Transcript Ch. 10 Gases

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Gases are defined by:
◦ The particles are free to move (fluid).
◦ The particles are far apart.
◦ The particles move very fast.
◦ The particles have no attraction nor
repulsion for each other.
◦ The fact that they have no definite shape
or volume – they take the shape of the
container.
◦ The fact that they are compressible.
Gases have four measurable variables.
1. Pressure (P) of a gas.
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◦ Measured in a variety of units like mmHg, atm, Pascals,
torr, bars, and psi.
2.
3.
4.
Volume (V) of a gas,
◦ Measured in mL or L.
Temperature (T) of a gas.
◦ Measured in Celsius or Kelvin.
◦ For all formulas, Kelvin temperatures must be used.
Quantity of a gas.
◦ Measured in grams or moles (n).
◦ Formulas will require moles.
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Pressure is the force per unit area.
As the molecules of a gas collide with the walls,
they exert a force.
Force
Pressure =
Area
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The column of air
above us exerts a
force due to the
gravitational force.
Atmospheric
pressure has two
main variables.
What are they?
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A barometer is used
to measure pressure.
A dish is filled with
liquid mercury.
A glass tube that has
been evacuated of all
air molecules is then
placed into the pool
of mercury.
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The height of the mercury in the tube over
the pool of mercury is measured with a ruler.
In weather, it is reported as inches of mercury
– inHg.
We will measure ours in millimeters of
mercury – mmHg.
On an average day, at sea level – the column
would measure 760mmHg = 1 atmosphere
(atm).
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As altitude
increases, the
pressure
decreases.
Denver, CO
Death Valley, CA
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Look at the relationship between two of the
four variables.
Boyle’s, Charles’, and Gay-Lussac’s Laws.
Direct (linear) relationship – as one variable
increase, so does the other.
Inverse relationship – as one variable
increase, the other decreases.
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Allows P and V to vary, while n and T are
constant.
Boyle found that as P , the V  and vice
versa.
What type of relationship – Direct or Inverse?
P x V = constant
Formula: P1V1 = P2V2
LEP #1
Pressure vs. Volume
Pressure, atm
5.00
4.00
3.00
2.00
1.00
0.00
0.00
10.00
20.00
Volume, mL
30.00
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Allows V and T to vary, while n and P are
constant.
Charles’ found that as V , the T .
What type of relationship – Direct or Inverse?
V
= constant, when T is in Kelvin
T
Formula: V
V
1
LEP #2
T1
=
2
T2
Volume vs. Temperature
Volume (ml)
600
400
200
0
0
100
200
Temperature (K)
300
400
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Allows P and T to vary, while n and V are
constant.
Gay-Lussac found that
Direct or Inverse?
Formula?
LEP #3
as P , the T .
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All three empirical gas laws can be merged
into a single formula.
This means that two of the three variables
can be changed.
P1 V1
P2 V2
=
T1
T2
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LEP #4
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The quantity of a gas (moles) and volume are
proportional.
STP = standard temperature (0oC) and
pressure (1 atm).
At STP, one mole of any gas will occupy
22.4L.
LEP #5
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The relationship between ALL four variables is
found in the ideal gas law.
Formula: PV = nRT.
R is called the gas law constant and equals
0.08206 L atm / mol K.
Because of the units, each of the variables
must be in the same units!
LEP #6
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Other derived versions of the ideal gas law
can be useful.
Since moles = grams (g) / molar mass (MW),
gRT
MW 
PV
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Density is grams / volume, so we can include
this in our formula as well.
DRT
MW 
P
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LEP #7
LEP #8
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Many reactions generate a gas as a product.
Grams of solid or liquid must be converted to
moles of gas.
Ex) 2 KClO3(s)  2 KCl(s) + 3 O2(g)
In this example, when 2 moles of KClO3
decompose, 3 moles of O2 gas are generated.
LEP #8
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Explosives like dynamite, TNT,
nitroglycerin, etc. are compounds that
produce mostly gases when allowed to
react.
Unlike hydrocarbons, most explosives
_________ rather than burn.
A ____________ is a very rapid chemical
reaction using oxygen that is contained in
the material rather than in the air.
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2 C7H5N3O6 → 3 N2 + 5 H2O + 7 CO + 7 C
CH3
O2N
NO2
NO2
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Ammonium nitrate / fuel
oil is commonly used in
mining.
It was also used in the
bombing of the Murrah
building in Oklahoma
City.
LEP #10
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Many gases we encounter are mixtures of two
or more substances.
The total pressure exerted by a mixture is
equal to the partial pressures of each of the
gases.
Ptotal = Pa + Pb + Pc + …
The partial pressure of a gas is proportional
to its mole fraction.
Pa = Ptotal ca.
LEP #11, #12
In the study of gases, five postulates
describing them were published by Rudolf
Clausius in 1857.
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Gases consist of large numbers of
molecules that are in continuous, random
motion.
1.
◦
Pressure is uniform on any surface.
The volume occupied by the molecules is
negligible when compared to the volume of
the container.
2.
◦
3.
A 5.0L container is assumed to be the volume
even though the gas molecules must occupy
some of that space.
Attractive and repulsive forces between gas
molecules are negligible.
4.
5.
Collisions of gas molecules are elastic.
Energy can be transferred between
molecules, but cannot be lost.
The average kinetic energy of a gas is
proportional to the temperature of the gas.
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This is an approximation of the average
speed of a gas molecule.
3RT
u
Mm
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LEP #13
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All gases deviate
from “ideal”
behavior under
different conditions.
At higher pressures,
gas molecules do
occupy a portion of
the volume.
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At low temperatures, gas molecules can have
attractive forces for each other.
Both of these factors can be corrected for in
the van der Waals equation.

n 2a

 P  V2

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V - nb   nRT
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