Chapter 5
Gases
Chapter Goals
Understand the basis of the gas laws and
know how to use each (Boyle‟s law, Charles‟
law, Avogadro‟s hypothesis, Dalton‟s law).
Use the (model) ideal gas law.
Apply the gas laws to stoichiometric
calculations.
Understand kineticmolecular theory of
gases, especially the distribution of
molecular speeds (energies).
Recognize why real gases do not behave
like ideal gases.
Solids, Liquids, and Gases
Density <
solids or liquids.
Liquids & gases are fluids; they flow easily
Density
(g/mL)
Solid
Liquid
Gas
H
2
O
0.917
0.998
0.000588
CCl
4
1.70
1.59
0.00503
Solid to liquid to gas, via heating.
Gases may be liquefied by cooling and compressing
General Properties of Gases
•
There is a lot of “free” space in
a gas.
•
Gases can be expanded
infinitely; compressed.
•
Gases occupy containers
uniformly and completely.
•
Gases diffuse and mix rapidly.
•
Gases exert pressure on their
surroundings
Importance of Gases
Airbags fill with N
2
gas in an accident.
Gas is generated by the decomposition of
sodium azide, NaN
3
.
2 NaN
3
(s)
2 Na(s)
+
3 N
2
(g)
Physical Properties of Gases
Gas properties can be
modeled
mathematically.
Model depends on four quantities
(parameters):
V = volume of the gas (L)
T = temperature (K)
n = amount (moles)
P = pressure (atmospheres)
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(arises from molecular collision)
Pressure is force per unit area.
force
F
Pressure =
────
P = ──
area
A
N
SI unit: pascal,
1 Pa =
───
= 1kg
m
−1
s
−2
m
2
Pressure
Atmospheric pressure (pressure of the
atmosphere) is measured with a
barometer
,
invented by
Torricelli
(1643).
Definitions of standard pressure
76 cm Hg =
= 760 mm Hg = 760
torr
=
= 1 atmosphere = 1 atm
1 atm = 101.3 kPa = 1.013
10
5
Pa
1 bar = 1
10
5
Pa = 0.987 atm
H
2
O density ~ 1 g/mL
Hg density = 13.6 g/mL
IDEAL GAS LAW
Brings together gas properties.
Can be derived from experiment and theory.
P V = n R T
Boyle‟s Law:
Calculations involving PV changes
PV = k
V
1
or
V= k
──
or
V
1/P
P
1/P
P
1
V
1
= k
1
for one sample of a gas.
P
2
V
2
= k
2
for a second sample of a gas.
k
1
= k
2
for
the same sample of a gas (same
number of moles at the same T.)
Thus we can write Boyle‟s Law
mathematically as
P
1
V
1
= P
2
V
2
for constant
n and T
Boyle‟s Law: The VolumePressure Relationship
Example: At 25
o
C a sample of He has a
volume of 4.00
10
2
mL under a
pressure of
7.60
10
2
torr.
What volume would it occupy
under a pressure of 2.00 atm at the same T?
Firstly
, we need to convert to same unit of P:
Then, using Boyle’s law
torr
1520
mL
400
torr
760
V
2
atm
1
torr
760
atm
2.00
= 1520 torr
2
1
1
2
P
V
P
V
2
2
1
1
V
P
V
P
Charles‟ Law:
The VolumeTemperature Relationship;
The Absolute Temperature Scale
0
5
10
15
20
25
30
35
0
50
100
150
200
250
300
350
400
Volume (L)
vs.
Temperature (K)
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 Spring '10
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