Gases V

1
Gases - V
2
Gas Particles Move FAST
1/2
v
Use J K-1 mol-1 for R Use kg mol-1 for MW
3
Gas molecules are moving FAST

• Gas v (m s-1)
• (at 25oC)
• O2 490
• N2 520
• H2O 650
• He 1360
• H2 1960

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O2
v(O2)
Number of Molecules
v(H2)
H2
0
4000
2000
speed, m s-1
5
To escape from the Earth...
...go up at gt 11.2 km/s
6
H2
Number of Molecules
Small but finite probability
0
20
10
speed, km s-1
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Mean Free Path, l

• How far does a gas molecule travel before
  colliding with another?
• More frequent collisions from bigger molecules
• higher pressure
• higher temperature

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Mean Free Path, l
RT
l
Ö2
pd2PNAV
size of molecule
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Mean Free Path

• at 25oC, 1 atm
• Species l, mm
• He 0.2
• N2 0.07
• O2 0.06

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v x/t

• time between collisions
• t x/v
• collisions per unit time
• t -1 v/x v/l
• Reactions occur when species COLLIDE

500 m s-1
5 x 109 s-1
t -1
.1 x 10-6 m
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Collisions in the air in this room

• 5 x 109 s-1 molecule-1 x 1.8 x 1028 molecule

2
(each collision involves 2 molecules)

• 4.5 x 1037 collisions s-1 in this room

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Effusion
Vacuum
Pgt0
13

• Rate of Effusion molecules s-1
• i.e., Rate of Effusion µ v

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Effusion
Grahams Law
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for example,

• Calculate the relative rates of effusion of H2(g)
  and He(g)

1.41
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Diffusion of Gases

• Diffusion - gradual dispersion of one substance
  through another

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Diffusion

• later...

P
P
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for example,

• C2F4 diffuses through a porous barrier at 4.6 x
  10-6 mol h-1
• An unknown gas composed only of B and H diffuses
  at 5.8 x 10-6 mol h-1. What is the unknown gas?

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Solving, MW?? 63 g mol-1
(B5H9)
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Another example...

• NH3(g) HCl(g) ? NH4Cl(s)

white solid
MW 17
MW 36
ratio of diffusion rates 1.45
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A practical example...

• Uranium Enrichment
• U is naturally 0.7 235U, 99.3 238U
• need 3-4 235U for most nuclear reactors
• 90 235U for bombs

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Enriching Uranium

• U ore UF6(g)

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a porous membrane
UF6(g), 0.7 235U
UF6, gt 0.7 235U
235UF6 diffuses faster than 238UF6
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235UF6 349.03 g/mol 238UF6 352.04 g/mol
1.0043
thus, need MANY such steps to enrich U
sufficiently (about 1400!)
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Other Neat Gas Stuff...

• 1. H2 balloons leak - H2 diffuses through rubber
  quickly
• 2. Deep sea divers breathe O2 and He

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Deep sea divers breathe O2 and He

• - Pgtgt1 atm
• - thus, breathing gas is compressed
• - N2 also dissolves in blood
• - upon decompression, N2(aq) N2(g)
• - He is not very soluble in blood

27
Divers often feel cold...

• He atoms move faster than N2
• Thus, the thermal conductivity of He is
• greater
• Thus, more heat is carried away from
• the diver

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Other Neat Gas Stuff...

• 1. H2 balloons leak - H2 diffuses through rubber
  quickly
• 2. Deep sea divers breathe O2 and He
• 3. Divers sound like Alvin the Chipmunk - He is
  less dense than air, hence vocal chords vibrate
  faster