L1.1: Intermolecular Forces

• Each physical state of matter is a phase, a physically distinct, homogeneous part of a system. The properties of each phase are determined by the balance between the potential and kinetic energy of the particles.
• The potential energy, in the form of attractive forces, tends to draw particles together.

  The kinetic energy associated with movement tends to disperse particles.
• Attractive Forces Kinetic Energy vs. Properties

  Gas | Attractive forces are weak relative to kinetic energy. | Particles are far apart. A gas has no fixed shape or volume.
• Attractive Forces Kinetic Energy vs. Properties

  Liquid | Attractive forces are stronger because particles have less kinetic energy. | A liquid can flow and change shape, but has a fixed volume.
• Attractive Forces Kinetic Energy vs. Properties

  Solid | Attractions dominate motion. Particles are fixed in place relative to each other. | A solid has a fixed shape and volume.
• Existing as a gas, liquid, or solid depends on:

  •The balance between the kinetic energy of its particles.

  •The strength of the interactions between the particles.
• Intramolecular or bonding forces are found within a molecule. The chemical behavior of each phase of matter is the same because the same basic particle is present in each case. H2O molecules are present whether the substance is in the solid, liquid, or gas phase.
• Intermolecular or nonbonding forces are found between molecules. The physical behavior of each phase of matter is different because the strength of these forces differs from state to state.
• •Intermolecular forces are the attractive forces that exist between molecules. •In order of increasing strength, these are:

  1. London dispersion forces
  2. Dipole–dipole interactions
  3. Hydrogen bonding
• The strength of the intermolecular forces determines whether a compound has a high or low melting point and boiling point, and thus whether it is a solid, liquid, or gas at a given temperature.
• London dispersion forces are very weak interactions due to the momentary changes in electron density in a molecule.

  •The change in electron density creates a temporary dipole.

  •All covalent compounds exhibit London dispersion forces.

  •The weak interaction between these temporary dipoles constitutes London dispersion forces.

  •The larger the molecule, the larger the attractive force, and the stronger the intermolecular forces.
• More e− density in one region creates a partial negative charge (δ−).

  Less e− density in one region creates a partial positive charge (δ+).
• Dipole–dipole interactions are the attractive forces between the permanent dipoles of two polar molecules.
• Hydrogen bonding occurs when a hydrogen atom bonded to O, N, or F is electrostatically attracted to an O, N, or F atom in another molecule.

  Hydrogen bonds are the strongest of the three types of intermolecular forces.
• •The boiling point is the temperature at which a liquid is converted to the gas phase.

  •The melting point is the temperature at which a solid is converted to the liquid phase.

  •The stronger the intermolecular forces, the higher the boiling point and melting point
• •Both propane and butane have London dispersion forces and nonpolar bonds.

  •In this case, the larger molecule will have stronger attractive forces.