1. Introduction to the Properties of Gases
Chapter 14

2. What do you know about gases?
Particle Diagrams
Ar(g), H2(g), CO2(g)
Gases take shape & volume of their container
Gases flow
Gases have low densities

3. Properties of Gases Gases have mass
Gases take shape & volume of container
Gases are compressible
Gases easily move through each other (perfume, skunks!)
“Diffusion”
Gases exert pressure

4. until they pop? Gases take the shape of their container
Why don’t the balloons on
the right keep expanding
until they pop?

5. What is pressure? Pressure = force per unit area P = Force Area
Force = mass x acceleration
Would you rather have your foot stepped on by someone wearing sneakers or stilleto heels?

6. What is air pressure?
pressure exerted on us by weight of gases above our heads (& all around our body)
At sea level:
air pressure = 1 atmosphere
this is standard pressure

7. Torricelli
Discovered air pressure & invented mercury barometer in 1643
1st person to propose correct explanation for wind

8. Mercury Barometer 1

9. reporting air pressure - take short-cut & report height of Hg column
At 1 atm: column height is
29.92 in Hg or
76 cm Hg or
760 mm Hg

10. How does downward pressure of Hg in column compare to pressure of atmosphere?

11. Can we use H2O in a barometer?
A water barometer has to be 13.6 times taller than a Hg barometer (DHg = 13.6 g/ml) because … Dwater = 1.0 g/ml

12. How does air pressure at top of Mt
How does air pressure at top of Mt. Whitney (14,494 ft) compare to air pressure at John Jay? What about Death Valley (86 m below sea level)?

13. When you drink through straw, you reduce pressure in straw so…
Why does liquid in cup go up the straw?
Could you drink a soda this way on the moon? Why or why not?

14. Units of pressure = 1 atm 14.7 lb/in2 U.S. pressure gauges
29.9 in Hg U.S. weather*
101.3 kPa SI Units (Regents)
(kPa = kilopascal)
1.013 Bars Physics & Astronomy
760 Torr or 760 mm Hg CHEMISTRY

15. What causes the pressure of a gas in a closed container?
Impacts of gas molecules with walls of container
Anything that increases # of impacts per second or force of each impact increases pressure
Microscopic View

16. Light molecules move faster and hit walls more often
Heavy molecules hit walls with greater force
These 2 effects essentially balance out
**Gas pressure doesn’t depend on the identity of the gas**

17. Pressure Depends on concentration or # gas molecules per unit volume
and
2) temperature

18. How fast do the molecules in the air move?
Depends on mass
Light molecules faster than heavy molecules at same temperature
Temperature = measure of average KE of particles in system

19. Molecular Speeds at 298 K H2 1.93 X 105 cm/sec He 1.36 X 105 cm/sec
O X 104 cm/sec *
Ar X 104 cm/sec
Xe X 104 cm/sec
48200 cm x 1 in x 1 ft x 1 mile x sec =
sec
2.54 cm
12 in
5280 ft
1 hour
1080 miles per hour

20. Speeds of various gasses according to mass

21. Molecular Speed vs. Temperature

22. Pressure – Microscopic View
Gas molecules hit walls of their container
Pressure depends on:
# impacts per unit time
Force each impact

23. Pressure – Macroscopic View
Pressure depends on:
how many gas molecules per unit volume &
temperature
same amount of gas exerts different pressure at different temperatures (tires)

24. Describing Gas Phase System
Need 4 variables to completely describe gas-phase system from macroscopic or lab view
1. Pressure
2. Volume
3. Temperature
4. Amount gas (moles)

25. Exit Ticket Name 5 common properties of all gases
Draw particle diagram of oxygen in gas phase
Use your particle diagram to explain at least 2 properties of gases
Bonus: At 0oC, a He atom is moving at meters/sec. How fast is this in miles per hour? Show all work! (Use the factor-label method)