States of Matter & Phase Changes

• Three Physical States: Solid, Liquid, Gas
• Phase Changes:
  • Melting: Solid → Liquid
  • Freezing: Liquid → Solid
  • Vaporization: Liquid → Gas
  • Condensation: Gas → Liquid
  • Sublimation: Solid → Gas
  • Deposition: Gas → Solid
  • Stronger intermolecular forces (IMFs) require more energy for phase changes.

Intermolecular vs. Intramolecular Forces

• Intramolecular: Within a molecule (e.g., covalent or ionic bonds) affecting chemical properties.
• Intermolecular: Between molecules affecting physical properties (e.g., boiling/melting points).

Types of Intermolecular Forces

• Dispersion (London) Forces: Present in all molecules; temporary dipoles.
  • Stronger in larger/more polarizable/linear molecules.
• Dipole-Dipole Forces: Between polar molecules; strength increases with greater dipole moment.
• Hydrogen Bonding: Special dipole-dipole interaction; occurs with H bonded to N, O, or F.

Properties of Liquids

• Viscosity: Resistance to flow; increases with stronger IMFs, decreases with temperature.
• Surface Tension: Energy needed to increase liquid’s surface area; related to cohesive forces.
• Capillary Action: Movement of liquid up a narrow tube due to adhesive and cohesive forces.

Enthalpies of Phase Changes

• Units: J/mol or kJ/mol
• Important Enthalpies:
  • ΔH_fus: Enthalpy of fusion (melting)
  • ΔH_vap: Enthalpy of vaporization (boiling)
  • ΔH_sub: Enthalpy of sublimation
• Phase changes occur at constant temperature; energy breaks/forms IMFs.

Vapor Pressure and Boiling Point

• Vapor pressure: Pressure of vapor in dynamic equilibrium with its liquid.
• Affected by temperature and IMFs.
• Boiling point: Temperature where vapor pressure equals external pressure.

Clausius-Clapeyron Equation

• Describes how vapor pressure varies with temperature: [ \ln\left(\frac{P_2}{P_1}\right) = \frac{-\Delta H_{\text{vap}}}{R} \left(\frac{1}{T_2} - \frac{1}{T_1}\right) ]
• Use to find enthalpy of vaporization or predict vapor pressure at different temperatures._

Heating and Cooling Curves

• Show temperature vs. heat added
• Use:
  • q = mcΔT for temperature changes
  • q = nΔH for phase changes
• Total energy = sum of all q values during heating/cooling

Phase Diagrams

• Graph of pressure vs. temperature
• Shows regions of solid, liquid, gas
• Lines represent equilibrium between phases
• Critical point: beyond this, no distinction between liquid and gas (supercritical fluid)

Types of Solids

• Ionic Solids: Cations + anions, electrostatic forces; high melting points, brittle, non-conductive (unless molten).
• Metallic Solids: Metal atoms in a “sea” of electrons; malleable, ductile, conductive.
• Covalent Network Solids: Atoms connected by covalent bonds; hard, high melting points, non-conductive.
• Molecular Solids: Neutral molecules held by IMFs; properties depend on polarity and size.

Practice Problems

• Identify strongest IMFs in a set of compounds.
• Rank substances by boiling/vapor pressure.
• Calculate heat required for full phase changes.
• Determine state of matter from phase diagram.
• Use Clausius-Clapeyron to estimate ΔHvap.

Example Problem Types

• Intermolecular Forces & Boiling Points

  1. Which has the highest boiling point? CH₄, CH₃Cl, CH₃OH, CH₃CH₃
  2. Arrange in order of increasing IMF strength: H₂O, CO₂, CH₃F
  3. Which substance exhibits hydrogen bonding? H₂S, CH₄, NH₃, CCl₄

• Vapor Pressure and Clausius-Clapeyron 4. Which liquid has the highest vapor pressure at room temperature? Water, Acetone, Glycerol, Ethylene glycol 5. Calculate ΔHvap using Clausius-Clapeyron.

• Heating Curves and Phase Changes 6. Energy required to convert 50.0 g of ice at -10°C to steam at 120°C?

• Phase Diagrams 7. What phase is present at 0.5 atm and 80°C using water’s phase diagram?

• Solids and Bonding Types 8. Classify NaCl, I₂, Diamond, Cu as ionic, molecular, metallic, or covalent network solids.

Flashcards

1. Dispersion Force → Weak attraction due to instantaneous dipoles; present in all molecules.
2. Hydrogen Bonding → Strong IMF when H is bonded to N, O, or F.
3. Viscosity → A liquid’s resistance to flow; increases with stronger IMFs.
4. Surface Tension → Energy required to increase the surface area of a liquid.
5. Vapor Pressure → Pressure exerted by a vapor in dynamic equilibrium with its liquid.
6. Boiling Point → Temperature at which vapor pressure equals atmospheric pressure.
7. ΔH_vap → Enthalpy change required to vaporize a substance at its boiling point.
8. Covalent Network Solid → Solid with atoms held by covalent bonds (e.g., diamond, SiO₂).
9. Phase Diagram → Graph showing phase stability as a function of temperature and pressure.
10. Clausius-Clapeyron Equation → Relationship describing how vapor pressure changes with temperature.