Unit I – Liquids, Solids, and Phase changes
Summary:
There are not only intermolecular attractions between molecules in the gas phase, but those forces of attraction are extremely important for liquids and solids.
The Kinetic-Molecular Theory is fundamental to the understanding of matter. The Kelvin temperature scale is an indicator of relative molecular motions, with the particles having no motion at absolute zero. Intermolecular forces are called van der Waals forces: London dispersion forces and dipole-dipole forces. A special case of dipole-dipole forces is hydrogen bonding, which have higher than expected melting and boiling points, enhanced solubility, unique molecular shapes, etc.
Ionic bonding is the result of a 3-dimensional array of positive and negative ions attracting each other, resulting in very high melting and boiling points.
Metallic bonding occurs because the valence electrons in metal atoms are free to move to any of the empty orbitals in the crystal and, therefore, belong to the whole crystal rather than just between two atoms. Substances that have network covalent bonding are generally very hard, strong, brittle, and have extremely high melting and boiling points. Most do not conduct electricity.
In order to undergo a phase change, the attractions between whole particles (atoms, ions, or molecules) in the substance must overcome interparticle attractions (van der Waals, metallic bonds, ionic bonds or covalent bonds).
Vocabulary:
| absolute zero |
hydrogen bond |
lattice energy |
van der Waals forces |
| deposition |
intermolecular forces |
London dispersion forces |
vapor |
| dipole-dipole forces |
interparticle attractions |
normal boiling point |
vapor pressure |
| hydrogen bond |
isoelectronic |
sublimination |
vaporization |
Review Questions:
Multiple-Choice:
1. Substances that are liquids at room temperature or below are
A. ionic.
B. nonpolar covalent molecular.
C. metallic.
D. covalent network.
2. Crystals such as diamonds (very hard, high melting point, nonconductors) are classified as
A. ionic crystals.
B. covalent molecular crystals.
C. covalent network crystals.
D. metallic crystals.
3. Metallic crystals characteristically have
A. good electric conductivity.
B. great hardness.
C. low melting points.
D. brittleness.
4. Because of the hydrogen bonds in water, the hydrogen atom of one water molecule may be
A. weakly attracted to the oxygen of a second water molecule.
B. weakly attracted to the hydrogen of a second water molecule.
C. strongly attracted to the second hydrogen of its own molecule.
D. strongly attracted to the oxygen of a second water molecule.
Unit I Objectives:
- Distinguish among covalent bonding, ionic bonding, network covalent bonding, and metallic bonding.
- Relate number of valence electrons to the kind of bonding and to bonding substances.
- Understand the nature of intermolecular forces.
- Distinguish among the main kinds of intermolecular forces collectively known as van der Waals forces (including London dispersion forces, dipole-dipole forces, and hydrogen bonding).
- Relate types of intermolecular forces to some properties of molecular substances.
- Use a simple and restricted theory of intermolecular forces to explain and predict relative properties such as solubility, viscosity, and boiling points.
- Understand what happens to the particles in matter as it undergoes phase changes.
- Explain various natural phenomena in terms of the Kinetic-Molecular Theory
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