- Physical Chemistry I
Spring 2012, Unique 52135
Lecture Summary, 9 April 2012
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,
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.