1. Nature of Gases
Kinetic Molecular Theory

2. We will now discuss the gas phase
So far we have primarily focused on reactions that happen in the liquid phase
We will now discuss the gas phase

3. Kinetic Molecular Theory States Gas Particles:
4. Have no interaction between atoms or molecules of a gas. Don’t attract or repel each other.
6. Have an avg. KE directly related to Kelvin temperature.
2. Go in a straight trajectory until
5. an elastic collision occurs
3. Have no volume: Mostly empty space
1. Consist of widely separated atoms or molecules in constant, random straight line motion

4. Properties of gases Gases have a unique set of physical properties:
Gases are translucent or transparent.
Gases have very low densities when compared to liquids or solids.
Gases are highly compressible compared to liquids and solids.
Gases can expand or contract to fill any container.
Mostly empty space

5. Properties of gases Gases have a unique set of physical properties:
Gases are translucent or transparent.
Gases have very low densities when compared to liquids or solids.
Gases are highly compressible compared to liquids and solids.
Gases can expand or contract to fill any container.
These can be explained by the kinetic molecular theory – (refer to your reading material for the KMT postulates)

6. Gases consist of atoms or molecules with a lot of space in between,
that are in constant, random motion
kinetic molecular theory: the theory that explains the observed thermal and physical properties of matter in terms of the average behavior of a collection of atoms and molecules.

7. This empty space makes gases good insulators (example: windows, coats)

8. Pressure Gases have pressure
Pressure = force per unit of area
A gas can easily change shape. When you push down on a balloon, sideways forces push the sides of the balloon outward.

9. Pressure and the number of molecules are directly related
More molecules means more collisions.
Fewer molecules means fewer collisions.
Gases naturally move from areas of high pressure to low pressure because there is empty space to move in.

10. Collisions of Gas Particles10

11. Kinetic theory of pressure
Pressure comes from the collisions of the many, many atoms inside and outside the balloon.
If the collisions occur frequently, the pressure is high. If the collisions don’t occur often, the pressure is low.

12. *Example: Blowing ______ air into a balloon makes it larger.
Here is the _____________ relationship between the # of gas particles in a container and the volume and pressure of the container:
As the # of gas particles _____________, the volume of a flexible container will ____________ if the temperature and pressure of the container remain constant.
# particles ___, V ___
*Example: Blowing ______ air into a balloon makes it larger.
qualitative
increase
increase ↑ ↑
more

13. As the # of gas particles ____________, the pressure of a
rigid container will ____________ if the temperature and volume of the container remain constant.
# particles ___, P ___
*Examples: Pushing the button on an aerosol can releases the gas and ___________ the pressure in the container. Adding too much gas into a rigid container could make it ___________ from too much pressure!
increase
increase ↑ ↑
decreases
explode

14. # of Gas Particles vs. Pressure

15. Units of pressure Convert 800 torr of pressure to atmospheres (atm).
Standard pressure
= atm = torr = mmHg = KPa
Convert 800 torr of pressure to atmospheres (atm).
Relationships: Standard pressure = 760 torr = 1 atm
Solve:
𝟖𝟎𝟎 𝒕𝒐𝒓𝒓 𝟏 𝒂𝒕𝒎 = torr
𝟕𝟔𝟎 𝒕𝒐𝒓𝒓

16. A gas will expand to fill any size
Kinetic theory of pressure
Remember:
Gases consist of atoms or molecules with a lot of space in between,
that are in constant, random motion
A gas will expand to fill any size
(or shaped) container

17. Kinetic Molecular Theory
Because we assume molecules are in motion, they have a kinetic energy.
At the same Temperature, all gases have the same average KE.
As Temperature goes up, KE also increases — and so does speed of the molecules.
Temperature = measure of the average kinetic energy of the particles of a substance

18. Kinetic theory of pressure
The average force from molecular impacts depends on two things:
1. Faster (hotter) molecules mean more force per impact and higher pressure.
Higher temperature means harder collisions and higher pressure

19. Kinetic theory of pressure
The average force from molecular impacts depends on two things:
1. Faster (hotter) molecules mean more force per impact and higher pressure.
2. More molecules per cubic centimeter (higher density) mean more impacts and therefore higher pressure.
Higher density means more collisions and higher pressure

20. Volume of Gas
In a smaller container, molecules have less room to move.
Hit the sides of the container more often.
As volume decreases, pressure increases. (think of a syringe)

21. Diffusion Gas molecules move around quite fast.
The average speed of a nitrogen molecule (N2) in air is 417 m/s, or 933 mph!

22. Diffusion
Gas molecules move around quite fast.
The average speed of a nitrogen molecule (N2) in air is 417 m/s, or 933 mph!
Random motion
A typical air molecule has 140,000 collisions in a centimeter

23. Diffusion Gas molecules move around quite fast.
The average speed of a nitrogen molecule (N2) in air is 417 m/s, or 933 mph!
The average speed of an oxygen molecule (O2) in air is 390 m/s, or 873 mph
Random motion
Heavier molecules move slower

24. Diffusion
diffusion: the spreading of molecules through their surroundings through constant collisions with neighboring molecules.

25. No interaction between atoms or molecules, except during collisions
The basis of kinetic molecular theory, which explains gas behavior
Straight trajectory until a collision occurs
Mostly empty space
Gases consist of widely separated atoms or molecules in constant, random motion

26. Pressure increases when:
Standard pressure
= atm = torr = mmHg = KPa
Pressure increases when:
the temperature (speed of molecules) increases.
the density (number of molecules) increases.
The energy of molecules only depends on temperature
therefore, heavier molecules move slower.
Diffusion
is the slow spreading of one type of molecules within another type.

27. Gases consist of widely separated atoms or molecules in constant, random motion
Mostly empty space
No interaction between gas atoms or molecules except in collisions
Straight trajectories until collision occurs

28. Gases consist of widely separated atoms or molecules in constant, random motion
Mostly empty space
No interaction between gas atoms or molecules except in collisions
Straight trajectories until collision occurs
Because gases can expand and contract they behave differently from solid and liquids.

29. Gas pressure is increased by
more frequent and/or harder collisions

30. You can affect the gas pressure by changing:
1. the density
More molecules means more impacts and a higher pressure.

31. You can affect the gas pressure by changing:
1. the density
More molecules means more impacts and a higher pressure.
2. the volume of the container
With less space to move around, there are more collisions and a higher pressure.

32. How KMT Explains the Gas Laws
You can affect the gas pressure by changing:
1. the density
More molecules means more impacts and a higher pressure.
2. the volume of the container
With less space to move around, there are more collisions and a higher pressure.
3. the temperature
With more kinetic energy, the molecules move faster. The collisions are harder and more frequent.

33. Atmospheric pressure
The air pressure at the top of Mt. Everest is only one third the air pressure at sea level (0.33 atm compared to 1.00 atm)

34. Earth is covered with a thin layer of air
Pressure
Earth is covered with a thin layer of air
Image credit: NASA (photo from Apollo 7 spacecraft)

35. Atmospheric pressure A barometer
Empty space
Gravity pulls mercury down the tube
Air pressure pushes mercury up the tube
A barometer
As air pressure changes, the height of the mercury column changes.
barometer: an instrument that measures atmospheric pressure.

36. Variations in atmospheric pressure affect the weather