Ideal gases

1
Everything Science www.everythingscience.co.za
7. Ideal gases
Chemistry
Grade 11

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The kinetic theory of gases
The kinetic theory of gases helps to explain the behaviour of gases under different
conditions.
The kinetic theory of gases states that gases are made up of constantly moving particles that
have attractive forces between them.
From the kinetic theory of gases we can define the pressure and the temperature of a gas.
The pressure of a gas is a measure of the number of collisions of the gas particles with each
other and with the sides of the container that they are in.
The temperature of a substance is a measure of the average kinetic energy of the particles.

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Ideal and real gases
An ideal gas has identical particles of zero volume, with no intermolecular forces between
the particles. The atoms or molecules in an ideal gas move at the same speed.
A real gas behaves like an ideal gas, except at high pressures and low temperatures. At low
temperatures, the forces between molecules become significant and the gas will liquefy. At
high pressures, the volume of the particles becomes significant.

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SI Units
The SI unit of temperature is the Kelvin (K). Temperature in degrees Celsius (°C) can be
converted to temperature in Kelvin (K) using the following equation:
The SI unit of pressure is the Pascal (Pa). The following conversions are useful:
The SI unit of volume is the cubic metre (m³). The following conversions are useful:
T K=TC+273
760 mmHg=1atm=101 325 Pa
1m
3
=1 000dm
3
=1 L

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Boyle's law
Boyle’s law states that the pressure of a fixed quantity of gas is inversely proportional to the
volume it occupies so long as the temperature remains constant.
p1V1=p2V 2

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Charles' law
Charles’ law states that the volume of an enclosed sample of gas is directly proportional to
its Kelvin temperature provided the pressure and amount of gas remains constant.
V 1
T1
=
V 2
T2

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Pressure-temperature relation
The pressure of a fixed mass of gas is directly proportional to its temperature, if the volume is
constant.
p1
T1
=
p2
T2

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General gas law
Combining Boyle’s law and the relationship between the temperature and pressure of a gas,
gives the general gas equation, which applies as long as the amount of gas remains constant.
The general gas equation is:
p1 V1
T1
=
p2V 2
T2

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Avogadro's law and the universal gas constant
Avogadro’s law states that equal volumes of gases, at the same temperature and pressure,
contain the same number of molecules.
The universal gas constant (R) is 8,314 J·K−1
·mol−1
. This constant is found by calculating pV
for 1 mol of any gas.

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The ideal gas law
Extending the above calculation to apply to any number of moles of gas gives the ideal gas
equation:
In this equation, SI units must be used. The SI unit for volume is m3
, for pressure it is Pa
and for temperature it is K.
pV =nRT

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Ideal gases

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