Untitled - Kelly Walsh High School

88 CHEMISTRY FOR THE UTTERLY CONFUSED
Don’t Forget!
Your answer must be reasonable. Hydrogen is the lightest gas (2.018 g/mol) so
that any molar mass less than this value is not reasonable.
5-4 Nonideal Gases
The KMT represents the properties of an ideal gas. However, there are no truly
ideal gases; there are only gases that approach ideal behavior. We know that
real gas particles do occupy a certain finite volume and we know that there are
interactions (attractions and repulsions) between real gas particles. These factors
cause real gases to deviate a little from ideal behavior. However, a nonpolar
gas at a low pressure and high temperature would come pretty close to ideal
behavior. It would be nice, however, to have a more accurate model/equation
for those times when we are doing extremely precise work or we have a gas that
exhibits a relatively large attractive or repulsive force. Johannes van der Waals
introduced a modification of the ideal gas equation that attempted to take into
account the volume and attractive forces of real gases by introducing two constants
a and b into the ideal gas equation. (Values for these two constants are
probably found in a table in your textbook.) The result is the van der Waals
equation:
(P an 2 /V 2 )(V nb) nRT
The attraction of the gas particles for each other tends to lessen the pressure
of the gas since the attraction slightly reduces the force of the collisions of the
gas particles with the container walls. The amount of attraction depends on the
concentration of gas particles and the magnitude of the intermolecular force of
the particles. The greater the intermolecular forces of the gas, the higher the
attraction is, and the less the real pressure. Van der Waals compensated for the
attractive force by the term: P an 2 /V 2 , where a is a constant for individual
gases. The greater the attractive force between the molecules, the larger the
value of a.
The actual volume of the gas is less than the ideal gas. This is because gas
molecules do have a finite volume and the more moles of gas present, the
smaller the real volume. The volume of the gas can be corrected by the V
nb term, where n is the number of moles of gas and b is a different constant
for each gas. The larger the gas particle, the more volume it takes up and the
larger the b value.