Chapter 13 - GASES
Properties of gases
1. are compressible
2. occupy all available volume
3. one mole of gas at 0oC and 1 atm pressure
occupies 22.4 liters
4. gases have mass
5. can move (diffuse) through each other
6. gases exert pressure
7. A gases pressure depends on temperature
The properties of gases are
explained by the
Kinetic-Molecular Theory of Gas
Gases are composed of small particles
that have mass.
The particles are far apart (compared to
the volume of the particles).
They are in rapid motion and have
perfectly elastic collisions
The collisions of gas molecules
against the sides of a container
create pressure.
The kinetic energy of a gas
molecule is given by the equation
KE = 1/2mv2
Temperature is a measure of the
average KE of gas molecules.
Lighter gas molecules, such as H2
move faster than heavier gas
molecules (O2) if they are at the
same temperature.
Measuring Gas - page 424
The four variables needed to completely
describe a sample of gas are the amount
of gas (moles), volume, temperature and
pressure.
All types of gas behave the same so we
do not need to know what type of molecule
the gas is.
Atmospheric pressure - the weight
of the atmosphere pushing on us.
1 atm = 14.7 lb/in2 = 101,325 N/m2 (Pa)
= 760 mm Hg
760 mm Hg corresponds to the weight per
square inch that atmospheric pressure can
support
If mercury is 13.6 times more
dense than water, what is the
maximum water height that the
atmosphere can support?
A well any deeper than this must use
a submersible pump.
Start assignment (practice problems
1 & 2 page 427)
Enclosed gases
Absolute pressure - the true pressure of a
gas (barometric pressure is an example)
Gage pressure - the difference in pressure
between the trapped gas and the
atmosphere (tire pressure is an example).
Manometer
A manometer can give the
true pressure of a gas by
adding or subtracting the
height of the Hg column from
the atmospheric pressure.
Draw a diagram and solve
practice problems 3 and 4 on
page 429.
Gas Laws
One mole of gas = 22.4 liters at STP
(The standard temperature and
pressure for gases is 0oC and 1 atm.)
Boyle’s Law
(pressure - volume relationship)
If we have a given quantity of gas
(moles) and the temperature is kept
constant while the pressure and volume
are changed, PV = constant (k).
P1V1 = P2V2
The pressure times volume before the
change is equal to the pressure times
volume after the change.
Do sample problem 3 page 433
Assignment continued (practice
problems 5,6 page 434)
Charles’s Law (temperature volume relationship)
If the pressure is kept constant, the
temperature and volume are directly
proportional. As temperature increases,
the volume also increases.
Experimental volume -temperature data
can be graphed and the line extended
(extrapolated) to zero volume.
V1T2 = V2T1
Charles’s Law
Absolute temperature scale
There are no negative temperatures
Absolute zero is the coldest possible
temp
(Kelvin scale, K) - nature’s
temperature scale
K = oC + 273.15
As the temperature of a gas is
changed, the volume of gas will be
changed by a ratio of the initial and
final temperature in oK.
Example problem: What will be the
new volume if 2 L of gas is heated
from 100oC to 300oC?
V1T2 = V2T1
Show how to solve sample problem
4 on page 438.
Assignment continued - page 438
practice problems 7,8
Avogadro’s Law
Equal volumes of gases at the same
pressure and temperature will have
equal number of gas particles.
Three moles of gas will occupy three
times the volume as one mole of gas.
Dalton’s Law of Partial Pressures
The sum of the partial pressures of all
the components in a gas mixture is
equal to the total pressure of the gas
mixture.
PT = pa + pb + pc + pd + - - - - Problem assignment continued (page
440 practice p. 9 & 10
THE IDEAL GAS LAW
PV = nRT
R = 8.314 J/(mol K)
R = .0821 (atm L)/(mole K)
R = 8.314 (Pa m3)/(mol K)
Use the R that has the correct units
for the problem
Problem assignment continued page 443 practice 11, 12
Real gases
Real gases can deviate from the
ideal gas equation at very high
pressure and at very low
temperatures. This is because of
the slight attractive forces
between real gases.
“Free-style Gas Calculations
Treat every problem as a conversion
problem with the ratio of the change being
a correction factor.
You must decide if each change will result
in an increase or decrease.
Example Problem: What will be
3
the new volume of 6 ft of gas if
the following changes are
made?
2.4 atm changed to 6.3 atm
Heated from 20 oC to 300 oC
5.2 moles changed to 12.6 moles
Chapter 13 assignment
Chapter questions on pages 452-453
(1– 19, 23, 24)
Problem Bank problems on pages
454 – 455 (27, 29, 31, 37, 39, 41, 42, 44,
52, 54) note that the answers are on page
944
You must show your work for credit.