Unit 4: Gases
Kinetic Molecular Theory
& Pressure
States of Matter
Solids:
Are they compressible?
No.
Liquids:
Are they compressible?
Essentially not.
Are Gases compressible?
Yes.
Look at particle distribution in each physical
state.
In which state are particles furthest apart?
In gas phase.
Kinetic Molecular Theory of Gases
Assumptions:
1. Volume of individual gas particles
(molecules or atoms) is negligible with
respect to the volume that they occupy.
i.e. Most of a gas is
empty space;
particles of gas are very
far apart.
2. No attractive or repulsive forces between
gas particles.
3. Gas particles are in rapid, random,
straight line motion in all 3-D.
4. Collisions of gas particles with
other gas particles or with
container walls are
perfectly elastic.
i.e. No loss of kinetic energy on collision
5. Average KE of a gas α Temperature. As
T ↑, so does average KE (i.e. speed) of
gas particles.
Gases at same T have same KEaverage.
Maxwell-Boltzmann Distribution of
Molecular Speeds:
KE = (½)mv2
At the same temperature, which of the
following gases molecules will have the
greatest average speed?
CO2, N2, H2, O2.
H2 — why?
It is the lightest
molecule.
Pressure
P = Force/Area . . . units are
N·m-2 or
Pascal (Pa)
The pressure of a gas arises from
collisions of gas molecules with the sides of
the container.
As T  ↑, pressure of gas
also increases.
Measuring Atmospheric Pressure
Evangelista Torricelli
built the first barometer ca. 1644
He inverted a long tube of Hg in a
bowl of Hg.
Column of Hg supported by Patm.
Variations in column height
due to variations in air pressure.
Can make a Torricelli Barometer
Using Water Instead of Hg.
density of Hg(l) = 13.5 g/mL
density of H2O(l) = 1.00 g/mL
How much taller would an equivalent column
of water have to be in order to be used as
a Torricelli barometer?
13.5 X taller.
Crescent’s Water Barometer
Built in atrium of Science Wing using plastic
tube, garbage can full of water, measuring
tape.
height of water column
= 10.16 m
equivalent ht of column of Hg =
10.16 m/13.5
= 0.753 m
= 753 mmHg
Classroom barometer read 754 mmHg.
Different Pressure Units
Standard atmospheric pressure (sea level)
can be expressed as:
1.00 atm
101.3 kPa (Pa = N·m-2)
760 mmHg (or torr)
14.7 psi (pounds per square inch)
Converting Pressure Units
1. Convert 102.3 kPa to mmHg.
Ans: 768 mmHg
2. Convert 0.978 atm to kPa.
Ans: 99.1 kPa
3. Convert 50 kPa to psi. No calculator
Ans: 100 kPa ≈ 15 psi and so
50 kPa ≈ 7 psi
How does the Patm at the top of
Mount Kilimanjaro compare to Patm at sea level?
Much lower—ca. 350 mmHg atop Kilimanjaro.
HW
Text sections 11.2, 11.2 to end p 509
P 506 RQ # 1 – 15. Read these questions
before you read the chapter. Then answer
the questions.
P 510 LC # 7 - 12