Liquids and Solids Flashcards

Chapter 16: Liquids and Solids

• Recommended Textbook Problems:
  • Intermolecular Forces and Physical Properties: 15, 17, 19, 21
  • Properties of Liquids: 27, 31
  • Phase Changes and Phase Diagrams: 83, 87, 91, 93, 95, 105

Dipole Moment and Molecular Dipole (16.1)

• Dipole Moment:
  • Positive and negative charges separated by a distance.
  • Molecular dipole is oriented towards the more electronegative atoms.
• Molecular Dipole:
  • The sum of bond dipoles.
  • \text{Molecular dipole moment} = \sum \text{of bonds}
• Polar Molecules:
  • Have a net dipole moment.
• Nonpolar Molecules:
  • Bond dipoles cancel out, resulting in no net dipole moment.

Intermolecular Forces

• Provide the driving force for molecules to enter the condensed phase.
• Represent interactions between molecules.
• Types of Intermolecular Forces:
  • Dispersive Forces (London Dispersion Forces):
    • Present in nonpolar molecules.
    • "Weakest", on the order of kJ/mol.
  • Dipole-Dipole Interactions:
    • Occur between polar molecules.
    • Strength is in the 10s of kJ/mol.
  • Hydrogen Bonding:
    • Occurs in molecules with N-H, O-H, or F-H bonds.
    • Also in the 10s of kJ/mol.
  • C-C bond: 348 \text{ kJ/mol}
  • Ion-dipole Interactions:
    • Occur in polar media.

Dispersive Forces (London Dispersion Forces)

• Driven by:
  • Temporary or induced dipole moments due to the random distribution of electrons.
• Temporary Dipole:
  • Arises from a slight excess of electrons on one side of an atom or molecule and a slight deficiency on the other side.
• Induced Dipole:
  • Surrounding molecules "feel" the temporary dipole, leading to induced dipoles in those molecules.
• Polarizability:
  • The more electrons a molecule has, the more polarizable it is.
• Properties:
  • Weak but additive.
  • Occur due to distortion of electron cloud in response to dipole.

Dispersion Forces Are Additive

• Must be overcome to transition from liquid to gas phase.
• Relationship between electrons/surface area and boiling point:
  • Ne (27 K)
  • Ar (87 K)
  • Kr (121 K)
  • Xe (166 K)
  • F₂ (85 K)
  • Cl₂ (239 K)
  • Br₂ (332 K)
  • I₂ (458 K)
• More electrons or more surface area lead to stronger IMFs and higher boiling points.
• Longer Chain vs Spherical:
  • Linear molecules (e.g., n-Pentane) have more points for dispersion forces to act, leading to higher boiling points.
  • n-Pentane, bp = 36.1 °C
  • Spherical molecules (e.g., Neopentane) have fewer points for dispersion forces to act, leading to lower boiling points.
  • Neopentane, bp = 9.5 °C
• Boiling points vs. number of carbon atoms of alkanes show a general trend of increasing boiling point with increasing chain length due to increased surface area for dispersion forces to act.
  • Methane to Octane

Dipole-Dipole Interaction

• Relies on a permanent dipole moment.
• About 10 times stronger than dispersive forces.
• Example:
  • H₂S is polar and exhibits dipole-dipole interactions.

Influence of Increasing Dipole Strengths

• Comparison of molecules with equivalent "molecular weight" (size and polarizability).
• The larger the molecular dipole, the stronger the attraction between molecules.
• Examples:
  • Propane (44.09 g/mol): Dipole moment = 0 D, Boiling point = 231 K
  • Dimethyl ether (46.07 g/mol): Dipole moment = 1.30 D, Boiling point = 248 K
  • Methyl chloride (50.48 g/mol): Dipole moment = 1.87 D, Boiling point = 249 K
  • Acetaldehyde (44.05 g/mol): Dipole moment = 2.7 D, Boiling point = 294 K
  • Acetonitrile (41.05 g/mol): Dipole moment = 3.92 D, Boiling point = 355 K
• Higher the polarity (dipole moment), higher the boiling point.

Hydrogen Bonding Interaction

• Dipole-dipole interactions made of molecules with N-H, O-H, or H-F bonds.
• Large electronegativity difference between H and N/O/F leads to a large molecular dipole.
• Electronegativity of Hydrogen = 2.1
• Differences in Electronegativity:
  • \Delta X{N-H} = 0.9 (XN = 3.0)
  • \Delta X{O-H} = 1.4 (XO = 3.5)
  • \Delta X{S-H} = 0.4 (XS = 2.5)
  • \Delta X{P-H} = 0.1 (XP = 2.2)
• With dipole-dipole interaction, dipole moment is just larger.

XH Boiling Points

• Anomalously high boiling points for molecules with hydrogen bonding (H₂O, HF, NH₃).
• Comparison of Group 14, 15, 16, and 17 hydrides:
  • Group 14 (Nonpolar - dispersive forces):
    • CH₄, SiH₄, GeH₄, SnH₄
  • Group 15:
    • NH₃ (H-Bonding), PH₃, AsH₃, SbH₃
  • Group 16:
    • H₂O (H-Bonding), H₂S, H₂Se, H₂Te
  • Group 17:
    • HF (H-Bonding), HCl, HBr, HI

Ion-Dipole Interaction

• Interaction between an ion (full charge) and the dipole (partial charge) of another molecule.
• Example:
  • Dissolution of NaCl in water:
    • Na⁺(aq) and Cl⁻(aq) ions interact strongly with the partial negative charge on oxygen and partial positive charge on hydrogen atoms of water molecules, respectively.

Identify the Strongest IMF

• Examples:
  • a) Ethanol (CH3CH2OH): Strongest IMF is Hydrogen Bonding
  • b) Acetaldehyde (CH3CHO): Strongest IMF is Dipole-Dipole
  • c) Butane (C4H10): Strongest IMF is London Dispersion Forces (L.D.)