Transcript Why Balloons Float (and why they don’t)

Why Balloons Float
(and why they don’t)
Unit 3: Phases of Matter
Lesson 3: Gases and Pressure
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How does a gas behave?
Kinetic Molecular Theory (KMT)Describes an “ideal” gas.
We imagine how it would behave.
It would have five properties:
1. Be made of particles with negligible volume
2. Particles move in random, straight-lines
3. Completely elastic collisions
4. No intermolecular attractive forces
5. Speed of particles is directly proportional to
Kelvin temperature
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Ideal is not Real
Real gases violate some/all of the KMT
ButOnly when the particles are moving
slow and are squeezed together.
When
would this
happen?
Low Temperature & High Pressure =
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Avogadro’s Hypothesis
Equal numbers of gas particles occupy
equal volumes of space under the same
conditions of temperature and pressure.
Amedeo Avogadro
(1776 – 1856)
They all contain equal
numbers of molecules!!!
How can this
be?!?
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Standard Temperature and Pressure (STP)
Because things happen differently at different
temperatures and pressures (particularly for
gasses), we have to set a standard reference
point.
Standard Temperature:
0° C = 273 K
Standard Pressure:
1.000 atm = 101.3 kPa = 760 mmHg (torr)
These are in Reference Table A.
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What is this “Pressure” of which we speak
Pressure =
Force exerted over an area.
Anything with mass can exert a force.
This includes the atmosphere.
Standard Pressure:
1 atmosphere of pressure (at sea level)=
14.7 pounds per square inch (psi).
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Brief notes on Torr.
Torr = millimeters of mercury (mmHg)
Refers to the column of mercury in a
barometer.
760 torr = Standard pressure
Why do we
use
mercury?
Evangelista Torricelli
(1608 – 1647)
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Pressure conversions
1.000 atm = 14.7 psi = 101.3 kPa = 760.0 mmHg
Convert 2.35 atm to kPa:
Convert 1.234 kPa to atm:
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Vapor Pressure
When a liquid in a sealed container is at vaporliquid equilibrium, the vapor exerts a pressure
(like any gas).
Stronger IMAF = Lower vapor pressure.
Higher vapor pressure = faster rate of evaporation.
Volatile=
Substances that evaporate quickly.
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Why do things boil?
Boiling happens when the vapor pressure of a
liquid is greater than the atmospheric
pressure the liquid is under.
How Can you
increase vapor
pressure?
Boiling Point =
Vapor pressure = atmospheric pressure.
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Normal Boiling Point
The boiling point of a liquid at Standard
Atmospheric Pressure.
What happens to boiling point if atmospheric
pressure increases? Decreases?
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Reference Table H
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Problem: What is the vapor pressure of ___ at
___°C?
Use Method A!
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Problem: What is the boiling point of ___ at a
pressure of ___kPa?
Use Method B!
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Problem: What is the normal boiling point of ___?
Use Method C!
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Equal numbers of chemistry
students, occupying equal
volumes of classrooms do
not possess equal numbers
of questions....You?
Amedeo Avogadro
(1776 – 1856)
What now?
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Things To Do Now:
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Passing Gases
Unit 3: Phases of Matter
Lesson 4: Partial Pressure and Effusion
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Dalton’s Law of Partial Pressures
The total pressure exerted by a
mixture of gases is equal to the sum
of the pressures exerted by each
gas in the mixture.
Ptotal = PgasA + PgasB + PgasC + ...
John Dalton
(1766 – 1844)
NOT in your Reference Tables
(memorize!)
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Practice Helps Us Learn!
1) What is the total pressure of a mixture of O2 (g), N2 (g) and
NH3 (g) if the pressure of the O2 (g) is 20. kPa, N2 (g) is 60. kPa
and the NH3 (g) is 15 kPa?
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2) A mixture of 1 mole of O2 and 2 moles of N2 exerts a pressure
of 150. kPa. What is the partial pressure of each gas?
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3) A mixture of 30.0% He and 70.0% Ar exerts a pressure of 150
kPa at 25oC. What is the partial pressure of each gas?
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4) A sample of NH3 (g) is decomposed into its component
elements. If the pressure of the nitrogen gas produced equals
40.0 kPa, what would the pressure of the hydrogen gas?
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The total questions asked by a class
of chemistry students equals the
sum of the questions asked by each
student in the class. Any Questions?
John Dalton
(1766 – 1844)
What now?
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Things To Do Now:
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Imagine a Piston...
Unit 3: Phases of Matter
Lesson 5: The Gas Laws
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Gases Obey Physical Laws
This should not surprise you.
The behavior of gases can be predicted and
expressed according to mathematical
relationships.
We will look at relationships of Pressure,
Volume, Temperature and the # of molecules
(aka moles) of a gas.
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A Brief Note on Units
We will use the following units:
Pressure-
Atmospheres(atm) & KiloPascals(kPa)
Volume-Liters(L) and milliliters(ml)
Temperature- Kelvin(K)
# of molecules-Moles(mol)
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The Beginning: Avogadro’s Hypothesis
All of the gas laws stem from Avogadro’s
Hypothesis:
Equal numbers of gas particles occupy
equal volumes of space under the same
conditions of temperature and pressure.
Amedeo Avogadro
(1776 – 1856)
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2 Illustrative Problems to Consider
1. Consider two 4.00 L containers, each at 298 K
and 1.00 atm. Container A holds nitrogen gas,
Container B holds carbon dioxide gas. If
container A holds 2.00 moles of nitrogen gas,
how many moles of carbon dioxide must be
present in container B?
2. Do equal volumes of gases under the same
conditions of temperature and pressure have
the same MASS? Why or why not?
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How To Solve Any Gas Law Problem
1. Get rid of the words!
Read the problem and pick out the variables.
Make a list of them.
Make sure your units are acceptable and agree.
2. Write down the particular Gas Law you need.
3. Rearrange to isolate the variable you’re
solving.
4. Plug in your numbers.
5. Solve for unknown
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Boyle’s Law: Pressure & Volume
As Pressure Increases, Volume Decreases
P1V1 = P2V2
Robert Boyle
(1627 - 1691)
Temperature must be constant
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A sample of gas occupies a volume of 2.00 L at STP. If the pressure is
increased to 2.00 atm at constant temperature, what is the new
volume of the gas?
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Charles’ Law: Temperature & Volume
As Temperature Increases, Volume
Increases
V1/T1 = V2/T2
Jacques Charles
(1746 - 1823)
Pressure must be constant
TEMP MUST BE KELVIN
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A sample of gas occupies a volume of 5.00 L at 300. K. If the
temperature is doubled under constant pressure, what will the
new volume of the gas be?
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Gay-Lussac’s Law: Temperature &
As Temperature
Increases, Pressure Increases
Pressure
P1/T1 = P1/T2
Joseph-Luis Gay-Lussac
(1778 – 1850)
Volume must be constant
TEMP MUST BE KELVIN
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A 10.0 L sample of gas in a rigid container at 1.00 atm and 200. K is
heated to 800. K. Assuming that the volume remains constant,
what is the new pressure of the gas?
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The Combined Gas Law
Puts all three gas laws together.
Any variable being held constant, can be ignored.
On Reference Table T!
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A 2.00 L sample of gas at STP is heated to 500. K and compressed to
200. kPa. What is the new volume of the gas?
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A 2.00 L sample of gas at 1.00 atm and 300. K is heated to 500.K and
compressed to a volume of 1.00 L. What is the new pressure of
the gas?
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A 2.00 L sample of gas at 300. K and a pressure of 80.0 kPa is placed
into a 1.00 L container at a pressure of 240. kPa. What is the new
temperature of the gas?
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The Ideal Gas Law
Relates the number of moles of a gas to it’s
pressure, volume and temperature:
PV = nRT
n = # of moles
R = Gas constant
NOT on your Reference Tables (MEMORIZE!)
Comes from the observation that 1 mole of any gas
occupies a volume of 22.4L at STP.
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What is the pressure exerted by 3.00 moles of gas
at a temperature of 300. K in a 4.00 L container?
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What is the volume of a sample of gas if 5.00 moles if
it exerts a pressure of 0.500 atm at 200. K?
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A sample of gas is contained in a cylinder with a
volume of 10.0 L. At what temperature will 2.50
moles of contained gas exert 20.0 atm of
pressure on the container?
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A sample of gas contained in a cylinder of 5.00 L
exerts a pressure of 3.00 atm at 300. K. How
many moles of gas are trapped in the cylinder?
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R!
Any Questions?
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