The Gas Laws
Chapter 13
Kinetic Molecular Theory
Gas molecules behave differently than those of solids
and liquids. To explain their behavior, the Kinetic
Molecular Theory provides a model and a special set of
assumptions about gases.
1. Gas particles do not attract or repel each other.
Kinetic Molecular Theory
2. Gas particles are much smaller than the distances
between them. This is why gases are compressible.
Kinetic Molecular Theory
3. Gas particles are in constant, random motion and
they quickly become thoroughly mixed in a closed
container. This phenomena is called dispersion and is
the reason why women wear perfume and men wear
cologne. When a person with Channel or Axe enters a
room, it doesn’t take long before everyone looks around
to see where the smell is coming from.
Kinetic Molecular Theory
4. Collisions between gas particles or the sides of their
containers are completely elastic. There is no kinetic
energy lost.
Kinetic Molecular Theory
5. All gases have the same average kinetic energy at a
given temperature.
The Nature of Gases
The assumptions of the Kinetic Molecular Theory are
based on 4 variables:
1. Temperature
2. Pressure
3. Volume
4. The Number of Gas Particles Present
These four variables work together to determine the
behavior of gases and when one variable changes, it
affects the other three. Also, the explanations as to
what happens when one of the variables changes is
explained by the Gas Laws
The Gas Laws
There are five gas laws that we will study:
1. Boyle’s Law – Pressure/Volume
2. Charles’ Law – Volume/Temperature
3. Gay-Lussac’s Law – Pressure/Temperature
4. Combined Gas Law – Pressure/Volume/Temperature
5. Graham’s Law of Diffusion – MM/Diffusion Rate
Boyle’s Law
In the 1600’s Robert Boyle determined that if the
pressure on a volume of gas was doubled, the volume of
gas decreased by half. More precisely, Boyle’s Law states
that the volume of a gas varies inversely with the
pressure, provided the temperature and the number of air
particles do not change.
Ouch! I hate when my ears pop!!
Boyle’s Law explains why we need to wear oxygen at high
elevations. There is less pressure the higher we are and
as a result, fewer oxygen molecules to breathe. Also,
Boyle’s Law explains why our ears pop when we go up in
an elevator or airplane. The pressure in our middle ear
must be equalized as the air pressure is decreased.
Measurement of Air Pressure
Air pressure is measured using several different systems:
1. The Non SI System of pressure is pounds per
square inch (psi)
2. The Metric System of pressure is the kilopascal
(kPa)
3. The Non SI System Meteorologists use is inches
of mercury (inHg)
4. The Metric System of pressure uses mm of
mercury (mmHg)
All of these systems are based on the pressure the
atmosphere exerts at sea level which is the atmosphere
(atm).
STP
To ensure uniformity worldwide in the measurement and
calibration of pressure, a set of standard conditions called
STP (Standard Temperature and Pressure) has been
established as One Atmosphere @ 0°C. However, in recent
years there has been a departure from STP by different
scientific disciplines. Whenever pressure measurements
are stated, the standard that is used must be referenced.
Pressure System Measurements
These are the equivalents between the different pressure
system measurements:
@ STP One Atmosphere = 14.7 psi
@ STP One Atmosphere = 101.3 kPa
@ STP One Atmosphere = 29.92 inHg
@ STP One Atmosphere = 760 mmHg
Boyle’s Law Calculations
Now that you have an understanding of pressure systems
let’s get back to Boyle’s Law. Since Boyle’s Law is an
inverse relationship (increase the pressure, decrease the
volume) the formula is:
P 1 V1 = P 2 V2
A typical Boyle’s Law Problem would look like this:
If a gas occupies 2 L at 1 atm, what will be the volume of
this gas at 4 atm?
P1V1 = P2V2
(2L)(1 atm) = (4 atm)V2
V2 =
Boyle’s Law Calculations
The greatest difficulty students have with Boyle’s Law is
moving from one pressure system to another. Look at this
problem. Notice that there are two different pressure
systems (kPa and psi). The system must have the same
units on each side of the equation, so you must change
either kPa or psi.
A gas occupies a volume of 5 L at 100 kPa. What volume
will it occupy at 100 psi?
100 psi x 101.3 kPa = 689.1 kPa
14.7
psi
P1V1 = P2V2
(5 L)(100 kPa) = (689.1 kPa)V2
V2 = .752 L