1. Kinetic-Molecular Theory
Kinetic Molecular Theory (KMT) is a model used to explain the behavior of gases.

2. Major Ideas of Kinetic Molecular Theory
Gases contain particles (usually molecules or atoms) that are in constant, random, straight line motion.

3. Major Ideas of Kinetic Molecular Theory
Gas particles collide with each other and with the walls of the container. The collisions transfer energy, but there is no net loss of energy.

4. Major Ideas of Kinetic Molecular Theory
Gas particles are separated by relatively great distances. Because of this, the volume occupied by the particles themselves is negligible.

5. Major Ideas of Kinetic Molecular Theory
Gas particles do not attract each other.

6. The Nature of Gases KMT Theory explains why gases exert pressure
Not only do the gas particles collide with each other, but they also collide with the walls.

7. The Nature of Gases
The collisions with the wall exert a force over the surface area of the wall.
That is they exert pressure on the wall…
Recall that pressure is a force per unit area.

8. Units of Pressure We will use two different units for pressure.
Atmospheres (atm)
kiloPascals
Animation

9. Temperature
Recall that temperature is the average kinetic energy of the particles.
So, the higher the temp, the faster the particles are moving.

10. Behavior of Gases There are four variables…
# of gas particles present
Pressure
Temperature
Volume
… that explain the behavior of gases.

11. Chemistry Lab
Boyles Law

12. Animations

13. Boyle’s Law
Boyle’s Law states that the volume of a given amount of gas held at constant temperature varies inversely with the pressure.
Mathematically…
P1V1 = P2V2

14. Pressure-Temperature Relationship in Gases
Chemistry Lab
Pressure-Temperature Relationship in Gases

15. Gay-Lussac’s Law
States that the pressure of a given mass of gas varies directly with the kelvin temperature when the volume remains constant.
Mathematically, …
P1 / T1 = P2 / T2

16. Practice Problem
A sealed rigid container has a volume of 2.5L, and a pressure of 2 atm at 400K. What will be the new pressure if the container is cooled to 300K?

17. Charles’s Law
States that the volume of a given mass of gas is directly proportional to to its kelvin temperature at constant pressure.
Mathematically, …
V1 / T1 = V2 / T2

18. Combined Gas Law Equation
The relationships among pressure temperature, and volume can be mathematically represented by an equation know as the combined gas law…
P1V1 / T1 = P2V2 / T2
(reference tables)

19. Practice Problem
What volume will a gas occupy if the pressure on 300 cm3 of gas at 5.0 atm is increased to 7.0 atm? Assume the temperature remains constant.

20. Standard Temperature and Pressure
Standard pressure is defined as 1 atmosphere
One atmosphere is equal to…
101.3 kPa
760 mm Hg
760 torr
Standard temperature is defined as 00C (273 K)

21. Lets Practice More…
If 120 cm3 of a gas is at STP, what volume will the gas occupy if the temperature is raised to 500C and the pressure is increased to 950 torr?

22. Ideal vs. Real Gases
When gas laws are used to solve problems, the results don’t always agree exactly with lab results
The explanation is that the assumptions of KMT are not exactly correct.

23. Ideal vs. Real Gases Gas particles really do attract one another!
In most cases, IMF’s are so small they can be ignored
But in extreme cases, the IMF’s become significant and important
For example, water becomes snow or rain when the temps get low enough.

24. Ideal vs. Real Gases Gas particles actually do occupy volume!
As pressure increases, the volume occupied by the by the gas particles can’t be ignored.
At high pressures, there are more frequent collisions.

25. Under What Conditions Is A Gas Nearly Ideal?
Low Pressure
High Temperature
Because those conditions allow the molecules to stay far away from one another.