Chapter 14 Introduction to the Properties of Gases
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
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
Gases take the shape of their container Why don’t the balloons on the right keep expanding until they pop?
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?
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
Torricelli • Discovered air pressure & invented mercury barometer in 1643 • 1st person to propose correct explanation for wind
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
How does downward pressure of Hg in column compare to pressure of atmosphere?
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
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)?
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?
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
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
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**
Pressure Depends on • concentration or # gas molecules per unit volume and 2) temperature
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
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
Pressure – Microscopic View • Gas molecules hit walls of their container • Pressure depends on: • # impacts per unit time • Force each impact
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)
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)
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 1200 meters/sec. How fast is this in miles per hour? Show all work! (Use the factor-label method)