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Unit 3 · 18–22% of the exam

Intermolecular Forces and Properties

Unit 3 is the heaviest-weighted unit. It explains how forces between particles control boiling points, solubility and the behavior of gases, solutions and solids. It also covers how matter interacts with light, from spectroscopy to the Beer–Lambert law used in lab analysis.

Gas particles in a box

Heat, squeeze or add gas and watch pressure follow PV = nRT, with the live speed distribution.

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Topics in this unit

  1. 3.1

    Intermolecular Forces

    London dispersion, dipole–dipole and hydrogen bonding are attractions between molecules; their strength depends on polarizability (electron count) and polarity.

  2. 3.2

    Properties of Solids

    Ionic, molecular, covalent network and metallic solids have different properties because different forces hold their particles together.

  3. 3.3

    Solids, Liquids, and Gases

    The state of a substance reflects the balance between particle motion (temperature) and the strength of attractions between particles.

  4. 3.4

    Ideal Gas Law

    PV = nRT relates the pressure, volume, moles and temperature of an ideal gas; in a mixture each gas contributes a partial pressure (Dalton's law).

  5. 3.5

    Kinetic Molecular Theory

    Gas particles are in constant random motion and their average kinetic energy is proportional to absolute temperature; lighter particles move faster at the same temperature.

  6. 3.6

    Deviation from Ideal Gas Law

    Real gases deviate from ideal behavior at high pressure and low temperature, where particle volume and intermolecular attractions matter.

  7. 3.7

    Solutions and Mixtures

    Solutions are homogeneous mixtures; molarity (M = mol solute / L solution) describes their concentration.

  8. 3.8

    Representations of Solutions

    Particle diagrams of solutions show the correct ratio of solute particles and ions surrounded by water molecules oriented toward them (ion–dipole).

  9. 3.9

    Separation of Solutions and Mixtures Chromatography

    Components of a mixture can be separated by differences in intermolecular interactions, as in chromatography and distillation.

  10. 3.10

    Solubility

    'Like dissolves like': a substance dissolves well when the solute–solvent interactions are comparable to the solute–solute and solvent–solvent interactions they replace.

  11. 3.11

    Spectroscopy and the Electromagnetic Spectrum

    Microwave radiation changes molecular rotation, infrared changes vibration, and UV/visible radiation causes electronic transitions.

  12. 3.12

    Photoelectric Effect

    Light behaves as photons with energy E = hν = hc/λ; a photon can eject an electron only if its energy exceeds the binding energy.

  13. 3.13

    Beer-Lambert Law

    Absorbance is directly proportional to concentration (A = εbc), so a calibration curve lets you find an unknown concentration.

Must know

  • PV = nRT, with R = 0.08206 L·atm·mol−1·K−1 (T in kelvin: K = °C + 273)
  • Dalton's law: P_total = P_A + P_B + …; P_A = X_A × P_total
  • Average kinetic energy ∝ T; at the same T, lighter particles have higher average speed
  • Real gases are most ideal at high T and low P
  • Hydrogen bonding requires H bonded to N, O or F; dispersion forces grow with the number of electrons (polarizability)
  • Molarity M = mol solute / L solution
  • E = hν and c = λν, so E = hc/λ (h = 6.626 × 10−34 J·s, c = 2.998 × 108 m/s)
  • Beer–Lambert law: A = εbc

Common mix-ups

  • When water boils, the bubbles are hydrogen and oxygen gas.

    Boiling overcomes intermolecular forces only; the bubbles are H2O vapor and no covalent bonds break.

  • Polar molecules always have higher boiling points than nonpolar ones.

    Dispersion forces can dominate: nonpolar I2 (a solid at room temperature) has a much higher boiling point than polar HCl because it has far more electrons.

  • Heavier gas particles have more kinetic energy at the same temperature.

    All gases at the same temperature have the same average kinetic energy; heavier particles simply move more slowly.

Practice questions

AP-style multiple choice. Every answer comes with the reasoning, including why the tempting wrong answer is wrong.

1 / 6

Which substance has the highest boiling point?

Free-response practice

A rigid 10.0 L flask at 25 °C contains 0.200 mol N2(g) and 0.300 mol O2(g). (a) Calculate the total pressure in the flask. (b) Calculate the partial pressure of O2. (c) The flask is heated to 323 °C. Calculate the new total pressure, and explain the change in terms of kinetic molecular theory.

Flashcards

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See it in real life

Everyday reactions that use Unit 3 ideas, animated atom by atom.