IT’S A GAS…

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ITS A GAS
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ITS A GAS
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The Nature of Gases

• Gases have some interesting characteristics that
  have fascinated scientists for 300 years.
• The first gas to be studied was air it was a
  long time before it was discovered that air was
  actually a mixture of particles rather than a
  single gas.

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The Nature of Gases

• But this realization did not make the study of
  gas behavior more difficult.
• Although air is a mixture of several different
  gases, it behaves much the same as any single gas.

• Regardless of their chemical identity, gases tend
  to exhibit similar physical behaviors

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The Nature of Gases

• Gas particles can be monatomic (Ne), diatomic
  (N2), or polyatomic (CH4) but they all have
  these characteristics in common

1) Gases have mass.
2) Gases are compressible.
3) Gases fill their containers.
4) Gases diffuse
5) Gases exert pressure.
6) Pressure is dependent on Temp.
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Kinetic Molecular Theory

• There is a theory that modern day chemists use
  to explain the behaviors and characteristics of
  gases - the Kinetic Molecular Theory of Matter.
• The word kinetic refers to motion.
• The word molecular refers to molecules

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Kinetic Molecular Theory

• The theory states that the tiny particles in all
  forms of matter in all forms of matter are in
  constant motion.
• This theory is used to explain the behaviors
  common among gases
• There are 3 basic assumptions of the 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 insignificant volume and
  are relatively far apart from one another.
• There is empty space between particles.
• No attractive or repulsive forces between
  particles.

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KMT Assumption 2

• The particles in a gas move in constant random
  motion.
• Particles move in straight paths and are
  completely independent of each of other
• Particles path is only changed by colliding with
  another particle or the sides of its container.

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KMT Assumption 3

• All collisions a gas particle undergoes are
  perfectly elastic.
• No energy is lost from one particle to another,
  and the total kinetic 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 much less than the
  density of a liquid or solid,
  however.

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Gases have mass.

• Its this very low density that allows us to be
  able to walk through the room without concerning
  ourselves with air resistance.
• Since it is so easy to swim across the room we
  dont put much thought into the mass of a gas.
• Really it is only noticeable if we have a large
  collection of gas in a container.

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• The Kinetic-Molecular theory explanation of it is
  that we assume that gases are composed of a
  collection of particles.
• You cant see these particles directly, so they
  are very tiny, and to notice any mass you must
  weigh a collection of the particles.
• It is usually necessary to have a mole or more of
  gas particles to have significant a significant
  change in mass.

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2nd Gases R squeezable

• If you squeeze a gas, its volume can be reduced
  considerably
• A gases low density allows for there to a lot of
  empty space between gas molecules.

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• Gas particles have a high velocity, relative to
  their masses.
• This gives them a lot of energy and movement.
• The movement causes the gases to spread out,
  which leaves a lot of space between molecules.
• That empty space can be compressed by pressure
  allowing gas particles less room to move around
  thus decreasing the volume.

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• This empty space can be compressed simply by
  adding pressure.
• We can use this ability of a gas to do work for
  us.
• Think of a shocks on a car. You really
  are riding on a pillow of air.
• A bump in the road compresses the gas in
  the shocks until the bumps
  energy is absorbed.

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3rd Gases fill their containers

• Gases expand until they take up as much room as
  they possibly can.
• Gases spread out to fill containers until the
  concentration of gases is uniform throughout the
  entire space.
• This is why that nowhere around you is there an
  absence of air.

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• The Kinetic-Molecular theory alludes to this by
  the fact that these particles are in constant
  random motion.
• Gases move in a straight line until it they
  collide with other particles or the sides of the
  container, which causes them to change directions
  until they collide with something else.
• This bouncing off of everything around them
  spread the particles out until they are uniform
  throughout the entire container.

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• If I opened up a bag of popcorn in front of the
  class you would soon be able to smell it in the
  back.
• The popcorn smell is a high energy molecule or
  group of molecules that is in the gas state.
• There are really two properties going on here
• - This property of gases spreading out until
  they have filled the room
• - And the property of diffusion

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4th Gases diffuse

• Gases can move through each other rapidly.
• The movement of one substance through another is
  called diffusion.
• Because of all of the empty space between gas
  molecules, another gas molecule can pass between
  them until each gas is spread out over the entire
  container.

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• The same logic from the observation that gases
  spread out applies here.
• If the gases are in constant random motion the
  fact that they are moving and colliding with
  everything around them then they will mix with
  other gases uniformly.
• This doesnt happen at the same speeds for all
  gases though.
• Some gases diffuse more rapidly then other gases
  based on their size and their energy.

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• Diffusion explains why gases are able to spread
  out to fill their containers.
• Its why we can all breath oxygen anywhere in the
  room.
• It also helps us avoid potential
  odoriferous problems.

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5th Gases exert pressure

• Gas particles exert pressure by colliding with
  objects in their path.

• The sum of all of the collisions makes up the
  pressure the gas exerts.

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• The Kinetic-Molecular theory alludes to this by
  the fact that these particles are colliding with
  anything in their path.
• Imagine a gas in a container as a room of hard
  rubber balls.
• The collisions of the balls bouncing around exert
  a force on the object that with which they
  collide.
• The definition of a pressure is a force per unit
  area so the total of all of the tiny collisions
  makes up the pressure exerted by the gas.

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• The gases push against the walls of their
  containers with a force.
• The pressure of gases is what keeps our tires
  inflated, makes our basketballs 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 exerts
• The reverse of that is true as well, a the
  temperature of a gas decreases the pressure
  decreases.
• Think about the pressure of a set of tires on a
  car

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Todays temp 35F
Pressure Gauge
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Todays temp 85F
Pressure Gauge
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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 pressure
  decreases.
• Think about the pressure of a set of tires on a
  car

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• Do you recall the definition of temperature?

• the average kinetic energy of the particles that
  make up an object

• The higher the temperature the more the energy
• The more the energy the more impacts the gases
  administer
• The more the impacts or collisions the more the
  pressure exerted.

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• The pressure increases when temperature increases
  because the molecules are moving with greater
  speed and colliding against the sides of their
  containers more often.
• Therefore, the pressure inside that container is
  greater, because there are more collisions.

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Measuring Gases

• The conditions under which a gas is studied is
  very important to its behavior.
• Experimental work in chemistry requires the
  measurement of such quantities as volume,
  temperature, pressure, and the amount of sample.
• These quantities are called variables and if they
  are not accounted for then the results of the
  experiment might be jeopardized.

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Gas variables

• In order to describe a gas sample completely and
  then make predictions about its behavior under
  changed conditions, it is important to deal with
  the values of

1) amount of gas
2) volume
3) temperature
4) pressure
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Amount (n)

• The quantity of gas in a given sample expressed
  in terms of moles of gas.
• This of course is in terms of 6.02 x 1023
  molecules of the gas.
• Dont forget to convert mass to moles you just
  divide by the molar mass of the gas.

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Volume (V)

• The volume of the gas is simply the volume of the
  container it is contained in.
• The metric unit of volume is the liter (L)
• There might also be problems that use cubic
  meters as the unit for volume.
• - 1 L 1 dm3

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Temperature (T)

• The temperature of a gas is generally measured
  with a thermometer in Celsius.
• All calculations involving gases should be made
  after converting the Celsius to Kelvin
  temperature.

Kelvin C 273
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Pressure (P)

• The pressure of a gas is the force exerted on the
  wall of the container a gas is trapped in.
• There are several units for pressure depending on
  the instrument used to measure it including

1) atmospheres (atm)
2) Millimeters of Mercury (mmHg)
3) Kilopascal (kPa)
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S T P

• The behavior of a gas depends very strongly on
  the temperature and the pressure at which the gas
  is held.
• To make it easier to discuss the behavior of a
  gas, it is convenient to designate standard
  conditions, called STP.

- Temperature 0C or 273K
- Pressure 1atm or 760mmHg or 101.3kPa
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Atmospheric Pressure

• The gases in the air are exerting a pressure
  called atmospheric pressure
• Atmospheric pressure is a result of the fact that
  air has mass is and is attracted by gravity
  producing a force.
• Knowing this atmospheric pressure and predicting
  changes in the atmospheric pressure is how
  forecasters predict the weather.

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Atmospheric Pressure

• Atmospheric pressure varies with altitude
• - the lower the altitude, the longer and heavier
  is the column of air above an area of the earth.
• Look on the back of a box of cake mix for the
  difference in baking times based on the
  atmospheric pressure in your region.

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Atmospheric Pressure

• Low pressure or dropping pressure indicates a
  change of weather from fair to rain.
• High pressure is an indication
  of clear skies and
  sun.
• It all has to do with
  the amount of air
  pressing down on us.

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Gas Laws

• Studies of the behavior of gases played a major
  role in the development of physical sciences in
  the 7th and 8th centuries.
• The Kinetic Molecular theory marked a significant
  achievement in understanding the behavior of
  gases.
• Observations have become mathematical laws which
  we can use to predict quantitative outcomes.

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Boyles Law

• Robert Boyle was among the first to note the
  relationship between pressure and volume of a
  gas.
• He measured the volume of air at different
  pressures, and observed a pattern of behavior
  which led to his mathematical law.
• During his experiments Temperature and amount of
  gas werent allowed to change

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How does Pressure and Volume of gases relate
graphically?
PV k
Temperature, of particles remain constant
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Boyles Mathematical Law
What if we had a change in conditions?
since PV k
P1V1 P2V2
Eg A gas has a volume of 3.0 L at 2 atm. What
is its volume at 4 atm?
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• determine which variables you have

• P1 2 atm
• V1 3.0 L
• P2 4 atm
• V2 ?

• determine which law is being represented

P and V Boyles Law
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3) Rearrange the equation for the variable you
dont know
4) Plug in the variables and chug it on a
calculator
V2 1.5L
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Charless Law

• Jacques Charles determined the relationship
  between temperature and volume of a gas.
• He measured the volume of air at different
  temperatures, and observed a pattern of behavior
  which led to his mathematical law.
• During his experiments pressure of the system and
  amount of gas were held constant.

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How does Temperature and Volume of gases relate
graphically?
V/T k
Pressure, of particles remain constant
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Charless Mathematical Law
What if we had a change in conditions?
since V/T k
Eg A gas has a volume of 3.0 L at 127C. What
is its volume at 227 C?
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• determine which variables you have

• T1 127C 273 400K
• V1 3.0 L
• T2 227C 273 5ooK
• V2 ?

• determine which law is being represented

T and V Charless Law
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4) Plug in the variables
5) Cross multiply and chug
(500K)(3.0L) V2 (400K)
V2 3.8L
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Gay Lussacs Law

• Old man Lussac determined the relationship
  between temperature and pressure of a gas.
• He measured the temperature of air at different
  pressures, and observed a pattern of behavior
  which led to his mathematical law.
• During his experiments volume of the system and
  amount of gas were held constant.

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Think of a tire...
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Think of a tire...
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How does Pressure and Temperature of gases relate
graphically?
P/T k
Volume, of particles remain constant
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Lussacs Mathematical Law
What if we had a change in conditions?
since P/T k
Eg A gas has a pressure of 3.0 atm at 127º C.
What is its pressure at 227º C?
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• determine which variables you have

• T1 127C 273 400K
• P1 3.0 atm
• T2 227C 273 500K
• P2 ?

• determine which law is being represented

T and P Gay-Lussacs Law
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4) Plug in the variables
5) Cross multiply and chug
(500K)(3.0atm) P2 (400K)
P2 3.8atm
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Summary