drawing dissolved salts in water, the partially positive hydrogen atoms (δ⁺) orient toward negative anions (Cl⁻), while the partially negative oxygen atom (δ⁻) orients toward positive cations (Na⁺).

Chemical separations exploit differences in intermolecular forces:

• Paper Chromatography: Solutes partition between paper (stationary phase) and solvent (mobile phase). Components with stronger attractions to the mobile phase travel farthest. Retention factor: R_f = d_solute / d_{solvent front}.
• Distillation: Separates liquid mixtures based on differences in boiling point, which reflect relative intermolecular force strengths.

Light energy is quantized: c = λν and E = hν = hc / λ. Different regions of the electromagnetic spectrum induce distinct molecular transitions:

• Microwave Radiation: Induces transitions in molecular rotational states.
• Infrared (IR) Radiation: Induces transitions in molecular vibrational states (bond stretching and bending).
• Ultraviolet / Visible (UV-Vis): Induces transitions in electronic energy levels (valence electrons promoted to higher orbitals).

Spectrophotometers measure how much light of analytical wavelength (λ_max) is absorbed by a colored solution: where A is absorbance (unitless), ε is molar absorptivity, b is cuvette path length (typically 1.0 cm), and c is molar concentration.