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Intermolecular Forces, Liquids, and Solids
At the end of Unit 10, we saw that gases do not exhibit ideal-gas behaviour, in part because of intermolecular attractive forces. Those forces are also the reason that liquids and solids exist. Unit 11 covers the different types of intermolecular forces, which provide a basis for understanding the properties of liquids and solids. The variations in melting and boiling points, heats of fusion and vaporization, surface tension, viscosity, and vapour pressure of different substances have explanations based on intermolecular forces. The last two sections of Unit 11 deal with solids. We know a lot about how solids are constructed on an atomic and molecular level because of tools like X-ray diffraction. So, this unit provides an introduction to the detailed study of the structure of solids.
After completing this unit, you should be able to
- explain the characteristic properties of gases, liquids, and solids at a particle (molecule, atom, or ion) level, based on the kinetic-molecular theory and the intermolecular attractive forces between particles.
- describe, and distinguish between, the various types of intermolecular (attractive) forces.
- predict, for a substance of known structure, which types of intermolecular forces may be operative, and which may be of major importance.
- describe the nature of the hydrogen bond, and identify those molecular systems where hydrogen bonding is expected to occur.
- relate surface tension and viscosity to the intermolecular forces in a liquid.
- explain phase changes, heat of fusion, and heat of vaporization, using the kinetic-molecular theory, or the nature of the intermolecular attractive forces involved, or both.
- calculate the heat absorbed or evolved when the temperature of a substance changes, given appropriate enthalpies and heat capacities.
- explain the relationships among critical temperature, critical pressure, and intermolecular attractive forces.
- explain the relationships among pressure, temperature, vapour pressure, and boiling point, using the kinetic-molecular theory, or the nature of the intermolecular attractive forces involved, or both.
- explain the dynamic equilibrium existing in a system consisting of two phases.
- predict the phase or phases present at a given temperature and pressure, given the phase diagram for a substance.
- draw and label a phase diagram for a substance, given the appropriate data.
- calculate the number of atoms or ions per unit cell for a substance crystallizing in a cubic unit cell.
- calculate the density, unit cell length, or unit cell volume for a substance crystallizing in a cubic unit cell.
- describe the primitive cubic, body-centred cubic, and face-centred cubic unit cells.
- explain the origin of the diffraction patterns obtained when X-rays are used to irradiate a single crystal.
- predict the type of solid (molecular, ionic, network-covalent, or metallic) expected for a given substance, and predict the general properties of the solid from the types of forces present.