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Credit Sprint's Guide to Chemistry Unit 3: Intermolecular Forces and Properties

AP® weight 18–22%CLEP® weight 15–20%

Forces between particles decide whether a substance is a gas, liquid or solid and how it dissolves. This unit also covers the gas laws, kinetic molecular theory, solutions and molarity, and how light is used to measure concentration.

Intermolecular forces and boiling pointCH₄dispersion−162 °C HCldipole-dipole−85 °C H₂Ohydrogen bonding100 °C NaClionic: 1465 °C Stronger forces → higher boiling point

What you need to know

  • Forces from weakest to strongest: London dispersion (all molecules, grows with electron count), dipole-dipole, hydrogen bonding (H on N, O or F), ion-dipole.
  • Stronger forces mean higher boiling points, lower vapor pressure, higher viscosity and higher surface tension.
  • Solid types: ionic, metallic, network covalent and molecular. Their properties follow their bonding.
  • Like dissolves like: polar and ionic solutes dissolve in polar solvents; nonpolar in nonpolar.
  • Phase diagrams show the triple point and critical point. Water's solid-liquid line leans left because ice is less dense than water.
  • Ideal gas law PV = nRT; at STP one mole is 22.4 L. Dalton: total pressure is the sum of partial pressures.
  • Kinetic molecular theory: average kinetic energy depends only on temperature; at the same temperature lighter molecules move faster. Graham's law: rate ∝ 1/√M.
  • Real gases deviate at high pressure and low temperature.
  • Molarity M = moles solute ÷ liters solution; dilution M₁V₁ = M₂V₂.
  • Beer's law A = εbc: absorbance is proportional to concentration. UV-visible light causes electronic transitions, IR causes vibrations, and microwaves cause rotations.
  • Colligative properties depend on particle count: ΔTf = iKfm and ΔTb = iKbm.

Equations and rules

PV = nRT
P₁V₁/T₁ = P₂V₂/T₂
M₁V₁ = M₂V₂
A = εbc
E = hc/λ
ΔT = iKm

Exam traps

Watch forTemperature must be in kelvin for gas laws.
Watch forHydrogen bonds are intermolecular; they are not bonds inside a molecule.
Watch forBoiling doesn't break covalent bonds; it overcomes intermolecular forces.
Watch forMolarity uses liters of solution, not liters of solvent.

Key terms

  • London dispersion forces: Attractions from temporary dipoles; present in all molecules and stronger with more electrons.
  • Hydrogen bonding: Strong dipole attraction when H is bonded to N, O or F and is attracted to a lone pair on another molecule.
  • Ion-dipole force: Attraction between an ion and a polar molecule, as when salt dissolves in water.
  • Vapor pressure: The pressure of vapor in equilibrium with its liquid; lower when intermolecular forces are stronger.
  • Triple point: The temperature and pressure at which solid, liquid and gas coexist.
  • Ideal gas law: PV = nRT.
  • STP molar volume: 22.4 L per mole of an ideal gas at 0 °C and 1 atm.
  • Dalton's law: Total pressure is the sum of the partial pressures of the gases.
  • Graham's law: Rates of effusion are inversely proportional to the square root of molar mass.
  • Kinetic molecular theory: Gas particles move randomly; average kinetic energy is proportional to Kelvin temperature.
  • Molarity: Moles of solute per liter of solution.
  • Dilution equation: M₁V₁ = M₂V₂.
  • Beer's law: Absorbance is proportional to concentration: A = εbc.
  • Colligative property: A property that depends on the number of dissolved particles, such as freezing point depression.
  • van't Hoff factor (i): The number of particles a solute produces per formula unit in solution.

Practice questions

Five of the 73 Unit 3 questions in Credit Sprint. Every choice is explained in the app.

1. Which substance has the highest boiling point?
  1. CH₄
  2. H₂O
  3. H₂S
  4. H₂
Show answer

H₂O. It forms extensive hydrogen bonds.

2. Which has the higher boiling point, CH₃CH₂CH₂OH or CH₃OCH₂CH₃ (similar molar mass)?
  1. CH₃CH₂CH₂OH
  2. Neither boils
  3. CH₃OCH₂CH₃
  4. They are equal
Show answer

CH₃CH₂CH₂OH. Its O–H group allows hydrogen bonding.

3. The solid-liquid line on the phase diagram of water has a negative slope. This means
  1. water boils at lower temperature at high pressure
  2. increasing pressure at constant temperature can melt ice
  3. water has no triple point
  4. ice is denser than water
Show answer

increasing pressure at constant temperature can melt ice. Ice is less dense than liquid water.

4. How does the rate of effusion of H₂ compare with that of O₂ at the same temperature?
  1. H₂ effuses 16 times faster
  2. O₂ effuses 4 times faster
  3. H₂ effuses 4 times faster
  4. They effuse at the same rate
Show answer

H₂ effuses 4 times faster. Rate ∝ 1/√M, so √(32/2) = 4.

5. Which gas is expected to deviate most from ideal behavior at a given T and P?
  1. Ne
  2. SO₂
  3. He
  4. H₂
Show answer

SO₂. It has the strongest attractions and largest size.