The forces between species at the atomic and molecular level can be broadly categorized as either
In this discussion we will be concerned with the intermolecular forces. The magnitude of these forces determine why ethanol boils at a lower temperature than water and why water tends to spread out on a surface more than mercury. These forces also cause water to rise in a capillary and mercury not to rise. These forces are responsible for the formation of the condensed states (solid and liquid) from the gaseous states. From just these examples, you can see that it is important that we understand some of the concepts of intermolecular forces so that we can better understand the world around us.
For a particular example we will consider trying to understand on the molecular level why methanol boils at a lower temperature than water, and what is the effect on their boiling points upon the addition of an ionic compound, NaCl. First it is important to understand that there is no such thing as one molecule of an ionic compound. An ionic compound exists normally either in the solid state as a crystal, or as the component ions in solution. So we will not find any NaCl molecules in a liquid solution. Secondly it is important to understand what happens when something like water boils. What happens on a molecular level is that as the temperature increases, the average kinetic energy of the molecules increases and finally there is so much internal energy that the intermolecular forces holding the water molecules together is not strong enough to maintain the coupling of the water molecules and some water molecules begin to break free of the surface of the water. This phenomena is what we call boiling. Some people might think that hydrogen and oxygen are being released, but this is not correct. The heating of the solution is not nearly energetic enough to break the intramolecular bonds, but it does furnish enough energy to break the intermolecular bonds.
Before we start our study in detail, let's look at a typical intermolecular potential energy diagram for the interaction of two molecules. When we look at the potential energy curves for such a situation, what we see is something like the following:
The total curve is given by the solid curve. The curve actually has two components as illustrated by the dotted curves. The attractive portion of the curve generally varies as 1/r6 and the repulsive portion of the curve varies as 1/r12. Let's examine this curve from r = infinity to r = 0. As r begins to decrease the two molecules may begin to attract each other in a typical 1/r6 variation so the curve goes deeper into the negative energy region (+ is repulsive and - is negative). As r becomes very small the 1/r12 portion of the curve dominates and the curve goes rapidly positive. At some point before the repulsive portion begins to dominate, the curve reaches a maximum negative value which is the most stable energy value for the molecular cluster.
We see the same sort of energetic situation as the preceding when we are studying the intermolecular forces between any two species. The two molecules (such as two water molecules) approach each other from infinity and begin to attract each other until they become so close that the repulsive forces dominate and then they repel each other. The minimum point in a similar curve as Figure 1 (solid line) for the two water molecules will be the energy of the stable water dimer.
So let's now begin to study the details of the intermolecular interactions so that we can better understand our macromolecular world in terms of molecular concepts. The calculation of the total potential energy of interaction of all the species in a mixture, solution, or whatever would be impossible to determine if all possible interactions and perturbations of interactions by other interactions were considered. A useful approximation used to determine the total potential energy of interaction is the pair-wise additivity approximation (PWAA) which considers that the total potential energy of interaction of a system can be taken to be the sum of all pair interactions:.....................(1)
For example for a 4 particle system, the PWAA states that
V(r) = V(r12) + V(r13) + V(r14) + V(r23) + V(r24) + V(r34) ....................(2)
The larger the number of particles the better this approximation so for an Avogadro number of particles the approximation is rather good. Using the PWAA, we then now need to classify all interactions into the various kinds of pair interactions and then see what the potential energy is for each kind. Then we would be able to understand how we could calculate the total potential energy for any system. Since the forces are between different molecules, we need to understand the various types of intermolecular forces. The types can be categorized as:
The first six types are attractive intermolecular forces and the last type is a repulsive intermolecular force. If it were not for the last force, everything in the universe would be in a single "clump"! Since we don't think that we are all clumped together, we have to have a repulsive force in the set of intermolecular forces.
Now let us examine each of the forces in a little detail. To continue the discussion now, go to the next section
Web Author: Dr. Leon L. Combs
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