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Tequila (don’t YOU drink it)
b-damascenone gives tequila some of its woody/fruity flavor
Chapter 11
Gases: what if there were no
intermolecular forces?
At a molecular level, the states of matter differ
primarily by the level of cohesion.
Solids (examples: a penny, ice, glass???)
•Molecules have attracted each other
•Atoms that are bound together by sea of electrons
•Sometimes in a specific array (crystals)
•Sometimes not (glasses)
•Hold their own shape indefinitely
Liquids (example: honey, water, glass???)
•Molecules have attracted each other
– permanent or temporary dipoles.
•Not in any pattern (exception: liquid crystals)
•Have a surface: adopt shape of bottom of container
Gases (example: air, helium in balloon)
•Molecules not attracted to each other (or weakly attracted)
•No pattern
•No surface: adopt shape of all of the container.
How things are: Everything wiggles, only
some things translate.
Solids: molecules or atoms can wiggle about fixed, central
location. They don’t translate far.
Liquids: molecules or atoms move along (translate) until
they bounce into something, and this happens frequently
because the molecules or atoms have cohesion.
Gases: like liquids, only collisions not as often. No cohesion.
How fast? Depends on molecule.
isopentyl acetate,
CH3COOCH2CH2CH(CH3)2
About 500 miles per hour—pretty fast!
Let’s measure it.
How do we KNOW that structure?
Why it took so long to get to
back of room: collisions!
The isopentyl acetate molecule only
goes about 200 nm before hitting
something, which then sends it in a
different direction. It takes forever for it
to spread around the room…..unless
there is a draft.
Diffusion: 10000 steps forward
and 10001 steps backward
The “hydrodynamic” world of diffusion—dominated by
collisions—is very different from our normal “ballistic” world. An
object set into motion comes almost immediately to a stop,
rather than coasting as we are used to seeing.
Gases—pressure matters!
Gas particle hits a piston, imparts
a force.
With a small number of gas atoms
in the cylinder, this force would be
quite random.
With a large number, the force
would be quite steady.
We usually see almost perfectly
steady pressure because there are
billions of billions of particles in
any normal sample.
You could measure pressure this way, but
it’s pretty darn inconvenient. We still show
it because it’s conceptually easy.
P = F/A = mg/p r2
Mass of bricks
9.8 m/s2
Radius of piston
Slightly more practical…using water
height to measure pressure
Vacuumreviewer.com
How high will the water rise?
•depends on the weather
•depends on whether you're in Louisiana
or Colorado
•is always a little more than 30 feet
Why? Because that is how far
atmospheric pressure can push it.
The vacuum pump doesn't suck;
the air pushes.
Yeah, but how far will the water rise?
Patm = rgh = 101,325
Density of the water
9.8 m/s2
2
nt/m
Rather than use the Super Hoover, we
could draw a hose full of water, sealed
at one end, out of the swimming pool.
•Fill a hose with water, taking care to introduce no bubbles.
•Put the hose in a large tub of water.
•Seal one end of the hose.
•Drag the sealed end out with a pulley.
•When the hose tip exceeds about 34 feet, you will see the
water goes no higher! You get an air gap at the top of the
hose.
•You now have a barometer. As weather changes, the height
of the column of water will fluctuate.
BOB = Big Ol’ Barometer
Remember: P = rgh
so… h = P  rg
Denser liquids mean shorter barometers!
Mercury density:
13.6 g/mL
Why mercury is a liquid
http://antoine.frostburg.edu/chem/senese/101/periodic/faq/why-is-mercury-liquid.shtml
http://www.madsci.org/posts/archives/may97/862179191.Ch.r.html
Experiments with barometers: #1
Pressure vs. temperature at fixed volume: put barometer
into fixed, rigid box and raise temperature.
Make pressure readings.
Rigid box
No matter what the gas,
always extrapolates to
-273.15 oC when P = 0
Kelvin concepts
T (K) = T(oC) + 273.15
Room temperature: 25oC = 298K
Ice water: 0.00 oC = 273.15 K
Boiling water: 100.00 oC = 373.15 K
All evidence suggests that NOTHING can be
colder than 0K = -273.15 C
If T is in Kelvins, P is directly proportional to
T. You can make a barometer into a
thermometer!
P T
Experiments with barometers: #2
Pressure vs. volume at fixed temperature: put barometer
into horror chamber with descending ceiling. Keep at same
Temperature while ceiling lowered. Make pressure readings.
Movable ceiling!
Lets you change
room volume.
P
V
P increases with inverse of volume
1
P
V
Experiments with barometers: #3
Pressure vs. mass of gas at fixed T and V.
More gas particles (e.g., weigh in more gas)
leads to more pressure, other things (like T and
V) being equal.
PN
Where N is the number of gas particles
Combined gas law: usually
called the ideal gas law
P T
1
P
V
PN
Combine
NT
P
V
We can rearrange that:
PV
 constant
NT
The constant is called the gas constant.
This relation only works for “ideal” gases (but most gases are ideal).
liter  atm
R  0.082
mol  K
The ideal gas law is good for
handling transitions in closed systems.
Before: “State 1”
(example: low T)
After: “State 2”
(example: high T)
P1V1 P2V2

N1T1 N 2T2
Suppose a gas is at 1.5 atm of
pressure and occupies 250 mL. If
the pressure is reduced to 0.5
atm, what volume does the gas
now occupy?
765 mL of a gas is held in a
container at a pressure of 0.95
atm. With temperature held
constant, the pressure is
decreased to 0.82 atm. What is
the new volume?
Ideal gas law is PV=nRT.
liter  atm
R  0.082
mol  K
But number of moles, n, is grams, g, divided by Mwt…..
g
n
M
So……
gRT
PV 
M
765 mL of a gas is held in a
container at a pressure of 0.95
atm. With temperature held
constant, the pressure is
decreased to 0.82 atm. What is
the new volume?
Standard Temperature & Pressure
(STP) is arbitrarily set at……
T = 0oC = 32oF = 273.15K
P = 1 atm = 101,325 Pa (1 Pa = 1 nt/m2)
1 mol of a “simple” gas occupies 22.4 liters at STP
16 g CH4 = 22.4 liters
44 g CO2 = 22.4 liters
32 g O2 = 22.4 liters
28 g N2 = 22.4 liters
http://generationsofaithfulness.wordpress.com/2011/08/03/dear-milk-man/
Mixing Gases, Partial Pressures
Gases can mix. When they do,
their pressures add (almost):
Patmospheric = PO2 + PN2 + PH2O + PAr
+ PCO2 + minor pressures from
trace gases
Air is approximately 80% N2, 20% O2
This is why Baton Rouge
is so uncomfortable:
PH2O is way too high! All
those water molecules crashing
into you replenish the heat your
body was trying to get rid of by
evaporating water molecules as
you sweat.
Wonderful things, everywhere you look!
Water crystals exclude
the “grape stuff” (color
and sugar and flavor” to
make beautiful and tasty
popsicle-like treat.
PE Container:
H-[CH2]n-H