Section 4.2 pg. 163-168
Curriculum Objectives:
1. Describe and compare the behaviour of real and ideal gases in terms of the kinetic
molecular theory.
2. Explain the law of combining volumes.

The central idea of the
kinetic molecular theory is
that the smallest entities of a
substance are in continuous
motion.

These entities may be
atoms, ions or molecules.

As they move about, the
entities collide with each
other and with objects in
their path.
Observation of microscopic particles,
such as a pollen grain, shows a
continuous, random motion. This is
known as Brownian motion, named
for Scottish scientist, Robert Brown.

According to Kinetic molecular theory, the motion of
molecules is different in solids, liquids and gases.
 Solids - primarily vibrational motion.
 Liquids - vibrational, rotational and some translational motion
 Gases – the most important form of motion is translational

Kinetic Molecular Theory explains:
1.
Gases are compressible (due to most of a sample of gas
being unoccupied space, thus particles can be forced closer
together)
2.
Gas pressure (due to pressure being the result of particle
collisions distributed over walls of a container causing a force
per unit area)
3.
Boyle’s Law (due to reduced volume, there is a shorter
distance between walls thus more frequent collisions,
causing increased pressure)
4.
Charles’ Law (due to increase in temperature, there is an
increase in particle speed causing more collisions with the
wall. The wall moves outward, thus volume increases)

Try pg. 164 #1 a), d), e)
a) According to the k.m.t., as the temperature increases, the average speed of
the gas particles increases. If the volume is kept constant, then faster-moving
particles will collide more often with the sides of the container. More collisions
mean a greater pressure.
d) According to the k.m.t., gases such as air are very compressible because most of the
volume is empty space. The fact that there is very little empty space between the
molecules of a liquid, such as oil, makes liquids not compressible. In a hydraulic system, the
pressure applied at one end (e.g., brake pedal) needs to be transmitted to the other end
(e.g., brakes). This will work only if the medium inside the system is not compressible.
e) A bullet moves in a straight line over a long distance before it hits its target.
According to the k.m.t., a gas molecule moves only a very short distance before
colliding with another gas molecule, and thus changing its direction.
Section 4.2
Pg. 164 - 166
Joseph Gay-Lussac
Amedeo Avogadro

The kinetic molecular theory explains many physical properties
of gases. But what about their chemical properties?

In 1809, Joseph Gay-Lussac, a colleague of Jacques Charles,
measured the relative volumes of gases involved in chemical
reactions.

His observations led to the Law of Combining Volumes, which
states that:
 “When measured at the same temperature and pressure, volumes
of gaseous reactants and products of chemical reactions are
always in simple ratios of whole numbers”
 This is also known as the Gay-Lussac’s Law

A simple example of this is the
decomposition of liquid water, in
which the volumes of hydrogen
and oxygen gas are always
produced in a 2:1 ratio

Which side is Hydrogen?
2H2O(l)  2H2(g) + O2(g)

Two years after Gay-Lussac’s Law, Avogadro
proposed an new explanation in terms of numbers
of molecules
 Avogadro proposed: “equal volumes of gases at the
same temperature and pressure contain equal
numbers of molecules”
 This means the mole ratios provided by a balanced
equation are also the volume ratios.
 This in now best called Avogadro’s Theory

When all gases are at the same temperature and pressure,
the law of combining volumes provides an efficient way of
predicting the volumes of gases in a chemical reaction.
Coefficients:
Chemical Amounts:
Volumes:
Example:
1
1 mol
1L
3
3 mol
3L
2
2mol
2L
2 mL
6 mL
4 mL
VH2: 2 ml x ( 3 ) =
1
6 mL
VNH3: 2 ml x ( 2 ) =
1
4 mL
Use the law of combining volumes to predict the volume of oxygen required for
the complete combustion of 120 mL of butane gas from a lighter.
1) The first step is to write the balanced chemical equation, including what you are
given and what you need to find:
2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(g)
120 mL
V=?
2) From this chemical equation you can see that 13 mol of oxygen is required for
every 2 mol of butane. Therefore, the volume of oxygen has to be greater than
120mL by a factor of 13/2.
VO2: 120 ml C4H10 x ( 13 mol O2)
2 mol C4H10
= 780 mL
To make sure that the ratio is used in the correct order, you could include the
chemical formula with each quantity as shown above. Note the cancellation of the
units and chemical formulas
A catalytic converter in the exhaust system of a car uses oxygen (from the air) to
convert carbon monoxide to carbon dioxide, which is released through the
tailpipe. If we assume the same temperature and pressure, what volume of
oxygen is required to react with 125L of carbon monoxide during a 100 km trip?
1) The first step is to write the balanced chemical equation, including what you are
given and what you need to find:
2CO(g) + O2(g) → 2CO2(g)
125 L
V=?
2) From this chemical equation you can see that 1 mol of oxygen is required for
every 2 mol of carbon monoxide. Therefore, the volume of oxygen has to be less
than 125L by a factor of 1/2.
VO2: 125 L CO x ( 1 mol O2)
2 mol CO
= 62.5 L O2
According to the law of combining volumes, 62.5L of oxygen is required.

This equivalence between the chemical
amounts (coefficients) and the volumes only
works for gases, and only if they are at the
same temperature and pressure.



Pg. 166 #5-6
Pg. 168 #1, 2, 4, 5
Law of combining volumes WS