1. Kinetic Molecular Theory of Gases
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Jeff Koons
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2. Kinetic Molecular Theory of Gases
A gas consists of very small particles, each of which has a mass.
Example:
An inflated basketball weighs more than a deflated basketball.

3. Kinetic Molecular Theory of Gases
The distances separating gas particles are large relative to the size of the particles.
The volume of the gas particles is assumed to be zero because it is negligible compared with the total volume in which the gas is contained.

4. Kinetic Molecular Theory of Gases

5. Kinetic Molecular Theory of Gases
Liquid

6. Kinetic Molecular Theory of Gases
Solid

7. Kinetic Molecular Theory of Gases
3. Gas particles are in constant, rapid, random motion.
Gases immediately fill a container and quickly diffuse from one area to another.
This is a good description of how gas molecules behave.

8. Kinetic Molecular Theory of Gases

9. Kinetic Molecular Theory of Gases
Collisions of gas particles with each other or with the walls of the container are perfectly elastic (no kinetic energy is lost).
Unlike “bouncing balls,” no energy of motion is lost.

10. Kinetic Molecular Theory of Gases
The average kinetic energy of gas particles depends only on the temperature of the gas.
The kinetic energy of gas molecules is proportional to their temperature in Kelvin -- a good description.
KE = ½ mv2
High T  Higher KE
Low T  Lower KE

11. Kinetic Molecular Theory of Gases
6. Gas particles exert no force on one another. Attractive forces between gas particles is assumed to be zero.
Depending on the gas, this can be good or bad assumption. For instance, H2O vapor has much stronger intermolecular interactions than He.

12. Ideal Gases
Gases that follow the KMT are called Ideal Gases. In reality, there are no ideal gases, only real gases.
Real gases slightly attract each other and the particles do have volume.
However, at low pressures and large volumes, gases behave more ideally.
Helium behaves the most ideally. Why?

13. Temperature
A measure of the average kinetic energy of the particles in a sample of matter.

14. Temperature
T (˚C)
Kinetic Energy (J)

15. Celsius (˚C) (or Centigrate)
Temperature
What are the units?
Fahrenheit (˚F)
Celsius (˚C) (or Centigrate)
Kelvin (K)

16. Temperature Where they place zero and the scale!
So what’s the difference between the units of temperature?
Where they place zero and the scale!

17. Temperature Celsius: Based on freezing and boiling points of water.
0˚ C = Freezing point of water
100 ˚C = Boiling point of water (at sea level)

18. Temperature 0 ˚F = Freezing point of sea water
Fahrenheit: Based on freezing point of sea water and average temperature of human.
0 ˚F = Freezing point of sea water
100 ˚F = Avg. temp. of human (measurement was off!)

19. Temperature
Kelvin: Same unit as Celsius, but zero is placed at absolute zero
0 K = absolute zero = temperature at which all molecular motion stops

20. Temperature
Conversions:
TF = 1.8(TC) + 32

21. TF = 1.8(TC) + 32
Practice:
25˚C = ? ˚F
Ans: 77 ˚F

22. TF = 1.8(TC) + 32
Practice:
100.˚F = ? ˚C
Ans: 37.8 ˚C

23. Temperature
Conversions:
TK = TC + 273

24. TK = TC + 273
Practice:
25˚C = ? K
Ans: 298 K

25. TK = TC + 273
Practice:
328 K = ? ˚C
Ans: 55 ˚C

26. Pressure The force per unit of area on a surface.
A gas uniformly fills any container, is easily compressed, and mixes completely with any other gas.
One of the most obvious and useful properties of a gas is that it exerts pressure on its surroundings. This is due to the particles hitting surfaces.
Atmospheric pressure, for example, is the result of a mass of air (a mixture of many gases in the atmosphere) being pulled down toward the center of the earth by gravity.

27. Mathematically, it looks like this:
Pressure
Mathematically, it looks like this:

28. Pressure
So is P directly related to F or A? P P F A

29. Pressure So what are some units of pressure? atm (atmosphere)
psi (pounds per square inch)
torr
mmHg (millimeters of mercury)
Pa (Pascal)
kPa (kilo Pascal)
bar

30. Pressure 1 atm = 14.7 psi = 760 torr = 760 mmHg = 29.92 in Hg
And here are the conversion factors:
1 atm = psi
= 760 torr
= 760 mmHg
= in Hg
= 101,325 Pa
= kPa
= bar

31. Pressure 689 torr = ? atm Ans = 0.907 atm
Let’s do some practice conversions:
689 torr = ? atm
Ans = atm

32. Pressure 156.1 kPa = ? psi Ans = 22.7 psi
Let’s do some practice conversions:
156.1 kPa = ? psi
Ans = 22.7 psi

33. T = 0 oC and P = 1 atm exactly STP
A common condition for gas law problems is called “STP” or “Standard Temperature and Pressure.” When a problem says a gas is at STP,
T = 0 oC and P = 1 atm
exactly

34. Good Job!!!
Now for your homework…