GASES
Introduction
Peter Chimtali
Gases and the Kinetic Molecular Theory
An Overview of the Physical States of Matter
Gas Pressure and Its Measurement
The Gas Laws and Their Experimental Foundations
The Ideal Gas Law and its applications
The Kinetic-Molecular Theory: A Model for Gas Behavior
Real Gases: Deviations from Ideal Behavior
The three states of matter
Matter: anything that occupies space and has
mass - the physical material of the universe
Matter is encountered as a collection of atoms or
molecules or ions
The three states of matter
Substances can exist in any of the three states of matter: solid,
liquid, or gas
Solid: strongest attractive forces. Atoms, molecules
or ions held together in a specific
arrangement.
Liquid: Strong attractive forces which pull particles close
together but still allow considerable
freedom to move.
Gas: Weak attractive forces, particles move about
freely and are far from each other.
• Physical properties depend, to a large extent, on state.
The three states of matter
E.g. Bromine
The three states of matter
Elements that exist as gases at 250C and 1 atmosphere
Some Important Industrial Gases
Name (Formula) Origin and use
Methane (CH4) Natural deposits; domestic fuel
Ammonia (NH3) From N2 + H2 ; fertilizers, explosives
Chlorine (Cl2) Electrolysis of seawater; bleaching
and disinfecting
Oxygen (O2) Liquefied air; steelmaking
Ethylene (C2H4) High-temperature decomposition of
natural gas; plastics
Gases
Gas particles can be monatomic (Ne), diatomic (N2), or
polyatomic (CH4)
Physical Characteristics of Gases
• Gases assume the volume and shape of their containers.
• Gases are the most compressible state of matter.
• Gas volume changes greatly with temperature
• Gases will mix evenly and completely when confined to the
same container.
• Gases have much lower densities than liquids and solids.
• Gas have relatively low viscosity.
• Gas particles are considered to have mass, but no volume
Physical Properties of a Gas
Gases are described in terms of:
Property Description Units of Measurement
Pressure (P) The force exerted by gas Atmosphere (atm); mmHg (
on the walls of container Torr), Pascal (Pa); psi;
Volume (V) The space occupied by Litre (L) = 1 dm3; mililitre
gas (mL)
Temperature The average kinetic Celcius (oC); Kelvin (K);
(T) energy and rate of motion Fareihnheit (F)
of the gas particles
Amount (n) The quantity of gas Grams (g); moles
present in the container
Pressure (P)
Force
Pressure = Area For a gas, the force arises
from the sum of collisions
with the walls of the cotainer
(force = mass x acceleration)
Units of Pressure
1 Pascal (Pa) = 1 N/m2 (Blaise Pascal
1623-1662)
= 1 kg/m s2
1 atm = 760 mm Hg = 760 torr
= 101,325 Pa
Pressure is the force exerted per
= 14.7 psi
unit area of surface by molecules
= 29.92 in. Hg in motion
A woman’s high heels sink into the soft ground,
but the larger shoes of the much bigger man do not.
Pressure =
force/area
Atmospheric Pressure
Atmospheric pressure is a result of the fact that
air has mass and is attracted by gravity
producing a force.
Knowing this atmospheric pressure and
predicting changes in the atmospheric pressure
is how forecasters predict the weather.
The pressure (force per unit area) exerted upon us
by the earth’s atmosphere
Atmospheric Pressure
The mass of a column of atmosphere of 1m2 in x-section area and
extending to top of the atmosphere (mass = 10, 300 kg) exerts a force of
1.01 x 105 N = 1 atm at the surface)
Effect of atmospheric pressure on a familiar object.
Atmospheric Pressure
– Changes with altitude and weather
- the lower the altitude, the
longer and heavier is the
column of air above an
area of the earth.
520 mmHg
760 mmHg = 1 atm
Measuring Gas Pressure
Barometer - A device to measure atmospheric
pressure. Pressure is defined as force divided by
area. The force is the force of gravity acting on
the air molecules.
Manometer - A device to measure gas
pressure in a closed container.
The barometer
(Evangelista Torricelli -1643)
Hg rises in tube until force of Hg (down)
balances the force of atmosphere
(pushing up).
Pressure of Hg pushing down is
related to
Hg density and column height
760 mm Hg = 760 Torr for std
Atmosphere (1 atm)
Pressure exerted by liquid column
force F mg hAg (F = ma = mg)
P
area A A A (m = ρhA)
P ρhg
Pressure is directly proportional to the height of the column
in a barometer or manometer.
Constructing a water barometer
What is the height of a column of water that would exert the
same pressure as a column of mercury that is 760 mm high
(use densities of 13.6 g/cm3 for Hg and 1.00 g/cm3 for water).
Solution:
PHg ρHg hHg g Pw ρw hw g
hw Hg
ρHg hHg g ρw hw g
hHg w
Hg
hw x hHg
w
13.6 g/cm3
hH 2O 760 mmHg x 3
10,300 mm
1.00 g/cm
10.3 m H 2 O
Standard temperature and pressure (STP)
STP allows us to compare Standard Pressure =
amounts of gases 1 atmosphere (atm) =
between different
760 mm Hg (or torr) =
pressures and
temperatures 14.7 psi =
101.3 kPa
Corresponds to typical pressure at sea level at 0 oC
Standard Temperature = 0 oC
(273 K)
Common Units of Pressure
Unit Atmospheric Pressure
Pascal (Pa); 1.01325 x 105 Pa SI unit
kilopascal (kPa) 101.325 kPa
Atmosphere (atm) 1 atm
Millimeters of mercury¨(mmHg) 760 mmHg
Torr 760 torr
Pounds per square inch (psi or lb/in2) 14.7 lb/in2
Bar 1.01325 bar
Gases and Gas Pressure
Force
Pressure:
Unit area
Conversions
1 atm = 760 mm Hg (exact)
1 torr = 1 mm Hg (exact)
1 bar = 1 x 105 Pa (exact)
1 atm = 101 325 Pa
Converting Units of Pressure
A Geochemist collects a sample of carbon dioxide from the
decomposition of Limestone (CaCO3) in a closed end manometer, the
height of the mercury is 341.6 mm Hg. Calculate the CO2 pressure in
torr, atmospheres, and kilopascals.
Plan: The pressure is in mmHg, so we use the conversion factors from
previous slide to find the pressure in other units.
Solution: converting from mmHg to torr:
1 torr
PCO2 (torr) = 341.6 mm Hg x = 341.6 torr
1 mm Hg
converting from torr to atm:
1 atm
PCO2( atm) = 341.6 torr x = 0.4495 atm
760 torr
converting from atm to kPa:
101.325 kPa
PCO2(kPa) = 0.4495 atm x = ________ kPa
1 atm
Converting Units of Pressure
Suppose the height of Hg in a barometer on a particular day is
760 mm (0.760 m), given that the density of Hg at 20 oC is
13.546 g/cm3 (13 546 kg/m3) and the acceleration of free fall is
9.80665 m/s2, what is the atmospheric pressure?
P = ρhg = (13546 kg/m3) x (0.760 m) x (9.80665 m/s2)
= 1.01 x 105 kg/m.s2 = 1.01 x 105 Pa
