1. Gas Laws Unit (part 1): Chapter 10
Gas Behavior Basics
Kinetic-Molecular Theory
Temperature and Pressure relationships
Gas Laws
Boyle’s Law,
Charle’s Law ,
Gay – Lussac’s Law
Combined Gas Law

2. Learning Objectives…
To describe the characteristics and behaviors that all gases have in common
To use Kinetic Molecular Theory (KMT) to describe how the behavior of gas particles explains the physical properties of gases

3. Gas Basics… Gases are all around us but we can’t always see them
A gas is usually not a single element or compound but a mixture of them

4. Gas Basics… Air is a mixture of gases that acts like a single gas
78% Nitrogen
21% Oxygen
1% Other gases

5. Gas Basics… Most gases are made of molecules
Can be diatomic or polyatomic MOLECULES
Examples: N2, O2, CO2, CH4, etc.
Some gases exist as single atoms
He, Ne, Ar, Kr, Xe, Rn
We use the term “gas particles” to refer to an individual atom OR a molecule of gas.

6. How Do Gases Behave?
Regardless of their chemical identity, gases have very similar behavior
Remember: 1 mole of any
Gas at 0o C & 1 atm occupies
A volume of 22.4 L
STP = 0o C & 1 atm.
Kinetic Molecular Theory (KMT)explains the behavior of gases

7. Temperature time…
Get up out of your seats and find a partner from some other area of the room and discuss the following question…
What are the temperatures at which water freezes and boils in Celsius, Fahrenheit, AND Kelvin temperature scales?
The next slide will help your discussion and or calculations…

8. Temperature conversions…
Kelvin temperature from Celsius:
0° C = kelvin temp
Temperatures in Celsius and Fahrenheit
[(° F) – 32] x (5/9) = ° C
(oC x 9/5) + 32 = o F
Make sure you write down the chart on the whiteboard in front of the classroom…

9. Kinetic-Molecular Theory…
1. Gases consist of small particles that have mass
Evidence of mass
a flat b-ball weighs less than one with air in it

10. Kinetic-Molecular Theory…
2. The particles in a gas are separated from each other by large distances
The volume of gas particles themselves is considered to be zero b/c they are so small compared to the volume of their containers

11. Kinetic-Molecular Theory…
Evidence of low density
Gases are easy to compress. Moderate squeezing (pressure) on a gas will decrease its volume.
Compressing a gas changes
Its density…

12. Kinetic-Molecular Theory…
3. The particles of a gas are in constant, rapid, random motion
They travel in straight lines, changing direction only when they collide with each other or the edges/walls of the container
KE (Kinetic Energy
depends only on
temp
Hotter = faster

13. Kinetic-Molecular Theory…
Evidence of constant random motion
Gases fill their containers completely
when a balloon fills with air, the air is distributed evenly throughout the balloon
Nowhere around you is there an absence of air

14. Kinetic Molecular Theory…
Gases “diffuse” and “effuse”
Diffusion = when gas molecules move through each other from areas of higher to lower concentration
This is why you can smell food cooking in another area of a house
Effusion = when gas molecules move through a hole so tiny that they have to pass through one particle at a time
This is why balloons deflate slowly even when tied off
The lighter the gas,(or smaller the atom)
the faster it will diffuse/effuse (Graham’s Law)
Nowhere around you is there an absence of air
The movement of one substance through another is called diffusion

15. Kinetic-Molecular Theory …
Which picture shows diffusion? Effusion?
Nowhere around you is there an absence of air
The movement of one substance through another is called diffusion
(b)
(a)

16. Kinetic Molecular Theory…
Do these show diffusion? Effusion? Both?
(a)
(b)

17. Kinetic-Molecular Theory …
A balloon filled with which gas will deflate the quickest and why?
O2: 32 g/mol
N2: 28 g/mol
H2: 2 g/mol
He: 4 g/mol

18. Kinetic-Molecular Theory…
4. Gases exert pressure because they constantly collide with each other and with the walls of the container in which they are held.
Gas particles undergo thousands of collisions with each other and with the walls of their container. When they collide with the container, the impact exerts a force on the inside wall. The force exerted over that area of wall space = pressure (pressure = force per unit of area)

19. Pressure…
Get up out of your seats and find a partner from some other area of the room and discuss any units of pressure (air pressure) that you may be aware of or familiar with…

20. Conversions for Pressure Units…
One (1) atmosphere of pressure equals…
1 atmosphere = kPa (kilopascals)
1 atm. = 760 mm Hg
1 atm. = 14.7 lbs./in2 (or psi)
1 atm. = 760 torr
1 torr = 1 mm Hg
101,325 Pascals = kPa
Put this slide into your notes!...

21. Kinetic-Molecular Theory …
Every time a gas particle collides with a container wall it exerts an outward push or force on the wall
The outward force spread over the area of the container is called “pressure”
e.g., the pressure a gas exerts on the inner walls of a balloon gives the balloon its shape
Nowhere around you is there an absence of air
The movement of one substance through another is called diffusion

22. Kinetic-Molecular Theory…
Evidence of pressure
a. The pressure a gas exerts on the inner walls of a balloon gives the balloon its shape.
b. Pressure increases when more air is added to a basketball
More gas particles  more collisions with the walls  greater force  increased pressure

23. Kinetic-Molecular Theory…
Assuming a constant volume and temp., the total pressure of a mixture of gases is the sum of the pressures of the component gases.
Dalton’s Law of Partial Pressures
Dalton observed that the particles of different gases in a mixture exert the same force on the walls of the container as if they were alone at the same temperature

24. Kinetic-Molecular Theory
The pressure of a gas depends on its temp
Higher temp = higher pressure (& vice-versa)
Higher temp = faster moving particles = collisions are more frequent and at higher speeds = more forceful = increased pressure.
Each of these balloons has the same amount of gas inside!
Why would you need to check your tire pressure in winter?
Nowhere around you is there an absence of air
The movement of one substance through another is called diffusion
Low Temp
Med Temp
High Temp

25. Kinetic-Molecular Theory…
5. When gas particles collide, they do not sloooow dooooown (slow down)
Collisions are “elastic.” In a perfectly elastic collision, no kinetic energy is lost

26. Kinetic-Molecular Theory…
6. Gas particles exert no force on one another.
Gas particles don’t slow down and condense into a liquid because the attractive forces between them are very weak

27. Real vs. Ideal Gases
Most of the time, gases behave exactly as the KMT predicts they will
When they do, we call them “Ideal Gases”
However, KMT makes some faulty assumptions…
1) gases have no attraction for each other (no Intra-molecular forces)
2) gases have no volume
For explanations of deviations from ideal behavior are on p. 445 in Prentice Hall

28. Real vs. Ideal Gases
So under certain conditions, real gases do not behave like KMT predicts they will
At very low temps, gas particles slow down enough that the IMFs they supposedly don’t have between them become significant
At very high pressures, the volume they supposedly don’t have is significant and compression becomes very difficult

29. Real vs Ideal Gases
Even though ideal gases do not really exist… “real gases” behave like ideal gases under most conditions so we can still use the gas laws to describe them and their behaviors.
For explanations of deviations from ideal behavior are on p. 445 in Prentice Hall