CH353 - Physical Chemistry I
Spring 2013, Unique 52575

Lecture Summary, 8 April 2013


Kinetic Model of Gases: The second major topic that we will cover in this class is chemical reaction kinetics, which deals with the rate at which chemical reactions occur.  Understanding how long it will take to get from reactants to products is necessary for determining if a spontaneous reaction will occur on a desirable or reasonable time scale.  In order for matter and energy to be transferred in a productive chemical reaction, the first thing that must happen is that two reactants must collide with an appropriate energy.  Therefore the first thing that we will do is figure out how fast molecules move under a given set of conditions of T, P, V, etc. 

We first need to make three assumptions about our system.  1) It is a gas that consists of atoms or molecules that are in ceaseless random motion in all directions; 2) The size of the molecule or atom is significantly smaller than the distance it travels between collisions; and 3) when collisions do occur, they are brief and completely elastic.  These three conditions are just another way of saying we have an ideal gas, but the aspects of that ideal gas that are important for the study of kinetics are emphasized. 

By thinking about how often a molecule collides with the walls of the container that it is being held in, we derived an expression for the root-mean-square speed of a particle in the gas:  c = (3RT/(FW))^(1/2).

Thus the average speed of a molecule is directly proportional to system temperature and indirectly proportional to its mass.