The Behavior of Gases
Properties of Gases (Review)
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No definite shape
No definite volume
compressible
Kinetic Molecular Theory
moving
molecules
well supported ideas
Basic Kinetic Theory of Gases
1.
Composed of particles like atoms
(ex: He) or molecules like (O2 and
CO2)
There are no attractive/repulsive
forces.
Lots of empty space!!
Basic Kinetic Theory of Gases
2. Particles move in
random, constant,
straight-line motion.
Move independently
of each other.
Basic Kinetic Theory of Gases
3. All collisions are elastic meaning
that KE is transferred without loss of
energy.
No change in kinetic energy.
Gases tend to diffuse towards areas of
lower concentration.
Gas Pressure
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Pressure- force exerted on container
walls by particles in a gas
Units used- kPa, atm, Torr, mmHg
STP (Standard Temperature and
Pressure) Table A
273 K or 0°C
and
101.3 kPa = 1 atm = 760 Torr (mmHg)
Factors Affecting Pressure
Amount of
Increasing amount Ex: bicycle tires,
Gas (number will increase P
car tires
of moles)
(and vice versa)
Temperature Increasing temp.
will increase P
(and vice versa)
Ex: Tires deflate
in winter
Volume
Ex: press down
on a balloon and
it pops
Decreasing
volume will
increase P,
increasing volume
decreases P
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Pressure and volume have an inverse
relationship, if temperature remains
constant.
If volume is increased, pressure is
decreased by the same factor.
Mathematically, the product of PV is constant or PV = k
(where k is some constant).
Boyle’ Law
P1 V1 = P2 V2 = P3 V3…
Summary
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Volume and temperature have a
direct relationship, if pressure is held
constant.
If temperature (K) is increased,
volume is increased by the same
factor.
Mathematically, the relationship of volume divided by Kelvin
temperature is constant or V/T = k.
Charles’ Law
V1 /T1 = V2 /T2 = V3 /T3 …
Summary
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Pressure and temperature have a
direct relationship, if volume remains
constant.
If temperature (K) is increased,
pressure will be increased by the
same factor.
Mathematically, the relationship of volume divided by Kelvin
temperature is constant or P/T = k.
Gay-Lussac’s Law
P
r
e
s
s
u
r
e
P1 /T1 = P2 /T2 = P3 /T3 …
Combined Gas Law Equation
P1 V1
T1
=
P2 V2
T2
Combined Gas Law Equation
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Steps:
Determine which variable (if any) is kept
constant.
Cancel those terms and remove them
from the equation (Ex: If the question
says that temperature remains constant
the new equation becomes P1V1 = P2V2).
Plug in values that are given.
Solve for the unknown.
Be sure to always use temperature in
Kelvins.
Ideal Gases vs. Real Gases
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“Ideal gases” behave as predicted
by Kinetic Molecular Theory.
Examples: H2 and He
Gases are most ideal at high
temperature and low pressure (also
have low mass and low polarity).
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“Real gases” deviate from ideal
behavior.
Why?
At low temps, gas particles
become attracted to each other
(KMT says they are not).
Under high pressure, gases occupy
a specific volume (KMT says they
don’t).
Avogadro’s Law
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Avogadro’s number: 6.02 x 1023
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Simply refers to the quantity of particles
found in a mole.
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At STP, 6.02 x 1023 particles of a gas
occupies 22.4 L.
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At STP, 3.01 x 1023 particles of a gas
occupies 11.2 L.
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Avogadro also
hypothesized that
equal volumes of
different gases at the
same temperature
and pressure contain
equal number of
particles (or equal
moles).
Vapor Pressure
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In a sealed container, vapor pressure
can be measured above a liquid.
Evaporation occurs when some
particles from the surface of a liquid
escape causing pressure to build up
above the liquid (not to be confused
with boiling).
Factors that Increase the Rate of
Evaporation
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Heating a liquid (not to
boiling point)
Increasing surface area
Create air currents
(blow across the surface)
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Liquid-Vapor Equilibrium
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Some of the gas particles condense
and then we find both evaporating
and condensing occurs at the same
rate.
Rate of Evaporation = Rate of Condensation
Related to Boiling
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Boiling occurs when the vapor
pressure becomes equal to the
external pressure.
At normal atmospheric pressure, we
call this normal boiling point.
Boiling and Attractive
(Intermolecular Forces)
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Boiling occurs when heat energy
overcomes attractive forces between
molecules.
The stronger the intermolecular forces,
the higher the boiling point.
The weaker the intermolecular forces, the
lower the boiling point.
Table H
Notice, increasing
temperature increases
vapor pressure.
Line drawn at 101.3 kPa
corresponds to normal
boiling point.