0.11 The kinetic molecular theory  (Page 6/7)

What do the deviations from ideality tell us about the gas particles? Starting with very low density andincreasing the density as in , we find that, for many gases,the value of $\frac{PV}{nRT}$ falls below 1. One way to state this result is that, for a given value of $V$ , $n$ , and $T$ , the pressure of the gas is less than it would have been for an ideal gas. This must be theresult of the interactions of the gas particles. In order for the pressure to be reduced, the force of the collisions of theparticles with the walls must be less than is predicted by our model of an ideal gas. Therefore, the effect of the interactions isto slow the particles as they approach the walls of the container. This means that an individual particle approaching a wall mustexperience a force acting to pull it back into the body of the gas. Hence, the gas particles must attract one another. Therefore, theeffect of increasing the density of the gas is that the gas particles are confined in closer proximity to one another. At thiscloser range, the attractions of individual particles become significant. It should not be surprising that these attractiveforces depend on what the particles are. We note in that deviation from the Ideal Gas Law is greater for ammonia than for nitrogen, and greater for nitrogen than for helium. Therefore,the attractive interactions of ammonia molecules are greater than those of nitrogen molecules, which are in turn greater than thoseof helium atoms. We analyze this conclusion is more detail below.

Continuing to increase the density of the gas, we find in that the value of $\frac{PV}{nRT}$ begins to rise, eventually exceeding 1 and continuing to increase. Under theseconditions, therefore, the pressure of the gas is greater than we would have expected from our model of non-interacting particles.What does this tell us? The gas particles are interacting in such a way as to increase the force of the collisions of the particleswith the walls. This requires that the gas particles repel one another. As we move to higher density, the particles are forcedinto closer and closer proximity. We can conclude that gas particles at very close range experience strong repulsive forcesaway from one another.

Our model of the behavior of gases can be summarized as follows: at low density, the gas particles aresufficiently far apart that there are no interactions between them. In this case, the pressure of the gas is independent of the natureof the gas and agrees with the Ideal Gas Law . At somewhat higher densities, the particles are closer together and the interactionforces between the particles are attractive. The pressure of the gas now depends on the strength of these interactions and is lowerthan the value predicted by the Ideal Gas Law . At still higher densities, the particles are excessively close together, resultingin repulsive interaction forces. The pressure of the gas under these conditions is higher than the value predicted by the Ideal Gas Law .

Observation 3: boiling points of simple hydrides

The postulates of the Kinetic Molecular Theory provide us a way to understand the relationship between molecular properties and the physical properties of bulk amounts ofsubstance. As a distinct example of such an application, we now examine the boiling points of various compounds, focusing onhydrides of sixteen elements in the main group (Groups IV through VII). These are given here .