Lecture 31 Gases VI NonIdeal Gases, The Atmosphere

1
Lecture 31 - Gases VI - Non-Ideal Gases, The
Atmosphere
2
Today...

• Non-ideal gases
• Structure of the atmosphere
• The Ozone layer

3
Non-ideal gases

• PV nRT only works at low P, high T

1 atm
³ 0oC
4
Non-ideal gases

• PV nRT assumed
• 1. Molecules have no volume
• (but they do)

5
at low P
at high P
V Vcontainer
V ltlt Vcontainer
6
Non-ideal gases

• PV nRT assumed
• 1. Molecules have no volume
• (but they do)
• 2. Molecules dont interact with one another
• (but they do)

7
raise P or lower T
LIQUID
GAS
same molecules, stronger interactions
no interactions
i.e. strength of interaction depends on distance
between molecules
8
Liquefaction of Gases
Low P
High P
molecules far apart
molecules closer
9
Liquefaction of Gases
Fµ 1/r6
r
e.g. decrease r by half, F increases 64 x
10
Liquefaction of Gases
Competition between kinetic energy of molecules
and attractive forces Higher kinetic energy ?
gas Higher attractive forces ? liquid
11
Liquefaction of Gases
increase P
molecules are closer
attractive forces dominate
gas liquefies
molecules are slower
decrease T
12
Liquefaction of polar gases
strong attractive forces
easier liquefaction
liquefaction at higher temperatures
13
Liquefaction occurs at the boiling temperature

• Gas Tb (oC)
• He -269
• CH4 -164
• H2O 100

strong attractive forces
14
Water is a polar molecule
H-bonds
15
P
V

2
n
P a
V - bn
nRT
V
accounts for molecular volumes
accounts for intermolecular forces
16
The van der Waals Equation
2
nRT
n
-
a
P
(V - nb)
V
17
van der Waals constants a and b

• Gas a (atm L2 mol-2) b (L mol-1)
• He 0.034 0.0237
• H2 0.244 0.0266
• O2 1.36 0.0318
• Cl2 6.49 0.0562

18
Comparison of Ideal and vdW equations

• 8 moles Cl2(g) at 27oC in a 4 L vessel

49.2 atm
19
according to van der Waals
29.5 atm (49.2 atm from ideal gas law)
20
The Atmosphere

• P 1 atm
• (thus PV nRT applies)
• but the chemistry is very complex

21

• The Atmosphere
• Starring
• N2, O2, H2O, CO2, O3
• Co-starring
• OH, NOx
• Villains
• SO2, CFCs, Hydrocarbons, etc
• Produced and Directed by
• The Sun
• Extras about 3,000

22
Atmospheric Problems

• Ozone depletion
• Smog
• Acid rain

23
Pressure and Mean Free Path
160
0
10-8
103
P (mbar) or l (m)
24
Temperature
100
z, km
30
Stratosphere
20
Troposphere
0
20
-120
T, oC
25
light from the Sun
200
depth of penetration of light
z, km
region 1
region 2
region 3
0
200
300
0
100
wavelength, nm
26
Importance of Solar Energy

• 1. MANY atmospheric reactions involve light
  energy
• e.g. O2 hn O O

ultraviolet light (high n, low l)
27

• In region 1 N2O hn N, O, O2, N2

light is consumed in these reactions
l lt 100 nm is required
hc
recall that E hn
l
28

• In region 1 N2O hn N, O, O2, N2
• In region 2 O2 hn O O
• In region 3 O3 hn O2 O

(l lt 100 nm)
(l lt 200 nm)
(l lt 325 nm)
29
Chapman Chemistry of Ozone

• 1. O2 hn O O
• 2. O O2 O3
• 3. O3 hn O2 O
• 4. O O3 2 O2

30
Ozone
60
low O2 reaction 1 doesnt go
40
Altitude, km
low light intensity reaction 1 doesnt go
20
0
1010
1011
1012
O3, molecules mL-1
31
Area Covered by Column
Compressed to STP
3 mm thick slab (300 DU)
32
Chapman chemistry is missing something...

• Something else must be destroying O3
• The villains - CxClyFz (CFCs)

33
(No Transcript)
34
(No Transcript)
35

• In the troposphere
• CFCs hn Cl atoms
• e.g. CClF3 hn CF3 Cl
• Cl atoms DIFFUSE to the stratosphere

36
In the stratosphere,

• Cl O3 O2 ClO (O3 is destroyed)
• ClO O Cl O2 (Cl is back !!!)
• O3 O 2 O2 (net reaction)
• a cyclic O3 destruction process
• Cl is a CATALYST