5 Steps to a 5 AP Chemistry 2019

104 ❯ STEP 4. Review the Knowledge You Need to Score High

PV=

T

PV

T

11

1

22

2

u ==kT

m

33 RT

rms M

KE per molecule = 1/2 mv^2

KE per mole =^3 RT

2

1 atm = 760 mm Hg

= 760 torr

n=m

M

Kinetic Molecular Theory

The Kinetic Molecular Theory attempts 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 fashion
   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 average kinetic 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 quali-
   tatively describes the motion of the gas particles. The average velocity of the gas particles is
   called the root mean square speed and is given the symbol urms. This is a special type of

KEY IDEA