0.15 Kinetic molecular theory  (Page 2/8)

Why should these materials, whose molecules do not seem all that different, behave so differently? What are the important characteristics of these molecules which produce these physical properties?

One of our first efforts at making a connection between molecular properties and macroscopic properties is the development of the Kinetic Molecular Theory. Among the very most important concepts in Chemistry is that atoms and molecules are constantly moving. These movements have a lot to do with how and when the atoms and molecules will react with each other. Also among these very important concepts is that the movements of atoms and molecules are related to the temperature. We shall see that higher temperature corresponds to faster molecules with more energy. Knowing this will help us understand a lot of Chemistry, including how chemical reactions are affected by temperature and energy.

Foundation

Since we wish to relate macroscopic properties to molecular properties, it is worth remembering what observations and conclusions we have already made about each type of property. In this study, we will focus mainly on the observations of gases which led us to the Ideal Gas Law. We will assume that we have made measurements of the pressure of a gas under different conditions. From these, we know that the pressure of a fixed sample of gas is inversely proportional the volume the gas is contained in, meaning that if we reduce the volume of the gas by compressing it, the pressure will rise. In a similar way, we know that the pressure of a fixed sample of gas in a fixed volume will increase in proportion to the temperature, provided that we measure the absolute temperature in degrees Kelvin. And finally, the pressure of the gas kept at fixed temperature is proportional the “particle density” of the gas, which is the number of gas particles (atoms or molecules) divided by the volume of the container.

We have also learned a great deal about molecular properties that will be useful in this concept development. We know that different atoms have different electronegativities and that, as a result, when these atoms are bonded together, the electron pair sharing is not equal. This can cause the molecule to have a molecular dipole moment. We also know about the geometries of molecules, and that a symmetric molecule may not have a dipole moment even if the bonds in the molecule are polar.

There are a few results from Physics which are important to our work. The first of these is that pressure P is the force exerted F divided by the area A on which it is exerted:

$P=\frac{F}{A}$

This sounds complicated but is actually commonly observed. Think of the differences you see when force is applied to a small area, like the point of a nail or a needle, instead of applied over a large area. For example, a stack of books piled on a desk creates a downward force due to gravity, but that force is applied over a large area which is the size of the bottom book in the stack. This means that it does not generate that much pressure, so the books have no effect on the desk. However, if the stack of books is somehow piled on top of a needle or a sharp nail, the pressure created is very high because the force of gravity has been applied to a very, very small area, namely the point of the needle or the nail. This high pressure means that the needle or nail may penetrate the surface of the desk. The force applied in either case is the same, but the pressure is quite different when the stack of books is placed on the sharp point.