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PressurePressure
Force applied to one unit of surface areaForce applied to one unit of surface area
Pressure = Pressure = ForceForce Area Area
More thoughts…More thoughts…1.1. When on thin ice should you lay When on thin ice should you lay
out and crawl or run like mad?out and crawl or run like mad?2.2. More pressure – a woman in high More pressure – a woman in high
heels or an elephant? heels or an elephant? 3.3. How many different ways could How many different ways could
you increase tire pressure? you increase tire pressure? 4.4. Which way will the book exert the Which way will the book exert the
most pressure flat or up and most pressure flat or up and down?down?
5.5. Which is the best tool for chopping Which is the best tool for chopping wood – a hammer or an ax of the wood – a hammer or an ax of the same weight? same weight?
6.6. Mud tires – Skinny or Wide? Mud tires – Skinny or Wide? 7.7. Bed of nails – 200 or 20 more Bed of nails – 200 or 20 more
comfortable? comfortable?
In a gas, pressure is…In a gas, pressure is…Caused by particles colliding with the sides of Caused by particles colliding with the sides of the container. the container.
The more collisions The more collisions particles have the particles have the greater the greater the pressure.pressure.
Units of PressureUnits of Pressure
psi (pounds per square inch)psi (pounds per square inch)
atm (atmospheres)atm (atmospheres)
mmHg (mm of Mercury)mmHg (mm of Mercury)
kPa (kiloPascal)kPa (kiloPascal)
Pa (Pascal)Pa (Pascal)
torrtorr
VolumeVolumeThe amount of space occupied by a The amount of space occupied by a
substancesubstanceIf a gas is a non-rigid container then its If a gas is a non-rigid container then its
volume can change.volume can change.Ex of non-rigid: balloonEx of non-rigid: balloon
If a gas is in a rigid container then its volume If a gas is in a rigid container then its volume cannot change.cannot change.
Rigid container: steel canisterRigid container: steel canister
Units of VolumeUnits of Volume
mL (milliliters)mL (milliliters)
L (Liters)L (Liters)
cmcm33 or cc (cubic centimeters) or cc (cubic centimeters)
How are pressure How are pressure and volume related and volume related when temperature when temperature is held constant?is held constant?
BoyleBoyle’’s Laws Law
At constant temperature, the product of At constant temperature, the product of the pressure and the volume of a gas the pressure and the volume of a gas
sample is a constant value.sample is a constant value.
BoyleBoyle’’s Laws Lawexplained using kinetic molecular theoryexplained using kinetic molecular theory
Relates pressure to volumeRelates pressure to volume
When you make the volume of a container When you make the volume of a container smaller:smaller:
Particles get closer togetherParticles get closer together
Therefore having more collisionsTherefore having more collisions
Which results in higher pressure.Which results in higher pressure.
So when V decreases, Pressure increases.So when V decreases, Pressure increases.
BoyleBoyle’’s Laws Law
An example of this:An example of this:
Think about the marshmallows in the syringe. Think about the marshmallows in the syringe. You pushed down on the syringe what You pushed down on the syringe what
happened to the marshmallows?happened to the marshmallows?
They got smaller – why? By pushing down They got smaller – why? By pushing down on the syringe you were increasing pressure, on the syringe you were increasing pressure, forcing the air particles in the marshmallow to forcing the air particles in the marshmallow to
get closer together.get closer together.
An inverse relationship is when one variable An inverse relationship is when one variable increases and the other variable decreases. increases and the other variable decreases. They arenThey aren’’t doing the same thing.t doing the same thing.
BoyleBoyle’’s Laws Law
In Equation form:In Equation form:
PP11VV11 = P = P22VV22
Temperature must be held constant!Temperature must be held constant!
We also assume that the number of gas We also assume that the number of gas molecules remains constant.molecules remains constant.
Practice with BoylePractice with Boyle’’s Laws LawGiven the following data find the missing Given the following data find the missing
variable.variable.
PP11 = 500mmHg = 500mmHg V V11 = 250mL = 250mL
PP22 = 1000 mmHg V = 1000 mmHg V22 = = ??
Practice with BoylePractice with Boyle’’s Laws LawA certain steel gas tank in a chemistry laboratory has a A certain steel gas tank in a chemistry laboratory has a
volume of 2.5L and it contains oxygen gas at a pressure volume of 2.5L and it contains oxygen gas at a pressure of 45atm. What volume would the gas from such a tank of 45atm. What volume would the gas from such a tank occupy at the classroomoccupy at the classroom’’s pressure, 1 atm? Assume the s pressure, 1 atm? Assume the
temperature remains constant.temperature remains constant.
Review - VolumeReview - Volume
The amount of space occupied by a The amount of space occupied by a substancesubstance
Units:Units:
mL (milliliters)mL (milliliters)
L (Liters)L (Liters)
cmcm33 or cc (cubic centimeters) or cc (cubic centimeters)
TemperatureTemperature
Extent of Extent of ““hotnesshotness”” or or ““coldnesscoldness”” of a of a sample, which is related to the kinetic energy sample, which is related to the kinetic energy
of the particlesof the particles
Increase in temperature meansIncrease in temperature means
more movement and greater kinetic energymore movement and greater kinetic energy
UnitsUnits
˚F, ˚C, and K˚F, ˚C, and K
Temperature vs Volume of a Gas
0
2
4
6
8
10
12
-300 -250 -200 -150 -100 -50 0 50 100
Temperature (degrees C)
Volum
e (L)
5 g of oxygen
10 g of oxygen
5 g of nitrogen
10 g of nitrgoen
Lord KelvinLord Kelvin
0K is absolute zero0K is absolute zero
Scientists believe this is the temp Scientists believe this is the temp were all particle motion stops were all particle motion stops and the volume of the gas is 0.and the volume of the gas is 0.
0K = -2730K = -273˚C˚C
Therefore to convert from ˚C to KTherefore to convert from ˚C to K
K = ˚C + 273K = ˚C + 273
WORD!
How are volume How are volume and temperature and temperature
related when related when pressure is held pressure is held
constant?constant?
CharlesCharles’’ Law LawThe volume of a gas sample at constant The volume of a gas sample at constant
pressure is directly proportional to its kelvin pressure is directly proportional to its kelvin temperaturetemperature
CharlesCharles’’ Law Law explained using kinetic molecular theoryexplained using kinetic molecular theory
As you increase the temperature:As you increase the temperature:
Particles move fasterParticles move faster
In order for pressure to remain constant those In order for pressure to remain constant those particles will have to move further awayparticles will have to move further away
Which results in larger volumeWhich results in larger volume
So when Temperature increases, Volume So when Temperature increases, Volume increases.increases.
ChuckChuck’’s Laws Law
0 Kelvin's = -273 C
pv simulaiton
sterling engine
CharlesCharles’’ Law Law
Think about the balloon you put in the hot pot Think about the balloon you put in the hot pot of water. What happened to it?of water. What happened to it?
It got bigger – why? It got bigger – why?
When the balloon is placed in the hot water, When the balloon is placed in the hot water, the air inside the balloon warms. As it warms the air inside the balloon warms. As it warms
the particles move faster causing it to the particles move faster causing it to expand.expand.
CharlesCharles’’ Law LawIn Equation form:In Equation form:
VV11 = = VV22 T T1 1
TT2 2
Pressure must be held constant!Pressure must be held constant!
Number of molecules must be the same.Number of molecules must be the same.
Temperature MUST be in Kelvin!Temperature MUST be in Kelvin!
Practice with CharlesPractice with Charles’’ Law LawIn the party store a balloon contains 3200mL of In the party store a balloon contains 3200mL of
helium gas at 18helium gas at 18C (65 C (65 F). What is the new F). What is the new volume of the balloon when you take it outside volume of the balloon when you take it outside
on a hot summer day 38on a hot summer day 38C (100 C (100 F)? F)?
Assume that the pressure remains constant.Assume that the pressure remains constant.
Practice with CharlesPractice with Charles’’ Law LawIf a sample of gas has a volume of 1000 mL when If a sample of gas has a volume of 1000 mL when
the pressure is 720 mmHg, what is its pressure the pressure is 720 mmHg, what is its pressure when the volume has been reduced to 500mL? when the volume has been reduced to 500mL?
Assume that the pressure remains constant.Assume that the pressure remains constant.
PressurePressureForced exerted by a gas Forced exerted by a gas which is caused by particle which is caused by particle colliding with the sides of the colliding with the sides of the container.container.
UnitsUnitspsi (pounds per square inch)psi (pounds per square inch)atm (atmospheres)atm (atmospheres)mmHg (mm of Mercury)mmHg (mm of Mercury)kPa (kiloPascal)kPa (kiloPascal)Pa (Pascal)Pa (Pascal)
TemperatureTemperature
Extent of Extent of ““hotnesshotness”” or or ““coldnesscoldness”” of a of a sample.sample.
Temperature MUST be in Kelvin!Temperature MUST be in Kelvin!
K = K = ˚C + 273˚C + 273
Gay LussacGay Lussac’’s Laws Law
The pressure of a gas sample at constant The pressure of a gas sample at constant volume is directly proportional to its kelvin volume is directly proportional to its kelvin
temperaturetemperature
Gay LussacGay Lussac’’s Laws Law explained using kinetic molecular theoryexplained using kinetic molecular theory
As temperature increasesAs temperature increases
Gas particles move fasterGas particles move faster
If in a rigid container, volume CANNOT changeIf in a rigid container, volume CANNOT change
Resulting in more collisionsResulting in more collisions
Creating a higher pressureCreating a higher pressure
Why does the Why does the label on this label on this can warn the can warn the user to not user to not put it in a put it in a
fire?fire?
What can What can happen to a car happen to a car
tire on a hot tire on a hot summer day, summer day,
especially if you especially if you have traveled for have traveled for many hundreds many hundreds
of miles? of miles?
Gay LussacGay Lussac’’s Laws LawIn Equation form:In Equation form:
PP11 = = PP2 2 T T1 1
TT2 2
Volume must be held constant!Volume must be held constant!
Number of molecules cannot change.Number of molecules cannot change.
Temperature MUST be in Kelvin!Temperature MUST be in Kelvin!
Practice Gay LussacPractice Gay Lussac’’s Laws Law
The gas in an aerosol can is at a pressure of 3.0 The gas in an aerosol can is at a pressure of 3.0 atm and 25 atm and 25 C. Directions on the can warn the C. Directions on the can warn the user NOT to keep the can in a place where the user NOT to keep the can in a place where the temperature can exceed 52temperature can exceed 52C (125C (125F). What F). What
would the gas pressure be at 52would the gas pressure be at 52C that causes C that causes the can to explode?the can to explode?
Practice with BoylePractice with Boyle’’s Laws LawWhat pressure is required to reduce the volume of a sample of What pressure is required to reduce the volume of a sample of
air from 1.0L to .25 L? The original pressure on the air from 1.0L to .25 L? The original pressure on the sample is 1.5 atm. Assume the temperature of the sample is 1.5 atm. Assume the temperature of the
sample remains constant.sample remains constant.