Gases
Some Common Properties of Gases
• They all assume the volume and shape of their
containers
• They are all compressible, i.e. reduce their
volumes considerably under even small
pressures
• Gases will mix thoroughly and evenly in a
container
• They have much lower densities than solids or
liquids
• They move constantly in their containers
General Chemistry I CHM 111
Dr Erdal Onurhan
Slide 1
Gases
Relationship between Force and Pressure
If a force of
magnitude F is
applied to a surface
with an area equal
to A, then we say
that the pressure on
surface is P and is
given by
F
P
A
General Chemistry I CHM 111
Dr Erdal Onurhan
Slide 2
Gases
A Simple Barometer
A barometer is a device for measuring
the pressure of the atmosphere.
This figure shows a mercury
barometer. The height of mercury in
the column is a measure of the
pressure of the atmosphere.
The general relationship between
pressure, (P) and the height of a liquid
column (h) in a barometer is
P  dhg
where d is the density of the liquid and
g is the gravitational acceleration.
At sea-level and 0C, d  760 mm
General Chemistry I CHM 111
Dr Erdal Onurhan
Slide 3
Gases
Two Types of Manometers
General Chemistry I CHM 111
Dr Erdal Onurhan
Slide 4
Gases
Boyles’ Law
Gases are compressible, i.e. they
can easily be compressed
(decrease in volume) by
applying even moderate
pressures to them. Liquids and
solids are not easily compressed,
i.e. they transmit the force
applied to them (by pressure)
instead of changing their
volume. This compressibility of
gases led to an empirical law,
known as the Boyle’ Law, which
states that the volume of a
sample of gas varies inversely
with pressure at a given
temperature..
General Chemistry I CHM 111
Dr Erdal Onurhan
Slide 5
Gases
Charles’s Law
Charles was the first scientist to study
the behaviour of hot gases. His and
later observations indicated a linear
relationship between volume and
temperature when expressed on the
Kelvin scale for a sample of a gas at
fixed pressure. Furthermore, plots of
volume vs. temperature showed that
the volume of a gas would vanish at 273.15 C (0 K, the absolute zero) if
the sample remains gaseous. This
empirical law is formulated as the
volume occupied by any sample of a
gas at constant pressure is directly
proportional to the absolute
temperature. This law is known as
Charles’s Law.
General Chemistry I CHM 111
Dr Erdal Onurhan
Slide 6
Gases
Gas Constant
General Chemistry I CHM 111
Dr Erdal Onurhan
Slide 7
Gases
Flowchart for Stoichiometric Calculations
Involving Gases
General Chemistry I CHM 111
Dr Erdal Onurhan
Slide 8
Gases
Partial Pressures of Components in a Mixture
General Chemistry I CHM 111
Dr Erdal Onurhan
Slide 9
Gases
Collecting Gases Over Water
The gas
collected in the
inverted tube is
not pure
hydrogen. It
inevitably
contains some
water vapour,
picked up
while passing
through water.
General Chemistry I CHM 111
Dr Erdal Onurhan
Slide 10
Gases
Speed Distribution in Gases
General Chemistry I CHM 111
Dr Erdal Onurhan
Slide 11
Gases
Diffusion
The green molecule will
eventually make its way
into the red zone.
It will take a while to reach
a given point, because of
a) its random motion
b) presence of like and
unlike molecules on its
path will result in
collisions, delaying the
molecule’s arrival at
the required spot
General Chemistry I CHM 111
Dr Erdal Onurhan
Slide 12
Gases
Effusion
Rate depends on
the speed of
molecules here,
since the
concentration
and the hole is
the same for all
of them. Since
the temperature
is constant, the
lightest will
effuse more in
unit time.
General Chemistry I CHM 111
Dr Erdal Onurhan
Slide 13
Gases
Real Gases
When the concentration is high (low temperatures
and high pressures), the free volume in which the
molecules move is lowered considerably and
interactions are much stronger. This means that
the pressure on the walls is lower than when we
have an ideal gas (no interaction). The pressure
term in the ideal gas equation must be modified to
reflect this drop. Also, the volume of the
container must be modified to reflect drop in the
free volume.
Hence, van der Waals’ Equation

n2 a 
P

(V  nb)  nRT

2 
V


General Chemistry I CHM 111
Dr Erdal Onurhan
Slide 14