0.15 Kinetic molecular theory

Introduction

Our understanding of the bonding between atoms in molecules tells us a great deal about the structures and properties of molecules. We can use the valence shell model of atoms and the Lewis model of bonding to explain and even predict what types of molecules will be stable, like CH ₄ , and which are not expected to exist, like CH ₅ . This is very powerful and quite beautiful. It means that the enormous number of known substances generally fit a fairly simple set of rules which describe their molecules. Without these rules, we would be lost trying to understand the properties of the more than 50 million different substances that chemists have identified. With these rules, though, we can not only understand the properties of each molecule but even predict properties of other molecules even if they have never been created before.

Chemistry, though, is about changes in matter. How do substances react with one another? If a molecule is stable under ordinary conditions, why does it become unstable when the temperature is raised or other molecules are added to the flask? We are interested in chemical reactions, but we are also interested in some physical changes which take place in matter. What happens when a substance changes from solid to liquid or liquid to gas? What do some substances do this so readily?

Answering these questions often requires us to understand how the properties of individual molecules create the properties we see in large quantities of a substance. This seems like a very challenging problem. It is hard enough to think of the structure of one molecule and try to visualize how this structure creates properties such as dipole moments or intermolecular forces. It seems much harder to imagine thinking of a mole of these molecules all interacting at the same time.

We need a way to relate macroscopic properties to molecular properties. As simple examples, let’s compare the substances water, carbon dioxide, and nitrogen. Each of these is composed of molecules with just a few atoms, and all of the atoms have rather small masses, so the molecules all have low molecular weight. These three molecules have very similar molecular properties. However, the physical properties of these three substances are very different. Carbon dioxide and nitrogen are gases at room temperature, but water is a liquid up to 100 °C. We can only get nitrogen to condense by cooling it to –196 °C. This means that the boiling temperatures of water and nitrogen are different by almost 300 °C. Water is a liquid over a rather large temperature range, freezing at 0 °C and boiling at 100 °C. Nitrogen is very different. It is only a liquid between –210 °C and –196 °C. Carbon dioxide is even more interesting. At normal atmospheric pressure, carbon dioxide gas cannot be condensed into a liquid at all, no matter how cold we make it. If we keep cooling carbon dioxide gas to –60 °C, it converts directly to solid “dry ice.” We can’t melt dry ice either. Warming dry ice above –60 °C does not produce any liquid, and instead the solid “sublimes,” meaning that it converts directly to gas.