1. What affects their speed?
Speed of Gases
How fast do they go?
What affects their speed?

2. Compare the speed of helium compared to carbon dioxide.
He is faster
CO2 is faster
He and CO2 travel at the same speed 2

3. Compare the speed of helium compared to carbon dioxide.
He is faster
CO2 is faster
Graham’s Law of effusion says that the rates of effusion for two gases is inversely proportional to their molar masses 3

4. Effusion and Diffusion
The escape of gas molecules through a tiny hole into an evacuated space.
Diffusion
The spread of one substance throughout a space or throughout a second substance.

5. Since T α KE = ½mv2
If gases are at the same temp and therefore have the same average KE, the equation above indicates that lighter gases (smaller molar mass) must have a larger speed (v).
The bottom line?
Low MM gasses go fast
High MM gases go slower

6. This is a graph of different gases all at 25ºC
This is a graph of different gases all at 25ºC. The blue line is water vapor. Which gas can be He? (Select all that apply.) #1 #2 #3 #4

7. This is a graph of different gases all at 25ºC
This is a graph of different gases all at 25ºC. The blue line is water vapor. Which gas is He?
Without more information either 3 or 4 could be He.
Light gases have higher velocity since KE = ½mv2.
Thus at the same temp lower mass means higher speed. #1 #2 #3 #4

8. Select the gas that has an average speed closest to Ne
Select the gas that has an average speed closest to Ne. All gases at same temp & pressure. H2 He
H2O
CO2 O2
They all have the same speed, since the conditions are the same.

9. Select the gas that has an average speed closest to Ne at STP.
H2O
CO2 O2
They all have the same speed, since the conditions are the same.
the mass of the gas will affect the speed.
thus the gas with the most similar molar mass will be the gas with the closest speed.

10. Select the gas that has the highest average kinetic energy
Select the gas that has the highest average kinetic energy. All gases at STP. H2 He
H2O
CO2 O2
All of these gases have the same KE
None of the gases have kinetic energy since they are all at 0ºC.

11. Select the gas that has the highest kinetic energy at STP.
H2O
CO2 O2
All of the gases have the same kinetic energy since they are at the same temperature.
None of the gases have kinetic energy since they are all at 0ºC.
T α KE

12. What’s so Ideal about the Ideal Gas Law?
PV = nRT

13. The “Ideal” Gas Law
Kinetic Molecular Theory: the temperature of a gas is proportional to its kinetic energy.
KMT is dependent upon a gas behaving as if the gas particles have NO IMFs.
KMT also considers gases to be “point masses.” That is to say, the size of each atom is insignificant.
If the statements above are true, the gases will behave “ideally.”
However, we know the 2nd & 3rd statements aren’t always so perfectly true.

14. Graphing PV/RT at various pressures
PV=nRT, thus for 1 mole of any gas,
PV/RT = 1
The graph of PV/RT vs P for one mole of an ideal gas would be the dashed line.
Ideal Gas
at ~300K

15. Real Gases Deviate from the Ideal
For one mole of the gases shown, the graph of PV/nT vs P actually looks like this.
at ~300K

16. The not-so-ideal “Ideal” Gas Law
If n is larger than 1, then at the pressure measured, V must be bigger than it “ideally” should be.
If n is smaller than 1, at the measured pressure, V must be smaller than it “ideally” should be.
at ~300K

17. What would make V smaller than it should ideally be at a given pressure?
If ratio is below the “1” line, IMFs are having an effect.
The V is smaller because the IMFs are playing a role.
The darker molecule is “feeling the love” and thus not pushing out as hard as it “ideally should” and causing less volume (if in a flexible container).

18. What would make V larger than it should ideally be at a given pressure?
Usually molecules are far apart and their personal particle size is irrelevant, however when “cramped” together, the particle’s personal size begins to matter.
At higher pressures, the volume of each individual molecule begins to matter and actually takes up some space, causing the volume to record as larger.

19. Put the gases in order of increasing intermolecular forces.
N2 < H2 < CH4 < CO2
CO2 < H2 < N2 < CH4
CO2 < CH4 < H2 < N2
H2 < N2 < CH4 < CO2
H2 < CH4 < N2 < CO2
CO2 < CH4 < N2 < H2
at ~300K

20. Put the gases in order of increasing intermolecular forces.
at ~300K
N2 < H2 < CH4 < CO2
CO2 < H2 < N2 < CH4
CO2 < CH4 < H2 < N2
H2 < N2 < CH4 < CO2
H2 < CH4 < N2 < CO2
CO2 < CH4 < N2 < H2
The molecule with the smallest IMFs deviates least below the line.
The deviation below the line increases with increasing IMFs
IMFs increase with increasing molar mass

21. Variations from the Ideal as a function of Temperature
Which line represents the highest temperature?
green
red
blue
Varying Temps
for N2

22. Variations from the Ideal as a function of Temperature
Which line represents the highest temperature?
green
red
blue
Turn to your mate, and tell them why.
Varying Temp
for N2

23. Variations from the Ideal as a function of Temperature
Remember that deviations below the line arise from intermolecular forces.
At higher temperatures, IMFs have less effect, thus the line does not dip below as much or at all.
Varying Temps
for N2