|Unit I – Liquids, Solids and Phase Changes
- In this unit bonding is extended to include not only covalent and ionic bonding, but also van der Waals, London dispersion forces, dipole-dipole attractions, hydrogen bonds, metallic bonds, and network covalent bonds.
- A more correct approach to intermolecular bonding is presented in this text than in most other texts. Van der Waals forces are a composite of two types of intermolecular forces: a) dipole-dipole forces and b) London dispersion forces. Of these two types, London dispersion forces are the more significant. Dipole-dipole forces require considerably more classroom time to learn than do London dispersion forces.
- This unit emphasizes the relationship between intermolecular forces and melting/boiling points. Students are taught to visualize what is happening to the particles with respect to particle movement and interparticle attractions as a substance is heated or cooled.
- Students have difficulty with the abstract nature of the attractions between particles. Be sure to do the demos and labs to help the students make the connections between the observed phenomena and theory.
- Distinguish among covalent bonding, ionic bonding, covalent network 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.
I.1 The Kinetic-Molecular Theory
Kinetic-Molecular Theory (KMT) was first introduced in Unit H during the study of gases. Students began to realize that there are attractive forces between particles. In this section, students now realize that the KMT can be extended to include liquids and solids as well.
I.2 Exercises: The Kinetic-Molecular Theory ***Worksheet Available***
Assign I.1 and I.2 together and let the students attempt the concepts on their own. It might be a good idea to discuss this assignment in class without taking it for a grade. Another idea might be to use the students to model the behavior of the particles in a substance: the students themselves would represent the particles, their arrangement would determine whether a solid, liquid, or gas is shown, and they could move around in a manner related to the phase they are modeling—“vibration” for solids, “rotation” or “movement from molecule-to-molecule” for liquids, and complete freedom and rapid motion for gases. (The problem with this model is that it does not necessarily show attractions between particles.)