Unit 5: Gases
Physical
Properties
Standards
 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 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 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 Gases
 Gases
expand to fill any
container
• Random constant motion, no
attraction
 very
low densities
C. Characteristics of Gases
 can
be compressed given a change
in Temp/Pressure
State Changes
Diffusion
 The
movement of one material
through another.
 The rate depends on the mass of
the particles
 Lighter = rapid diffusion
Effusion
 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
Effusion
 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 Gases
 Gases
can be described by their:
• Temperature
•K
• Pressure
• atm
• Volume
•L
• Number of molecules/moles • #
E. Temperature
 Always
use absolute temperature
(Kelvin) when working with gases!
ºF
-459
ºC
-273
K
0
C  F  32
5
9
32
212
0
100
273
373
K = ºC + 273
F. Pressure
force
pressure 
area
Which shoes create the most pressure?
F. Pressure
 Barometer
• measures atmospheric pressure
 exact
height of
the Hg depends
on atmospheric
pressure
 usually measured
in mm Hg
F. 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. Pressure
 KEY
EQUIVALENT UNITS
101.325 kPa (kilopascal)
1 atm
760 mm Hg
760 torr
14.7 psi
N
Pa  2
m
G. STP
STP
Standard Temperature & Pressure
0°C
273 K
-OR-
1 atm
101.325 kPa
H. 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 760 mm Hg = 631
1 atm
mm Hg
(b) 0.830 atm 101.325 kPa= 84.1
1 atm
kPa
H. Pressure Problem 2
 Convert
a pressure of 1.75 atm to
kPa and mm Hg.
(a) 1.75 atm
101.325 kPa
= 177
1 atm
kPa
(b) 1.75 atm
760 mm Hg
1 atm
= 1330
mm Hg
H. Pressure Problem 3
 Convert
a pressure of 570. torr to
atmospheres and kPa.
(a) 570 torr
1 atm
760 torr
(b) 570 torr
= .750
atm
101.325 kPa
= 76.0
760 torr
kPa