1. Introduction: Properties of Gases

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

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

4. Gases take shape of container
Why don’t these balloons
keep expanding until
they pop?

5. What is pressure? Pressure = force per unit area 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

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

8. Mercury Barometer 1

9. To report air pressure: take short-cut & report height of Hg column
at 1 atm column is: in Hg
76 cm Hg
760 mm Hg

10. How does the downward pressure of the Hg in the column compare to the pressure of the atmosphere?
water barometer has to be times taller than Hg barometer (DHg = 13.6 g/ml) because …
DH2O = 1.0 g/ml

11. 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?
It’s less
What about Death Valley (86 m below sea level)?
It’s greater

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

13. Units of pressure = 1 atm 14.7 lb/in2 U.S. pressure gauges
29.9 in Hg U.S. weather service
1.013 bars Physics & Astronomy
CHEMISTRY:
1 atm = 760 torr = 760 mm Hg = kPa SI Units

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

15. Light molecules move faster & 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**

16. Pressure Depends on concentration and 2) temperature
# of gas molecules per unit volume (moles!!)
and
2) temperature

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

18. 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
1080 miles per hour

19. Pressure – Microscopic View
Gas molecules hit walls of container
Pressure depends on
Number of impacts per unit time
Force of each impact

20. Pressure – Macroscopic View
Pressure depends on:
how many gas molecules per volume and
temperature
same amount of gas exerts different pressure at different temperatures
Tires
Bike
Car
etc

21. Describing Gas Phase System
Need 4 variables to describe gas
phase system from macroscopic or
lab view
1. Pressure (P)
2. Volume (V)
3. Temperature (T)
4. Amount of gas (# moles)

22. Exit Ticket List 5 common properties of all gases?
Draw a particle diagram substance in gas phase
Use picture from #2 to explain at least 2 properties of gases
Bonus:
At 0oC, a He atom is moving at 120 meters/sec.
How fast is this in miles per hour? Show all
work!