Physical Properties Unit 5: Gases Unit 5: Gases. StandardsStandards b 4a. Students know the random motion of molecules and their collisions with a surface.
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StandardsStandards
4a. Students know the random motion of molecules and their collisions with a surface create the observable pressure on the surface
4b. Students know the random motion of molecules explains the diffusion of gases
A. Kinetic Molecular TheoryA. Kinetic Molecular Theory
Used to predict and explain the behavior of a theoretical gas or ‘ideal gas’
Particles in an ideal gas…• have no volume or elastic
collisions• in constant, rapid, random,
straight-line motion• don’t attract or repel each other
‘Ideal gases’- are elastic (do not lose energy upon collision)
Cannot be compressed given a change in temperature
Can be measured using the eq. KE= 1/2mv2
B. Real GasesB. Real Gases
Particles in a REAL gas…• have their own volume• attract each other
Gas behavior is most ideal…• at low pressures• at high temperatures• in nonpolar atoms/molecules
C. Characteristics of GasesC. Characteristics of Gases
Gases expand to fill any container• Random constant motion, no
attractionvery low densities
C. Characteristics of GasesC. Characteristics of Gases
can be compressed given a change in Temp/Pressure
State Changes
DiffusionDiffusion
The movement of one material through another.
The rate depends on the mass of the particles
Lighter = rapid diffusion
EffusionEffusion
When a gas escapes through a tiny opening
Rate of effusion can be calculated according to Graham’s law of effusion:
Rate of effusion = 1/SQRT MM
EffusionEffusion
Using Graham’s Law, you can also set up a proportion to compare the diffusion rates for two gases
** see eq on board.
Ammonia has a molar mass of 17.0 g/mol; hydrogen chloride has a molar mass of 36.5 g/mol. What is the ratio of their diffusion?
D. Describing GasesD. Describing Gases
Gases can be described by their:
• Temperature
• Pressure
• Volume
• Number of molecules/moles
• K
• atm
• L
• #
E. TemperatureE. Temperature
ºF
ºC
K
-459 32 212
-273 0 100
0 273 373
32FC 95 K = ºC + 273
Always use absolute temperature (Kelvin) when working with gases!
F. PressureF. Pressure
Barometer• measures atmospheric pressure
exact height of the Hg depends on atmospheric pressure
usually measured in mm Hg
F. PressureF. Pressure
Manometer• measures contained gas pressure
Difference in height in two arms of U-tube is measure of pressure of gas sample
measured in various different units
F. PressureF. Pressure
2m
NPa
KEY EQUIVALENT UNITS
101.325 kPa (kilopascal)
1 atm
760 mm Hg
760 torr
14.7 psi
H. Pressure Problem 1H. Pressure Problem 1
The average pressure in Denver, Colorado, is 0.830 atm. Express this in (a) mm Hg and (b) kPa.
(a) 0.830 atm
1 atm
760 mm Hg= 631
mm Hg
(b) 0.830 atm
1 atm
101.325 kPa= 84.1
kPa
H. Pressure Problem 2H. Pressure Problem 2
Convert a pressure of 1.75 atm to kPa and mm Hg.
(a) 1.75 atm
1 atm
101.325 kPa= 177
kPa
(b) 1.75 atm
1 atm
760 mm Hg= 1330
mm Hg
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