1. Non-Ideal Behavior (Real gases):
All the gas laws and the previous equations are based on the assumptions made in KMT (no IM Forces)
The problem:
Most gases DO have some sort of weak intermolecular force
Gases composed of molecules with stronger IM forces will deviate from ideal behavior MORE
Gases also deviate from ideality at extreme conditions, such as low T’s (approaching condensation)

2. Non-Ideal Behavior (Real gases):
High Pressure deviations:
Draw a model showing the same 4 particles at extremely high pressure
(keep the temperature the same)
At extremely high pressures, the gas particles have less space to move around in, IM forces start to take hold …
1 atm
25oC
Flexible container

3. Non-Ideal Behavior (Real gases):
Low Temperature deviations:
What can we do to condense (liquify) the gas shown?
At very low T’s, the molecules go much slower, and the weak IM forces begin to take hold . . .
1 atm
25oC
Rigid container

4. Non-Ideal Behavior (Real gases):
What are the observed effects of “real” behavior?
Assume the particles at the right have relatively strong IM forces …
As particles collide with the walls, they do so with LESS FORCE because they are drawn back inwards by the IM Forces to the other molecules.
Real gases tend to have lower than normal P’s

5. Non-Ideal Behavior (Real gases):
Conditions for more “real” and less “ideal” behavior:
1. Low Temp’s
More ideal

6. Non-Ideal Behavior (Real gases):
Conditions for more “real” and less “ideal” behavior:
1. Low Temp’s
Less ideal

7. Non-Ideal Behavior (Real gases):
Conditions for more “real” and less “ideal” behavior:
1. Low Temp’s
Even less ideal
If the particles move slow enough, they will start to experience the IM forces when they collide

8. Non-Ideal Behavior (Real gases):
Conditions for more “real” and less “ideal” behavior:
1. Low Temp’s
More ideal
2. High pressures

9. Non-Ideal Behavior (Real gases):
Conditions for more “real” and less “ideal” behavior:
1. Low Temp’s
Less ideal
2. High pressures

10. Non-Ideal Behavior (Real gases):
Conditions for more “real” and less “ideal” behavior:
1. Low Temp’s
Even less ideal
2. High pressures
At high P’s the particles are forced closer together, so will begin to experience IM forces

11. How do we correct for these “real” gas behaviors?
The behavior of real gases usually agrees with the predictions of the ideal gas equation to within 5% at normal temperatures and pressures. At low temperatures or high pressures, real gases deviate significantly from ideal gas behavior. In 1873, while searching for a way to link the behavior of liquids and gases, the Dutch physicist Johannes van der Waals developed an explanation for these deviations and an equation that was able to fit the behavior of real gases over a much wider range of pressures.

12. Van deWaals Equation
Example: Calculate the pressure exerted by 10.0 moles of He gas in a 1.00-L container at
25oC using the Ideal Gas Law and Van derWaals equation separately. Which value is more
accurate and what accounts for the difference?
( aHe = atm L2 / mol2 , bHe = L/mol)
ANSWER: P = 245 atm by IGL, P = 320 by VDW, VDW is more accurate

13. Partial Pressure:
In a mixture of gases, each type of gas exerts it’s own partial pressure.
= gas A
= gas B
Dalton’s Law of Partial Pressures …