1. How does a gas differ from a solid and a liquid?
Remember…
How does a gas differ from a solid and a liquid?
A gas has no definite shape and no definite volume
It takes the shape and volume of its container
Its particles are spread far apart

2. Reviewing Temperature
Temperature is a measure of average kinetic energy
When temperature increases, average kinetic energy increases, and particles move faster
When the particles of two substances have the same average kinetic energy, the substances are at the same temperature
Temperature Unit Conversion: K = °C + 273
Standard Temperature: 0°C or 273 K
At Absolute Zero (0 K) there is no molecular motion

3. Reviewing Volume
Volume is a measure of the amount of space that an object occupies
The particles of a gas expand to occupy the entire volume if their container
Volume Unit Conversion: 1 L = 1000mL

4. Pressure
Pressure exerted by a sample of gas is the result of collisions between the particles of the gas and the inner walls of the container
Standard Pressure: 1 atm, kPa, torr, 760 mmHg

5. Avogadro's Principle:
**Equal volumes of gases at the same temperature and pressure contain an equal number of particles.
Remember how the coefficients of a balanced equation represent the volumes of gases that react with each other.
**One mole of any gas at STP occupies a volume of 22.4L

6. Vapor Pressure
The pressure exerted by vapor above the surface of a liquid in a closed container
Stronger IMFs result in lower vapor pressure (and higher BP)
Weaker IMFs result in higher vapor pressures (and lower BP)

7. Essential Questions: How does a real gas differ from an ideal gas?
When do real gases behave most like ideal gases?

8. Ideal Gases Gases that obey the laws of the kinetic molecular theory
(No gases are actually ideal gases, but ideal gas laws help to explain the behavior of real gases)

9. Kinetic Molecular Theory of Gases
Gases consist of individual particles that move in random straight lines
Gas particles have no attraction for one another

10. Kinetic Molecular Theory of Gases
Gas particles are separated by such large distances that the particles themselves have negligible volume
When gas particles collide, the collisions are perfectly elastic (particles may transfer energy, but there is no net loss of energy)

11. Explain the behavior of gases using the Kinetic Molecular Theory (KMT)
Why are gases compressible?
The particles are separated by very large distances
Why do gases expand to fill a container?
Gas particles move in random straight lines and are not attracted to each other
Why do gases diffuse and effuse?

12. Explain the behavior of gases using the Kinetic Molecular Theory (KMT)
Why do gases exert pressure on the walls of their containers?
Gas particles expand in random straight lines until they collide with the walls of their containers exerting a pressure on the surface
More gas particles exert more pressure because there are more collisions against the walls

13. The Behavior of Real Gases Deviates from that of Ideal Gases
Particles of a real gas are attracted to one another (and can condense)
Real gas particles do have volume
When gas particles collide, the collisions are not perfectly elastic (there is some loss of energy)

14. When do gases behave most like ideal gases?:
Real gases behave more like ideal gases under conditions of low pressure and high temperature
(Think about it…gases under these conditions are less likely to condense)
Lighter (smaller) gases behave more like ideal gases than heavier ones

15. Graham’s Law of Diffusion/Effusion:
Lighter gases diffuse/effuse more quickly than heavier gases
To decide which gas diffuses the most quickly, consider the masses from the PT
H2 will diffuse more quickly than CO2 because each H2 molecule has less than and a CO2 molecule

16. How do changes in pressure affect the volume of a gas?
If pressure increases, what happens to the volume of a gas?
The volume decreases
If volume decreases, what happens to the pressure of a gas?
The pressure increases
As pressure increases, volume decreases, and vice versa

17. Boyle’s Law P1V1 = P2V2 PV = k initial final
At constant temperature, the volume times the pressure of a gas is equal to a constant.
PV = k
Pressure and Volume have in inverse relationship
P1V1 = P2V2
initial
final
What would a graph of P vs V look like?

18. Standard Pressure
Check Reference Table A for standard temperature and pressure
For standard pressure: add the values 760 mmHg and 760 torr

19. Essential Question:
How do changes in temperature affect the pressure and volume of a sample of gas?

20. Temperature is a measure of average kinetic energy
What will happen to the speed of gas particles if temperature increases?
Particles move faster
What will happen to the number of collisions against the walls of the container?
Collide against wall more frequently
What will happen to the intensity of collisions against the walls of the container?
Collide against wall with more energy

21. What happens to the pressure exerted by a sample of gas if temperature increases?
Pressure increases
BECAUSE…
The particles collide with the walls more frequently and with more energy

22. What happens to the pressure exerted by a sample of gas if temperature decreases?
Pressure decreases
BECAUSE…
The particles collide with the walls less frequently and with less energy

23. Guy-Lussac’s Law P1 P2 = T1 T2 initial final
At constant volume, the pressure of a fixed amount of gas is equal to the temperature, in degrees Kelvin, times a constant.
P = T k
P and T have a direct relationship P1 P2 = T1 T2
initial
final
What would a graph of P vs T look like?

24. What happens to the volume of a gas if temperature increases and pressure remains the same?
Volume increases
BECAUSE…
The particles collide with the walls more frequently and with more energy

25. What happens to the volume of a gas if temperature decreases and pressure remains the same?
Volume decreases
BECAUSE…
The particles collide with the walls less frequently and with less energy

26. Charles’s Law V1 V2 = T1 T2 initial final
At constant pressure, the volume of a fixed amount of gas is equal to the temperature, in degrees Kelvin, times a constant.
V = T k
V and T have a direct relationship V1 V2 = T1 T2
initial
final
What would a graph of V vs T look like?

27. Combined Gas Law Temperature must be in Kelvin!
Example Problem: A sample of gas at 745mmHg and 25°C occupies a volume of 2.4L. What would the volume be at STP?

28. Density of Gases (at STP)
Molar Mass
Density =
Molar Volume
Molar volume: 22.4 L/mol
Which gas is more dense? O2 or H2
What is the molar mass of a gas that has a density of g/L at STP?

29. Density of Gases Finally calculate density as m/V Challenge Problem:
Calculate the density of 0.625g of CO2 at 26˚C and 1.03 atm.
First determine the molar density of CO2 at STP:
Then determine the volume of 0.625g CO2 at STP:
Then determine the corresponding volume under the given conditions:
Finally calculate density as m/V

30. Dalton’s Law of Partial Pressures
The total pressure of a mixture of gases is equal to the sum of the pressures of each gas
PTotal = P1 + P2 + P3 …

31. Dalton’s Law Problems
In a mixture of dry air, the partial pressures of N2, O2, and CO2 are 593.4mmHg, 159.2mmHg, and 7.4mmHg respectively. What is the total pressure of this mixture?
In a gas mixture of He, Ne, and Kr, the total pressure is 125kPa. The partial pressure of Ne is 73kPa, and the partial pressure of Kr is 21kPa. What is the partial pressure of He?

32. Pressure Fraction
The partial pressure of a gas divided by the total pressure of the mixture
Often written as a percentage
In a mixture of dry air, the partial pressures of N2, O2, and CO2 are 0.75atm, 0.90atm, and 1.35atm respectively. What is the pressure fraction of each gas?

33. Mole Fraction
The number of moles of a gas divided by the total number of moles in the mixture
*Is the same value as Pressure Fraction
A mixture of gases totaling 12 moles contains only H2 and He. If 25% of the gas is H2, how many moles of He are there in the mixture?

34. Collecting a Gas Over Water
Patm= PTotal= Pwater + Pgas
If the atmospheric pressure is 745.8mmHg, what is the partial pressure of the dry gas?
The vapor pressure of water at 25˚C is 23.8mmHg.
Temperature = 25˚C