Gases
Properties of ideal gases
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Gases are evenly distributed in a volume.
They have very weak IMF, intermolecular
forces. (van der Waal)
They have frequent elastic collisions
They have high kinetic energy due to
their high velocities.
Most of the volume they occupy is empty
space; they are easily compressed.
Kinetic molecular theory of
gases
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Kinetic energy (KE) is the energy a particle
has as a result of its molecular movement.
KE=1/2(molar mass)velocity2
Temperature is directly proportional to KE.
For example: an increase in molecular
movement causes an increase in
temperature.
This theory is best applied to gases since they
are free moving particles and are subject to
great changes in velocity.
http://www.youtube.com/watch?v=EzWRvK0
Applying the kinetic molecular
theory to gases
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An example: Two gases, He and H2 are at the
same temperature of 20C, have the same
mole value, but are in separate but identical
containers.
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Both gases have the same KE due to their same
temperature.
Both gases will exert the same pressure since
there is the same number of molecules of each
gas. (same mole value) and are in the same
volume.
The hydrogen will move faster than the He since it
is a lighter gas H2= 2g/mol, He=4g/mol
Applying the kinetic molecular
theory to gases continued
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Summary:
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Remember KE= 1/2mv2. So if gases are at the
same temp. then they have the same KE.
If gases have different masses, then their
velocities must be different. The rates of diffusion
will differ.
Regardless of the mass of the gas, gases at the
same temperature have the same force of
collision/molecule. Heavy gases hit just as hard as
light gases if they are at the same temp.
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Heavier gases have slower velocities, lighter gases have
higher velocities. This equalizes the force of collision.
Kinetic energy videos.
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http://www.youtube.com/watch?v=UNn
_trajMFo KE and size of molecules
http://www.youtube.com/watch?v=EzW
RvK0zhQk KE and Temp. (too long)
Applying the kinetic molecule
theory to solids and liquids.
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This theory does not apply as easily to
solids and liquids due to their restricted
motion.
It is safer to apply the KE theory to
solids and liquids if they are at the
same temp.
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H2O and alcohol at 20C have similar KE
Or apply it to the same solid or liquid.
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H2O at 20C has less KE than H2O at 100C
Kelvin temperature scale/
absolute temperature scale.
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Properties of this temperature scale
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Based upon the molecular movement of
gases
At zero Kelvin there is no molecular
movement. Since temp. is based upon
molecular movement, O Kelvin is the
coldest temp. possible anywhere.
O Kelvin = -273°C
Add 273 to C to get Kelvin
Non Ideal Gas- SF6
http://www.youtube.com/watch?v=xQov_F1P9U
 Mythbusters inhale SF6
Properties of gases
Pressure
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Pressure= force/area
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For gases: the force is created by the
strength and the number of collisions.
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The strength of the collisions is determined by
the kinetic energy of the molecules. Higher KE
= higher velocity= greater force of collisions.
The number of collisions is determined by: the
mole value/volume of the gas and the velocity
of the gas.
For gases: the area is defined by the
pressure unit. (walls of the container).
Units of pressure
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Millimeters of mercury (mmHg) = based upon
the height of a mercury column in a
barometer. Pressure at sea level=760mmHg
Torr (named after Torricelli) is the same unit
as the mmHg. You will see this unit in the
texts. 760 torr= pressure at sea level.
Atmospheres (atm)= 1atm is equal to the
pressure at sea level.
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Pascal (Pa)= SI unit, =1newton/meter2,
101,325 Pa = pressure at sea level. kPa
is most often used. 101.3kPa
Pounds per square inch (psi) =14.7 psi
is the pressure at sea level.
http://www.youtube.com/watch?v=GgBE8_SyQCU
A Torrecilli Barometer
Summary of pressure at sea
level
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1 atm
101,325 Pa or 101.3kPa
760mmHg
760 torr
14.7psi (will not be using this unit.)
Standard temperature and
pressure: STP
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For research and convenience a
standard temp and pressure are used
when working with gases.
Standard temperature: 273K or 0
celcius
Standard pressure: sea level= 1atm,
760mmHg, 760torr, 101.3 kPa, 14.7psi
MEMORIZE STP!!!!!!!!!!!
Avogadro’s Hypothesis
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Equal volumes of gases at the same
temperature and pressure have the same
number of molecules.
At S.T.P. the volume of one mole of any ideal
gas is 22.4L.
At STP the volume of a gas can be easily
determined using a proportion as long as the
amount (moles) of gas is known.
Pressure and volume
relationship
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An increase in pressure will decrease volume, or
decreasing the volume will increase the
pressure.
When the volume is decreased, the number of
collisions increase per time. The force of
collisions stays the same.
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Pressure and volume are inversely related. The
equation: P=k/V or P1V1=P2V2 when moving the
gas into another container. Amount and
temperature of gas remains constant.
Pressure and volume
Pressure and volume: Boyles
law
Effects of temperature on
pressure and volume
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Temperature and pressure in a fixed
volume
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Increasing the temperature increases the
pressure.
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Molecules have more KE which means their
velocity increases. (mass stays constant)
Higher velocity= more collisions and greater
force in the collisions so the pressure increases
Effects of temperature on
pressure and volume
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Temperature and volume (container can
expand)
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Increasing temperature will increase the volume,
the pressure remains constant.
Temperature is directly proportional to volume.
T=kV (charles law)
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Molecules have more KE which means their velocity
increases.
Higher velocity=more collisions and greater force in the
collisions. This expands the container. The container will
expand until the pressure inside=the pressure outside.
Gas Laws so far that you will
be using on your homework.
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Boyle’s Law: P1V2 = P2V2
Charle’s Law: V1/T1 =V2/T2
Gay-Lussac law: P1/T1 =P2/T2
Combined Gas Law: uses the above
laws: P1V1/T1 =P2V2/T2
Important note: you must use the
Kelvin temperature scale for all “T’s”
IDEAL GAS LAW: uses only ideal
gases and is the only law that allows
for a change in moles.
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PV=nRT
P=pressure
V=volume(L)
n=moles of gas
R=constant
T=temperature(K)
R Constants