Gases Chemistry Worksheet - Chapter 13, An Introduction To Chemistry Page 3
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13.1 Gases and Their Properties
Ideal Gases
The model described above applies to real gases, but chemists often simplify the model
further by imagining the behavior of an ideal gas. An ideal gas differs from a real gas
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in that
The particles are assumed to be point masses, that is, particles that have a
mass but occupy no volume.
There are no attractive or repulsive forces at all between the particles.
When we add these assumptions to our model for gases, we call it the ideal gas model.
As the name implies, the ideal gas model describes an “ideal” of gas behavior that
is only approximated by reality. Nevertheless, the model succeeds in explaining and
predicting the behavior of typical gases under typical conditions. In fact, some actual
gases do behave very much in accordance with the model, and scientists may call
them ideal gases. The ideal gas assumptions make it easier for chemists to describe the
relationships between the properties of gases and allow us to calculate values for these
properties.
Properties of Gases
The ideal gas model is used to predict changes in four related gas properties: volume,
number of particles, temperature, and pressure. Volumes of gases are usually described
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in liters, L, or cubic meters, m
, and numbers of particles are usually described in
moles, mol. Although gas temperatures are often measured with thermometers that
report temperatures in degrees Celsius, °C, scientists generally use Kelvin temperatures
for calculations. Remember that you can convert between degrees Celsius, °C, and
kelvins, K, using the following equations.
? K = °C + 273.15
? °C = K - 273.15
To understand gas pressure, picture a typical gas in a closed container. Each time a
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gas particle collides with and ricochets off one of the walls of its container, it exerts a
force against the wall. The sum of the forces of these ongoing collisions of gas particles
against all the container’s interior walls creates a continuous pressure upon those walls.
Pressure is force divided by area.
Force
Pressure =
Area
Force due to particle collisions with the walls
Gas pressure =
Area of the walls
The accepted SI unit for gas pressure is the pascal, Pa. A pascal is a very small amount of
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pressure, so the kilopascal, kPa, is more commonly used. Other units used to describe
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gas pressure are the atmosphere (atm), torr, millimeter of mercury (mmHg), and bar.
The relationships between these pressure units are
1 atm = 101,325 Pa = 101.325 kPa = 760 mmHg = 760 torr
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1 bar = 100 kPa = 0.9869 atm = 750.1 mmHg
The numbers in these relationships come from definitions, so they are all exact. At sea
level on a typical day, the atmospheric pressure is about 101 kPa, or about 1 atm.
In calculations, the variables P, T, V, and n are commonly used to represent pressure,
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temperature, volume, and moles of gas.
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