It's a Gas.ppt

7/28/2019 It's a Gas.ppt

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ITS A GAS


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ITS A GAS


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Gases have some interestingcharacteristics that havefascinated scientists for 300 years.The first gas to be studied was air& it was a long time before it wasdiscovered that air was actually amixture of particles rather than asingle gas.

The Nature of Gases


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But this realization did not make thestudy of gas behavior more difficult.

Although air is a mixture of severaldifferent gases, it behaves much thesame as any single gas.

Regardless of their chemicalidentity, gases tend to exhibit similarphysical behaviors

The Nature of Gases


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The Nature of Gases

Gas particles can be monatomic (Ne),diatomic (N 2), or polyatomic (CH 4)

but they all have thesecharacteristics in common:1) Gases have mass.2) Gases are compressible.3) Gases fill their containers.4) Gases diffuse5) Gases exert pressure.6) Pressure is dependent on Temp.


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Kinetic Molecular Theory

There is a theory that modern daychemists use to explain the

behaviors and characteristics ofgases - the Kinetic MolecularTheory of Matter.

The word kinetic refers tomotion.

The word molecular refers tomolecules


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Kinetic Molecular Theory

The theory states that the tinyparticles in all forms of matter inall forms of matter are inconstant motion .This theory is used to explain the

behaviors common among gasesThere are 3 basic assumptions ofthe KMT as it applies to gases.


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KMT Assumption #1

A gas is composed of small hard particles.The particles have an insignificantvolume and are relatively far apartfrom one another.

There is empty space betweenparticles.No attractive or repulsive forcesbetween particles.


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KMT Assumption #3

All collisions a gas particleundergoes are perfectly elastic.No energy is lost from oneparticle to another, and the totalkinetic energy remains constant.


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Gases have mass. Gases seem to be weightless, but

they are classified as matter , which

means they have mass. The density of a gas the mass per

unit of volume is muchless than the density of aliquid or solid, however.


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Gases have mass.Its this very low density that allowsus to be able to walk through theroom without concerning ourselveswith air resistance.Since it is so easy to swim acrossthe room we dont put muchthought into the mass of a gas.Really it is only noticeable if we havea large collection of gas in acontainer.


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2nd Gases R squeezable

If you squeeze agas, its volume can

be reducedconsiderablyA gases low density

allows for there toa lot of emptyspace between gas

molecules.


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Gas particles have a high velocity,relative to their masses.

This gives them a lot of energy andmovement.The movement causes the gases tospread out , which leaves a lot ofspace between molecules.That empty space can becompressed by pressure allowinggas particles less room to movearound thus decreasing the

volume.


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3rd Gases fill their containers

Gases expand until they take up asmuch room as they possibly can.

Gases spread out to fill containersuntil the concentration of gases isuniform throughout the entire

space.This is why that nowhere aroundyou is there an absence of air.


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The Kinetic-Molecular theory alludesto this by the fact that theseparticles are in constant randommotion.Gases move in a straight line until itthey collide with other particles orthe sides of the container, whichcauses them to change directionsuntil they collide with something else.

This bouncing off of everythingaround them spread the particles outuntil they are uniform throughoutthe entire container.


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4th Gases diffuseGases can move through each otherrapidly.

The movement of one substancethrough another is called diffusion.Because of all of the empty spacebetween gas molecules, another gasmolecule can pass between themuntil each gas is spread out over theentire container.


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The same logic from theobservation that gases spread outapplies here.If the gases are in constant randommotion the fact that they aremoving and colliding witheverything around them then theywill mix with other gases uniformly. This doesnt happen at the same

speeds for all gases though.Some gases diffuse more rapidlythen other gases based on their sizeand their energy.


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5th Gases exert pressure

The sum of allof the

collisionsmakes up thepressure the

gas exerts.

Gas particles exert pressure bycolliding with objects in their

path.


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The Kinetic-Molecular theory alludesto this by the fact that theseparticles are colliding with anythingin their path.Imagine a gas in a container as aroom of hard rubber balls.The collisions of the balls bouncingaround exert a force on the objectthat with which they collide.

The definition of a pressure is aforce per unit area so the total ofall of the tiny collisions makes up thepressure exerted by the gas.


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The gases push against the walls oftheir containers with a force.

The pressure of gases is what keepsour tires inflated, makes ourbasketballs bounce, makes hairspray

come out of the can, etc.


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6th Pressure depends on Temp

The higher the temperature of a gas-the higher the pressure that the gas

exertsThe reverse of that is true as well,a the temperature of a gas decreases the pressure decreases .Think about the pressure of a set oftires on a car


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Pressure

Gauge

Todays Temp: 85 F


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6th Pressure depends on Temp

The reverse of that is true as well,a the temperature of a gas

decreases the pressuredecreases.Think about the pressure of a setof tires on a car


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- the average kinetic energy of theparticles that make up an object

Do you recall the definition oftemperature?

The higher the temperature themore the energyThe more the energy the more

impacts the gases administerThe more the impacts or collisionsthe more the pressure exerted.

The pressure increases when


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The pressure increases whentemperature increases because themolecules are moving with greaterspeed and colliding against the sidesof their containers more often.Therefore, the pressure inside thatcontainer is greater, because thereare more collisions.


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Gas variables

In order to describe a gas samplecompletely and then make

predictions about its behavior underchanged conditions, it is importantto deal with the values of:

1) amount of gas2) volume3) temperature

4) pressure


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Amount ( n)

The quantity of gas in a given sampleexpressed in terms of moles of gas.This of course is in terms of6.02 x 10 23 molecules of the gas.Dont forget to convert mass tomoles you just divide by the molarmass of the gas.


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Volume ( V)

The volume of the gas is simply thevolume of the container it is

contained in.The metric unit of volume is theliter (L)

There might also be problems thatuse cubic meters as the unit forvolume.

- 1 L = 1 dm3


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Temperature ( T)

The temperature of a gas is generallymeasured with a thermometer inCelsius.All calculations involving gasesshould be made after converting theCelsius to Kelvin temperature.

Kelvin = C + 273


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Atmospheric Pressure The gases in the air are exerting apressure called atmospheric pressure

Atmospheric pressure is a result ofthe fact that air has mass is and isattracted by gravity producing a force.

Knowing this atmospheric pressureand predicting changes in theatmospheric pressure is howforecasters predict the weather.


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Atmospheric Pressure Atmospheric pressure varies withaltitude

- the lower the altitude, the longerand heavier is the column of airabove an area of the earth.

Look on the back of a box of cakemix for the difference in baking timesbased on the atmospheric pressure inyour region.


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Atmospheric Pressure

Low pressure or dropping pressureindicates a change of weather fromfair to rain.High pressure is anindication of clearskies and sun.It all has to do withthe amount of airpressing down on us.


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Boyles Law

Robert Boyle was among the first tonote the relationship between

pressure and volume of a gas.He measured the volume of air atdifferent pressures, and observed apattern of behavior which led to hismathematical law.During his experiments Temperature and amount of gas werent allowed to

change


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As the pressureincreases

Volumedecreases


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1) determine which variables youhave:

P and V = Boyles Law

2)determine which law is beingrepresented:

P 1 = 2 atmV1 = 3.0 L

P 2 = 4 atmV2 = ?


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Charless Law

Jacques Charles determined therelationship between temperature

and volume of a gas.He measured the volume of air atdifferent temperatures, andobserved a pattern of behavior whichled to his mathematical law.During his experiments pressure ofthe system and amount of gas were

held constant V l f b ll V l f b ll


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Volume of balloonat room

temperature

Volume of balloonat 5C


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Charless Mathematical Law:

since V/T = k

Eg: A gas has a volume of 3.0 L at

127C. What is its volume at 227 C?

V 1 V 2

T 1 T 2=

What if we had a change in conditions?

) d h h bl


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T and V = Charless Law

2)determine which law is beingrepresented:

T1 = 127C + 273 = 400KV1 = 3.0 L

T2 = 227C + 273 = 5ooKV2 = ?

4) Pl i h i bl


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4) Plug in the variables:

(500K)(3.0L) = V2 (400K)V2 = 3.8L

3.0L V 2400K 500K

=

5) Cross multiply and chug


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Gay Lussacs Law

Old man Lussac determined therelationship between temperature

and pressure of a gas.He measured the temperature of airat different pressures, and observeda pattern of behavior which led to hismathematical law.During his experiments volume of thesystem and amount of gas were held

constant


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PressureGauge

Car before a trip

Think of a tire...

Lets get on the road

Dude!


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Car after a long trip

Think of a tire...

WHEW!

PressureGauge

How does Pressure and


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Temp

P r e s s u r e

How does Pressure andTemperature of gases relate

graphically?

P/T = k

Volume,

# of particlesremain constant


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Lussacs Mathematical Law:

What if we had a change in conditions?since P/T = k

P 1 P 2

T 1 T 2=

Eg: A gas has a pressure of 3.0 atm at

127 C What is its pressure at 227 C?

1) d t i hi h i bl


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T and P = Gay- Lussacs Law

T1 = 127C + 273 = 400KP 1 = 3.0 atm

T2 = 227C + 273 = 500KP 2 = ?


2)determine which law isbeing represented:


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LAW RELAT-IONSHIP LAWCON-

STANT

Boyles P V P 1V1 = P2V2 T, n

Charles V T V1/T 1 = V2/T 2 P, nGay- P T P 1/T 1 = P2/T 2 V, n
http://localhost/var/www/apps/conversion/tmp/Local%20Settings/Temporary%20Internet%20Files/Content.IE5/6M06IQCW/Combined%20and%20ideal%20gas%20laws.ppt

It's a Gas.ppt

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