Chemistry Reference
In-Depth Information
between pressure and temperature. All of these laws are expressed in
the combined gas law,
P
1
V
1
/
T
1
=
P
2
V
2
/
T
2
. The pressure of a combination
of gases can be determined using Dalton's law of partial pressures (
P
total
=
P
1
+
P
2
+
P
3
. . . ), where the partial pressures of all gases can be added
together to equal the total pressure of a gas.
The ideal gas law ties in the relationship between temperature, pressure,
volume, and the number of moles of gas in a closed system. However,
for a gas to be considered an “ideal gas” that can be calculated by this
equation, it must follow the rules of the kinetic theory. The kinetic theory
has five parts:
1. Gas molecules are in constant, random motion.
2. Most of the volume of a gas is empty space and the volume of
the molecules is negligible.
3. The molecules of a gas experience no forces of attraction or
repulsion.
4. The impact of gas molecules is completely elastic and therefore
no energy is lost in collision between molecules.
5. The temperature of a gas is equal to the kinetic energy of all its
molecules.
Under normal conditions, the ideal gas law is reasonably accurate.
However, when a gas is near its
condensation point
, its
critical point
, or
is highly pressurized, the ideal gas equation will not be accurate. In such
cases, a real-gas equation such as the van der Waals gas equation (Figure
3), which accounts for attractive forces, can be used.
(
p
+
a
•
n
2
liquid
)(
v
-
n
•
b
) =
n
•
R
•
T
V
2
gas
where:
p
= pressure
V
= volume
T
= temperature
R
= gas content
a
and b = specific constants
for each gas
volume
Figure 3
Van der Waals equation for real gas and a graph of the equation
