1. Welcome (Oct 25)
Lab 7 out in lab books! Ready to go RIGHT AWAY!

2. Mini-Lab 7a: Real vs Ideal Gases
QUESTIONS (in lab book): What is the shape of the graph? Why is the shape of the graph not as you expected? THINK CAREFULLY!

3. Ideal Gases vs Real Gases (5.8)
Ideal gases are gases that obey the Kinetic Molecular Theory perfectly.
The gas laws apply to ideal gases, but in reality there is no perfectly ideal gas.
Under normal conditions of temperature and pressure many real gases are close to ideal gases.
This works for us in lab!
Under more extreme conditions (P and T) gases show deviations from ideal behavior.
This is more important in industry and physics

4. Kinetic Molecular Theory (5.6)
Matter consists of particles that are in continuous, random, rapid motion
Collisions between particles are elastic
Gases have essentially no volume and take the shape of their container
The average kinetic energy of the particles is proportional to their Kelvin temperature

5. Kinetic Energy KE=3/2 RT=1/2 mv2 Consider two equal flasks:
Which flask has molecules moving more quickly?
Which flask has the larger pressure? He
4.0 g/mol N2
28.0 g/mol
Same pressure, but speeds of molecules are different because the heavier molecules hit harder, but less often!

6. Maxwell-Boltzman Distribution
Not all particles have the same energy
The average kinetic energy is related to the temperature
An increase in temperature spreads out the distribution and the mean speed is shifted upward
Higher temperature = more randomness in speeds because of collisions with each other and wall

7. Root Mean Squared Velocity of a Gas
What governs how fast a gas particle is moving?
urms =
3RT M 

8. BUT WHY?! Velocity of a Gas The distribution of speeds
of three different gases
at the same temperature
At higher temp, on average the molecules are moving faster (RMS)
At same temp, the lightest molecule will be most spread out (highest RMS velocity)
The distribution of speeds
for nitrogen gas molecules
at three different temperatures

9. Diffusion – DEMO from last year!
Gas diffusion is the gradual mixing of molecules of one gas with molecules of another by virtue of their kinetic properties.
NH4Cl
Which will diffuse faster into the other?
HCl or NH3?
NH3
17.0 g/mol
HCl
36.5 g/mol

10. DIFFUSION VS. EFFUSION (5.6-5.7)
Diffusion is the gradual mixing of molecules of different gases.
Effusion is the movement of molecules through a small hole into an empty container.

11. Graham’s Law – not on AP Exam
Graham’s law governs effusion and diffusion of gas molecules.
The rate of effusion is inversely proportional to its molar mass.
Thomas Graham, Professor in Glasgow and London.

12. Effusion – why do we care?

13. Problem: Hydrogen Storage!
In fuel cell technology, the hydrogen effuses out of the tanks and also is super dangerous! What could we do stop this?

14. Multiple Choice #1- 3
Questions refer to the following gases at 0oC and 1 atm:
(A) Ne (B) Xe (C) O2 (D) CO (E) NO
1. Has an average atomic or molecular speed closest to that of N2 molecules at 0oC and 1atm.
2. Has the greatest density
3. Has the greatest rate of effusion through a pinhole
D, B, A

15. Another application!
We smell something based on how quickly it diffuses through the air to our noses.
Consider the following AWFUL smells – which one is going to get to your nose the most quickly? WHY?
C4H9SeH (skunk)
CH4 (farts)
C4H8O2 (vomit)
H2S (sewer smell)
It’s all about molar mass = farts (CH4) are the lightest so they travel at the fastest rate!

16. In an Ideal Gas …
The volume occupied by the particles is essentially zero when compared to the volume of the container
The particles are neither attracted to each other nor repelled
The average kinetic energy of the particles is proportional to their Kelvin temperature

17. But, in a Real Gas… These deviations occur because
For ideal gases the product of pressure and volume is constant. Real gases deviate somewhat as shown by the graph pressure vs. the ratio of observed volume to ideal volume below.
These deviations occur because
Real gases do not actually have zero volume
Polar gas particles do attract if compressed

18. At high pressures… Volume of particles is significant
So, volume is actually bigger than anticipated at high pressure for all molecules because the gas doesn’t have as much room to move around!
REAL VOLUME IS BIGGER

19. With polar molecules…
At low(er) pressures (low Temperatures), polar molecules (like CO2 in this example) are attracted to each other, so exert less pressure on the container that an ideal gas would.
REAL PRESSURE LESS THAN EXPECTED

20. Great Video (watch at home)!
From Chem Crash Course

21. van der Waals Equation (1910)
The van der Waals equation shown below includes corrections to allow the equation to work with real gases:
(Pobs + n2a/V2)(V - nb) = nRT
where a => attractive forces between molecules
b => residual volume of molecules
The van der Waals constants for some elements are shown below
Substance
a (dm6atm mol-2)
b (dm3 mol-1) He
0.0341
CH4
2.25
0.0428
H2O
5.46
0.0305
CO2
3.59
0.0437
See pages for good explanation. To correct the real observations back to ideal, we add some pressure (because less is observed with polar molecules), and subtract some volume so that we’re only considering the actual volume the gas can move around in due to particles taking up important space.

22. Multiple Choice #4
When do real gases deviate from the ideal gas equation most severely?
Why do real gases act like ideal gases at high temperatures?
A) Polar, High P
B) Polar, Low P and T
C) Non-polar, High P
D) Non-polar, Low P
E) All molecules, High P
B – because at Low P, there is Low T, and the polar molecules are attracted, and E; because high P means low V, so molecules have significant volume now
Most important is probably B (polarity and attractive forces)

23. Important Equations KE avg = urms = This is PER MOLE R =
This equation tells us that heavier gas particles move more slowly for a given temperature
Mass in kg!

24. Multiple Choice #5
A 2.00-liter sample of nitrogen gas at 27oC and 600 mm Hg is heated until it occupies a volume of 5.00 liters. If the pressure remains unchanged, the final temperature of the gas is:
(A) 68 oC
(B) 120 oC
(C) 477 oC
(D) 677 oC
(E) 950 oC
Answer: C

25. Vapour Pressure
Rank the following from lowest vapour pressure to higher vapour pressure:

26. Answer It’s all about IMFs and mass of the molecule!
It’s all about IMFs and mass of the molecule!

27. Wrapping up! Quizzes Back Work time on:
WS 6 – good practice problems
Practice Test Packet
Quiz corrections (due at test FRIDAY)
Today in lab: Lab 7 – APPLY what you’ve learned and be creative!
And some demos if we didn’t have time in class!
Questions on Practice Test

28. Homework Day 6 Prepare for the test on FRIDAY!
Quiz corrections due at the test, too!
Finish up Lab 7 in lab books – due Tuesday
Unit 3 starting Tuesday!
You DO have homework over the weekend to prepare for Unit 3 since we need to SCOOT (and are missing a lot of days in November)

29. 4 NH3(g) + 5 O2(g)  4 NO(g) + 6 H2O(g)
Sample Problem 1
1 mole of oxygen gas and 2 moles of ammonia are placed in a container and allowed to react at 850oC according to the equation:
4 NH3(g) + 5 O2(g)  4 NO(g) + 6 H2O(g)
Using Graham's Law, what is the ratio of the effusion rates of NH3(g) to O2(g)?

30. Sample Problem 2
What is the rate of effusion for H2 if cm3 of CO2 takes 4.55 sec to effuse out of a container?
Rate for CO2 = cm3/4.55 s = cm3/s

31. Hot Air Balloons! Heat air to lift – why? What PVnRT variables change?
To lift about 1000 pounds, about 65,000 ft3 are needed!
Heating air 40 C makes it ¼ lighter

32. Webassign with the KE qs?

33. DEMO On your own piece of paper:
Observe what happens (draw pictures, too!)
Give me a good inference/explanation of what happened BASED ON YOUR OBSERVATIONS
Can Crush Demo, then canon

34. EXPLAIN THE CAN!
How can we explain the can crush AND the water inside?

35. Cartesian Diver! Can you explain what is going on?
Now, what about SCUBA diving?