- Principles of Chemistry I: Honors
Fall 2015, Unique 49310
Lecture Summary, 10 November 2015
we started talking about ideal gasses. Ideal gasses are
defined as noninteracting point spheres that are arranged such
that the distance between molecules is much larger than the size
of an individual molecule, and collisions between molecules are
rare, but when they do occur they are perfectly elastic. We
performed thought experiments to derive Boyles' law, Charles' law,
and the ideal gas (or sometimes called "perfect gas") law:
This is our first example of a "state" function; i.e. a relationship that depends only on the properties of a state, not on how the state acquired those properties.
Because there is no chemical information in our definition of an ideal gas (it is a noninteracting point sphere), this state function applies both to pure species and mixtures of different species. This is Dalton's law: an ideal gas behaves the same if it is composed of one type of atom or molecule, or mixtures of atoms or molecules.
Kinetic Model of Gasses: Now that we know how ideal gasses behave, we have to know how often and with what energy two molecules collide. To do this, we imagined a box filled with molecules moving in the x direction and colliding with the walls of the box. By calculating the total change in momentum that occurs with each collision, we worked out an expression for the root mean square speed, crms, of each molecule:
crms = (3RT/(FW))1/2
This is a remarkable result because it describes the kinetic energy of a molecule (through its velocity) in terms of constants (3, R, FW), and an easily measured property, T.