CH301H - Principles of Chemistry I: Honors
Fall 2012, Unique 51390

Lecture Summary, 27 November 2012

Real Gasses: Because interesting molecules actually do have to interact, we need to incorporate what we know about intermolecular forces into a new gas law.  There are many real gas state functions, but the most famous and general is the van der Waals gas law:

    P = (nRT/(V-nb)) - a(n/V)2

The first part of this term modifies the ideal gas law by reducing the total volume available to any given atom or molecule.  This accounts for repulsive forces, which will increase the effective pressure of the gas.  The second term accounts for how often we find molecules near each other, and because it reduces the effective pressure of a gas, accounts for attractive forces.  The constants a and b are properties of the individual atom or molecule.

In order to determine the relative importance of attractive vs. repulsive forces, it is often useful to compare the predictions of the ideal and van der Waals gas laws for species under certain conditions.  If P(ideal) < P(vdW), then repulsive forces dominate the gas.  If P(ideal) > P(vdW), then attractive forces dominate the gas.  With a little bit of understanding of the structure of the particular molecul
e, we can also make a good guess at what intermolecular force is responsible. 

Phase Transitions and Diagrams:  One of the most instructive ways to investigate any given phase is to study transitions is to study the transition between that phase and something else.  This information is wrapped up in a phase diagram, which plots the phase space of an atom or molecule as a function of pressure and temperature.  Phase diagrams describe where transitions between phases occur, which in turn helps us figure out how much energy is necessary to cause a phase transition. 

Aside About Water:  There are three interesting properties of water that are of central importance to life on earth as we know it:

   1.  It has a high melting point compared to other molecules of similar shape and formal weight (for example methane);
   2.  The solid is less dense than the liquid;
   3.  The triple point of water is very near the ambient temperature and pressure of earth.  This means that the solid, liquid, and gas can all appear under near ambient conditions with very small inputs or outputs of energy.

We will see a couple interesting consequences of these properties in this class, but keep this in mind as you move forward in your chemistry and biology classes.