Transcript Regents Unit 5: Intro to Gases

Properties of Gases
Chapter 14
What do you know about gases?
• Particle Diagrams
• Ar(g), H2(g), CO2(g)
• Gases take the shape and
volume of their container
• Gases flow
• Gases have low densities
Properties of Gases
1. Gases have mass.
2. Gases take the shape and volume of
their container (Mickey Mouse
balloon).
3. Gases are compressible.
4. Gases move through each other
easily (perfume, skunks!).
Gas mixing
“Diffusion”
5. Gases exert pressure.
Gases take the shape of their container.
Why don’t the balloons on the right expand
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?
• The pressure exerted on us by
the weight of the gases above
our heads.
• At sea level, air pressure = 1
atmosphere.
• Tiger Graphic
Torricelli
• Discovered air pressure and
invented the mercury barometer in
1643.
• 1st person to propose correct
explanation for wind.
V2, CCA: Mercury Barometer 1 & Mercury Barometer 2
To report air
pressure, we take a
short-cut & report
the height of the
Hg column.
At 1 atm, the
column is 29.92 in
or 76 cm or 760
mm.
1. How does the downward
pressure of the Hg in the
column compare to the pressure
of the atmosphere?
2. A water barometer has to be
13.6 times taller than a Hg
barometer (DHg = 13.6 g/ml)
because …
Large Barometers
How does the air pressure at
the top of Mt. Whitney (14,494
ft) compare to the air pressure
at John Jay?
What about Death Valley (86 m
below sea level)?
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?
Units of pressure = 1 atm
• 14.7 lb/in2
• 29.9 In. Hg
• 101.3 kPa
U.S. pressure gauges
U.S. weather*
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 the
walls of the container.
Microscopic View
Anything that
increases the number
of impacts per second
or the force of each
impact increases the
pressure.
Light molecules move
faster and hit the walls
more often.
Heavy molecules hit the
walls with greater
force.
These 2 effects exactly
balance out.
**Gas pressure doesn’t depend on the
identity of the gas.**
Pressure Depends on
1) the concentration or # of gas
molecules per unit volume
and
2) the temperature.
How fast do the molecules in the
air move?
• Depends on the mass.
• Light molecules are faster than
heavy molecules at the same
temperature.
• Temperature = measure of the ave.
translational K.E. of the particles of
a system.
Molecular Speeds at 298 K
•
•
•
•
•
H2
He
O2
Ar
Xe
1.93 X 105 cm/sec
1.36 X 105 cm/sec
4.82 X 104 cm/sec
4.31 X 104 cm/sec
2.38 X 104 cm/sec
48200 cm X 1 in X 1 ft X 1 mile X 3600 sec =
sec 2.54 cm 12 in
5280 ft
1 hour
1080 miles per hour
Average Speeds
• Boltzmann Applet
• Maxwell-Boltzmann Velocity Distribution
Molecular Speed vs. Temperature
Pressure – Microscopic View
• Gas molecules hit the walls of their
container.
• Pressure depends on
– Number of impacts per unit time
– Force of each impact
Pressure – Macroscopic View
• Pressure depends on how many gas
molecules per unit volume and on
the temperature.
• The same amount of gas exerts
different pressure at different
temperatures (tires).
Describing a Gas Phase System
1.
2.
3.
4.
Need 4 variables to completely
describe a gas-phase system from
the macroscopic or lab view.
Pressure
Volume
Temperature
Amount of gas in moles.
Exit Ticket
• What are 5 common properties of all
gases?
• Draw a particle diagram of the gas
phase.
• Use this picture to explain at least 2 of
the 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.)