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Transcript of 1 Chapter 4 - Gases Movie – phase diagrams Kinetic Theory Pressure Measurement –Units –Unit...
1
Chapter 4 - Gases
• Movie – phase diagrams• Kinetic Theory• Pressure Measurement
– Units– Unit Conversions
• Boyle & Charles• Combined Gas Law
2
Movie – Phase Diagrams
• Take home message:– how gases move around– distances between molecules
• Other– There is more information in the
movie – it’s all chemistry.– Much of it is “common sense” with
scientific labels.
3
Kinetic Theory
• Define Temperature• Define Source of Pressure• Relate Molecular Motion to Gas
Behavior.
4
Kinetic Theory – Define Temperature
• Temperature relates to the speed of molecular motion.
• Fast moving molecules = higher temp.– Solids vibrate– Liquids wiggle (movie)– Gases move in straight lines
• Actually, temp. relates to kinetic energy which is mass times speed.– Fat molecules move slower at same temp.– Skinny molecules move faster– (Geo Metro vs. GMC Suburban)
5
Kinetic Theory - Source of Pressure
• Pressure is the force of gas atoms bouncing off the walls of a container.
POW
BIFF
6
Kinetic Theory: Molecular Motion Gas Behavior
• Hit the wall = pressure• Gas collisions are perfect bounces.
• T = Moving faster P__• T = moving slower P__• More molecules P__• Smaller Bottle P__
7
How do we measure pressure?
• Definition: P = force area
• Wall Barometer (Fig. 4.8)– air pushes on a diaphram against a
spring– calibrate the scale
• Manometer– Air pushes on a mercury column.
10
Pressure Units
Name Source Unit Normal Day
Pascals SI (like in lab) kPa 102 kPa
Pounds-per-Square-Inch
Life psi 14.7 psi
Atmospheres Lab atm 1.00 atm
Torr Lab/Barometer
torrmmHg
760 mmHg
11
Pressure Unit Conversions• We always get to convert these!
• 1 atm 760 torr = 760 mmHg = 14.7 psi = 101.3 kPa – Forget about “bar”
psi
kPa
psi
torr
psi
mmHg
kPa
atm
psi
atm
torr
atm
mmHg
atm
7.14
3.101,
7.14
760,
7.14
760
3.101
1,
7.14
1,
760
1,
760
1
12
Vacuum
• A vacuum is a truly empty space.
• No gas molecules.
• Pressure = 0.0000000 atm.– Real experiments can only get to
0.000 000 000 000 000 01 atm or so.
13
What is vacuum?• Vacuum- literally translated, the word describes
the volume from which all the air has been totally removed.
• However, so far nobody has succeeded in achieving such a condition. Scientifically the highest vacuum yet reached is in the range of 10-14 torr. This result has been achieved during a test by IBM Thomas J. Watson Research Center, New York, USA, October 1976, in a cryogene system where the temperature dropped to minus 269 °C.
• Even at this low pressure thousands of gas molecules were detected per cm3. Also far out in space the presence of gas has been measured.
http://www.busch.com.au/whatisvacuum.htm
14
Absolute vs. Gauge
• Bike Tires? 45 psi• Car Tire?
• The psi values we use are usually relative to atmospheric pressure.
• “Gauge” pressure means 1 atm + the reading.• “Absolute” means 0.000 atm + the reading• Actual pressure in bike tire is 45 psi + 14.7 psi
= 59.7 psi!• Chemistry lab experiments use absolute
pressure.
15
Pressure - Volume
• Boyles’ Law– Change the volume of a gas sample– See what the pressure does (graph
it).
• Avogadro’s Law– Trap different volumes of gas in the
syringe.– See what the pressure does.
16
Pressure – Volume Math
• Boyle’s Law says that: PV = k, P= k/V
• If you do 2 experiments: – 1st Experiment: 1.0 atm, 5.0 mL– 2nd Experiment: ? atm, 25.0 mL
atmmL
mLatm
V
VPPforSolve
VPVPsokVPkVP
20.00.25
0.50.1
,
2
112
22112211
18
Pressure - Temperature
• Charles’ Law says that:
Pressure Temperature– P T– P T– Double temperature double pressure
• Careful:– You have to use Kelvin units for
temperature.
22
Pressure – Temperature Math
• Charles’ Law says that: P = kT, k = P/T
• If you do 2 experiments: – 1st Experiment: 1.0 atm, 0 °C– 2nd Experiment: ? atm, 100 °C
atmKelvin
Kelvinatm
T
TPPforSolve
T
P
T
Psok
T
Pk
T
P
4.115.273
15.3730.1
,
1
212
2
2
1
1
2
2
1
1
23
Combined Gas Law
• We can shove Boyle & Charles together:
2
22
1
11
2
22
1
11 ,T
VP
T
VPsok
T
VPk
T
VP
24
Combined Gas Law - Math• These problems are all about setting
them up right• At 56°C and 868 torr, a gas takes up
4.30 L. What will the pressure be if the volume is changed to 6.36 L and cooled to 12 °C?
Case 1 Case 2P _868 torr_ _???_____V _4.30 L___ _6.36 L_T _56°C=329K_ _12°C=285K_
25
Combined Gas Law - Math• Once you make the table, fill into the
big equation.
• Answer: 508.4 torr
SolveSimplify
L
LP
K
Ltorr
T
VP
T
VP
&
285
36.6
329
30.4868 2
2
22
1
11