Introduction to the Properties of Gases

1. Chapter 14 Introduction to the Properties of Gases

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 move through each other easily (perfume, skunks!) • “Diffusion” • Gases exert pressure

4. 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

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, we 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? A water barometer has to be 13.6 times taller than a Hg barometer (DHg = 13.6 g/ml) because …

11. 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)?

12. 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?

13. Units of pressure = 1 atm • 14.7 lb/in2U.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

14. 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

15. 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**

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

17. 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

18. 12 in Molecular Speeds at 298 K • H2 1.93 X 105 cm/sec • He 1.36 X 105 cm/sec • O2 4.82 X 104 cm/sec * • Ar 4.31 X 104 cm/sec • Xe 2.38 X 104 cm/sec 48200 cm X 1 in X 1 ft X 1 mile X 3600 sec = sec 2.54 cm 5280 ft 1 hour 1080 miles per hour

20. Molecular Speed vs. Temperature

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

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

23. 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)

24. Exit Ticket • Name 5 common properties of all gases • Draw particle diagram of gas phase • Use your particle diagram to explain at least 2 properties of gases • Bonus: At 0oC, a He atom is moving at 1200 meters/sec. How fast is this in miles per hour? Show all work! (Use the factor-label method)