Ch11

Key Concepts

Kinetic Molecular Theory, States of Matter, and Phase Changes

  • Liquids and solids are condensed phases (11.1a).
  • The physical state of a substance is controlled by pressure, temperature, and intermolecular forces (IMFs) (11.1a).
  • The strength of the IMFs in a substance affects many of its properties (11.1a).
  • Converting an ordered phase to a less ordered phase requires the addition of energy to overcome the IMFs, and energy is released when a less ordered phase is converted to a more ordered phase (11.1b).
  • The magnitude of a liquid’s enthalpy of vaporization is directly related to the strength of the IMFs in the liquid (11.1c).

Vapor Pressure

  • Equilibrium vapor pressure for a liquid is the pressure of the vapor over a liquid when a dynamic equilibrium is reached at a given temperature (11.2a).
  • As temperature increases, the vapor pressure for a liquid increases (11.2b).
  • As the strength of the IMFs in a series of liquids increases, the vapor pressures of the liquids decrease (11.2b).
  • The temperature at which the vapor pressure for a liquid is equal to the external pressure is the boiling point of the liquid (11.2c).
  • As the strength of the IMFs in a series of liquids increases, the boiling points of the liquids increase (11.2c).
  • Vapor pressure and temperature are related mathematically by the Clausius–Clapeyron equation (11.2d).

Other Properties of Liquids

  • Surface tension is a measure of force required to “break” the surface of a liquid. Liquids with strong IMFs generally have a strong surface tension (11.3a).
  • Liquids with strong IMFs generally have high viscosity, or resistance to flow (11.3b).
  • Capillary action occurs when the forces within a bulk sample of liquid are overcome by strong IMFs between the liquid and the surface of the material in contact with the liquid (11.3c).

The Nature of Intermolecular Forces

  • Dipole–dipole IMFs are the attractive forces that occur between two polar molecules (11.4a).
  • Hydrogen bonding is an unusually strong type of dipole–dipole IMF that occurs between molecules with H–N, H–O, or H–F bonds (11.4a).
  • Dipole–induced dipole IMFs are attractive forces that occur between polar and nonpolar molecules (11.4b).
  • Polarizability, which is directly related to molar mass and molecular size, is the ease with which the electron cloud in a molecule can be distorted and a dipole can be induced (11.4b).
  • Induced dipole–induced dipole IMFs, also called London dispersion forces, are attractive forces that occur between nonpolar molecules (11.4c).

Intermolecular Forces and the Properties of Liquids

  • As molar mass increases in a series of small nonpolar compounds, enthalpy of vaporization and boiling point generally increase (11.5a).
  • As molar mass and the length of the carbon chain in a series of compounds with similar IMFs increases, enthalpy of vaporization and boiling point generally increase (11.5a).
  • Hydrogen bonds have a significant impact on the physical properties of compounds (11.5b).
  • It is possible to quantify the relationship between IMFs and the physical properties of liquids, but the relationships are not always simple (11.5c).

Key Equations

ln P=−ΔHvapRT+Cln P2P1=−ΔHvapR(1T2−1T1)(11.1)(11.2)

Key Terms

  • condensed phase - A phase in which the particles are packed in close proximity to one another
  • intermolecular forces - Interactions between molecules, between ions, or between molecules and ions
  • fusion - The state change from solid to liquid
  • vaporization - The state change from liquid to gas
  • condensation - The movement of molecules from the gas to the liquid phase
  • enthalpy of vaporization, ΔHvap - The quantity of energy as heat required to convert one mole of a liquid to a gas at constant temperature
  • vapor pressure - The pressure of the vapor of a substance in contact with its liquid or solid phase in a sealed container
  • dynamic equilibrium - A condition in which the forward and reverse reaction rates in a physical or chemical system are equal
  • equilibrium vapor pressure - The pressure of the vapor of a substance at equilibrium in contact with its liquid or solid phase in a sealed container
  • boiling point - The temperature at which the vapor pressure of a liquid is equal to the external pressure on the liquid
  • normal boiling point - The boiling point when the external pressure is 1 atm
  • cohesive forces - A force of attraction between molecules of a single substance
  • surface tension - The energy required to disrupt the surface of a liquid
  • viscosity - The resistance of a liquid to flow
  • capillary action - The result of the cohesive forces within a bulk sample of liquid being overcome by adhesive forces
  • adhesive forces - A force of attraction between molecules of two different substances
  • electrostatic forces - Forces of attraction or repulsion caused by electric charges
  • dipole–dipole intermolecular forces - The electrostatic force between two neutral molecules that have permanent dipole moments
  • hydrogen bonding - Attraction between a hydrogen atom and a very electronegative atom to produce an unusually strong dipole–dipole attraction
  • dipole–induced dipole intermolecular forces - The electrostatic force between two neutral molecules, one having a permanent dipole and the other having an induced dipole
  • induced dipole - Separation of charge in a normally nonpolar molecule, caused by the approach of a polar molecule
  • polarizability - The extent to which the electron cloud of an atom or molecule can be distorted by an external electric charge
  • induced dipole–induced dipole intermolecular forces - Intermolecular attractions between two neutral molecules, both having induced dipoles
  • London dispersion forces Intermolecular attractions between two neutral molecules, both having induced dipoles