CH 13 The Chemistry of Gases
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Gases are elements (He), elemental
substances (O2), or compounds (CO2) in
which the particles of the substance are
widely separated, moving randomly, and
have very little attraction to each other.
Gases
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Are fluids
Have low density
Are highly compressible
Fill their container
Describing Gases
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Aside from the type of gas, there are four
physical properties that describe a gas.
Each of these properties can have an
affect on the others.
They are:
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Amount of the gas
Temperature of the gas
Pressure of the gas
Volume of the gas
Property 1: Volume
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Gases expand to fill the container which
contains them.
The volume of a gas is the space it
occupies which is typically the space of
the container.
Volume is expressed in
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Liters, L
Cubic centimeters, cm3
Milliliters, mL
1.0 L = 1000 cm3 = 1000 mL
Property 2: Temperature
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The temperature of a gas is an indirect
measure of the average kinetic energy of
the particles.
The higher the temperature, the greater
the movement of the gas particles.
Temperature is measure on 3 scales
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Fahrenheit, which we will NOT use.
Celsius, which is a relative scale.
Kelvin, which is the absolute temperature
scale.
Tk = Tc + 273.15
Tf = [9/5]Tc + 32
Property 3: Pressure
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The pressure exerted from a gas is a result of the
force of the collisions of the gas particles as they
strike the sides of the container.
These collisions are elastic (conserve energy).
Pressure is the ratio of force to unit area.
Units of Pressure
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Atmospheres (atm) defined by the
pressure of the atmosphere at sea level
which is 1.0 atm.
Pascal (Pa) the metric unit of pressure
from the definition of the force of 1.0 N
over 1.0 square meters of area.
Pounds per square inch (psi) British unit
1.0 atm = 101.3 kPa = 14.7 psi
The Barometer
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The barometer measures
atmospheric pressure.
The mercury barometer
was one of the first
apparatus to do this.
The atmosphere can
support a column of
mercury 760 mm high.
1 atm = 760 mmHg
Also called the Torr after
its inventor Torricelli.
Aneroid Barometer
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A more practical
barometer is the
aneroid barometer.
As pressure changes
the vacuumed disc,
called the aneroid,
expands and
contracts.
This indicates relative
pressure on a
calibrated scale.
Property 4: Amount of Gas
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The amount of gas present is the actual
number of particles in the volume of gas.
This is measured in moles (n).
The moles of gas is the mass of the gas
divided by the molar mass of the
substance.
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n = m/M
Particles = n * NA