CHM.CHP.10

Chapter 10.1: Liquids and Solids, Intermolecular Forces

Intermolecular Forces

Intermolecular forces (IMFs) are the attractive forces that exist between molecules or atoms in a substance. These forces play a crucial role in determining the physical properties of liquids and solids, such as boiling point, melting point, and solubility.

• Definition: Intermolecular forces are forces of attraction or repulsion which act between neighboring particles (atoms, molecules, or ions).

• Types of Intermolecular Forces:

  • Dispersion Forces (London Forces): Present in all molecules and atoms, these are the weakest IMFs, arising from temporary fluctuations in electron distribution.

  • Dipole-Dipole Interactions: Occur between polar molecules due to the attraction between positive and negative ends of dipoles.

  • Hydrogen Bonding: A special, stronger type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms (N, O, or F).

  • Ion-Dipole Forces: Occur between an ion and a polar molecule; important in solutions of ionic compounds in polar solvents.

• Relative Strengths: Generally, ion-dipole > hydrogen bonding > dipole-dipole > dispersion forces.

• Effect on Physical Properties: Stronger IMFs lead to higher boiling and melting points, greater viscosity, and lower vapor pressure.

Comparison of Intermolecular Forces

The strength and type of intermolecular forces present in a substance determine its physical state and properties. The following table summarizes the main types of IMFs and their characteristics:

Type of IMF	Occurs Between	Relative Strength	Example
Dispersion (London)	All molecules/atoms	Weakest	Ar, I2
Dipole-Dipole	Polar molecules	Intermediate	HCl, CH3Cl
Hydrogen Bonding	H bonded to N, O, or F	Strong	H2O, NH3
Ion-Dipole	Ions and polar molecules	Strongest	Na+ in H2O

Examples and Applications

• Example: Ranking IMFs

  • Given the following substances, list them in order of increasing IMF strength: Ammonia (NH3), Krypton (Kr), HCl, CH3Cl, XeF4.

  • Solution:

    1. Krypton (Kr): Only dispersion forces (weakest).

    2. XeF4: Larger atom, stronger dispersion than Kr, but still only dispersion.

    3. HCl: Dipole-dipole interactions.

    4. CH3Cl: Dipole-dipole and dispersion (slightly stronger than HCl due to larger size).

    5. Ammonia (NH3): Hydrogen bonding (strongest among these).

Trends in Properties

Intermolecular forces influence trends in boiling and melting points, viscosity, and other physical properties.

• Halogens (Cl2, Br2, F2):

  • All are nonpolar molecules; only dispersion forces are present.

  • Boiling point increases with molecular size and polarizability: F2 < Cl2 < Br2.

• Boiling Point and IMFs: Substances with stronger IMFs have higher boiling points because more energy is required to separate the molecules.

• Viscosity: Liquids with stronger IMFs are generally more viscous.

Practice Questions

• For each pair, which has the higher boiling point?

  • CH3CH2OH (ethanol) or CH3OCH3 (dimethyl ether)? Answer: Ethanol, due to hydrogen bonding.

  • C6H6 (benzene) or C6H14 (hexane)? Answer: Benzene, due to its larger, more polarizable π system.

  • NH3 or PH3? Answer: NH3, due to hydrogen bonding.

Key Equations

• Clausius-Clapeyron Equation: Relates vapor pressure and temperature: $\ln P = -\frac{\Delta H_{vap}}{R} \left( \frac{1}{T} \right) + C$

Additional info: The notes above expand on the brief points and images in the provided slides, adding definitions, examples, and context for clarity and completeness.