5 Steps to a 5 AP Chemistry

KEper molecule = 1/2mv^2

KEper mole =

1 atm =760 mm Hg

= 760 torr

Kinetic Molecular Theory

The Kinetic Molecular Theoryattempts to represent the properties of gases by modeling

the gas particles themselves at the microscopic level. There are five main postulates of the

Kinetic Molecular Theory:

1. Gases are composed of very small particles, either molecules or atoms.


2. The gas particles are tiny in comparison to the distances between them, so we assume
   that the volume of the gas particles themselves is negligible.


3. These gas particles are in constant motion, moving in straight lines in a random fash-
   ion and colliding with each other and the inside walls of the container. The collisions
   with the inside container walls comprise the pressure of the gas.


4. The gas particles are assumed to neither attract nor repel each other. They may collide
   with each other, but if they do, the collisions are assumed to be elastic. No kinetic
   energy is lost, only transferred from one gas molecule to another.


5. The averagekinetic energy of the gas is proportional to the Kelvin temperature.

A gas that obeys these five postulates is an ideal gas. However, just as there are no ideal

students, there are no ideal gases: only gases that approach ideal behavior. We know that

real gas particles do occupy a certain finite volume, and we know that there are interactions

between real gas particles. These factors cause real gases to deviate a little from the ideal

behavior of the Kinetic Molecular Theory. But a non-polar gas at a low pressure and high

temperature would come pretty close to ideal behavior. Later in this chapter, we’ll show how

to modify our equations to account for non-ideal behavior.

Before we leave the Kinetic Molecular Theory (KMT) and start examining the gas law

relationships, let’s quantify a couple of the postulates of the KMT. Postulate 3 qualitatively

describes the motion of the gas particles. The average velocity of the gas particles is called the

root mean square speedand is given the symbol urms. This is a special type of average speed.

n m

M

=

3

2

RT

urms==^33 kT

m

RT

M

PV

T

PV

T

11

1

22

2

=

Gases  103

KEY IDEA