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properties of gases
Gases are easily expandable and compressible
unlike solids and liquids. Gases have a measurement of pressure.
Pressure is defined as force exerted per unit area of surface. It
can be measured in several units such as kilopascals (kPa),
atmospheres (atm), and millimeters of Mercury (mmHg). Gas has a low
density because its molecules are spread apart over a large volume.
A gas will fill whatever container that it is in. An example of
this is a bottle of ammonia being opened in a room and the smell
traveling throughout the room.
The Kinetic Molecular Theory is the basis of the many properties of
gases. The five postulates to the Kinetic Theory are as
follows:
- Gases are composed of molecules whose size is
negligible compared to the average distance between
them.
- Molecules move randomly in all directions and
at various speeds.
- The forces of attraction or repulsion between
two molecules in a gas are very weak or negligible, except when
they collide.
- When molecules collide with one another, the
collisions are elastic; no kinetic energy is lost.
- The average kinetic energy of a molecule is
proportional to the absolute temperature.
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five postulates to the Kinetic
Theory
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Effusion and Diffusion are the two ways that gases
mix with other gases. Diffusion is a process in which a gas enters
a container with another gas and the two mix to form a uniform
mixture. Effusion occurs when a gas moves through a small hole in
its current container into another container. An example of
diffusion is the ammonia mentioned earlier where the ammonia moves
into the room with the air. An example of effusion is if a coke
bottle had a small hole in it just small enough for the gas inside
to escape.
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| Since one of the properties of a gas is
compressiblity, a gas at a certain volume can be compressed by
adding pressure. The mass of the gas will remain unchanged. Since
the mass remains the same and the volume decreases, the density of
the gas is greater. This can be observed by using the density
equation D=m/V. If the mass of the gas is .50 grams and the volume
of the gas is one liter then the density of the gas is .50
grams/liter. However, if the gas is compressed to only take up one
half a liter then the density will change to 1 gram/liter. The
picture illustrates the compressing of a gas. |
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Many of the properties of gases can be measured in
different ways. Conversion from one unit of pressure to another is
very important. To achieve this there has to be a conversion factor
to move from one unit to another. Here is a list of equivalent
amounts of pressure:
- 1 atm
- 760 mmHg
- 76 cmHg
- 101.3 kPa
- 760 torr
- 29.92 inches Hg
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equivalent amounts of pressure
If you are in atms and want to convert to mmHg then # of atms *
(760 mmHg / 1 atm) = # of mmHg because the atms cancel. This
dimensional analysis method of canceling the units will work for
changing from any unit to another.
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