ITS A GAS

1
ITS A GAS
2
ITS A GAS
3
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.
• 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.

4
The Nature of Gases

• Regardless of their chemical identity, gases tend
  to exhibit similar physical behaviors
• Gas particles can be monatomic (Ne), diatomic
  (N2), or polyatomic (CH4) but they all have
  some common characteristics

• Gases have mass.
• Gases are compressible.
• Gases fill their containers.
• Gases diffuse.
• Gases exert pressure.
• Pressure is related to Temperature

5
Kinetic Molecular Theory

• There is a theory that modern day chemists use
  to explain the behaviors and characteristics of
  ideal gases - the Kinetic Molecular Theory of
  Matter.
• The theory states that the tiny particles in all
  forms of matter in all forms of matter are in
  constant motion.
• There are 3 basic assumptions of the KMT as it
  applies to ideal gases.
• Ideal gases are perfect gases that are used as
  a model to describe characteristics of real gases.

6
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.

7
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.

8
KMT Assumption 3

• All collisions a gas particle undergoes are
  perfectly elastic.
• They exert a pressure but dont lose any energy
  during the collisions.

9
The Kinetic Molecular Theory
10
Gases have mass.

• Gases appear weightless, but they are classified
  as matter, therefore, they must have mass.
• The density of a gas is much less than the
  density of a liquid or solid, however.
• Its this very low density that allows us to be
  able to walk through the room without concerning
  ourselves with air resistance.
• The mass is really only noticeable if we have a
  collection of gas in a container.

11
Gases have mass.
12
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.
• Gas particles have a high
  velocity, relative to their
  masses.
• They are in constant
  motion

13
Gases R squeezable

• The movement causes the gases to spread out,
  which leaves a lot of space between molecules.
• That empty space can
  be compressed by pres-
  sure allowing gas
  particles less room to
  move around thus
  decreasing the volume.
• The empty space can be
  compressed by adding a pressure.

14
Gases R squeezable

• 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 compres-
  ses the gas in the shocks
  until the bumps energy is
  absorbed.

15
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 no-
  where around you is
  there an absence of air.

16
Gases fill their containers

• Since the particles are in constant random
  motion, according to the KMT, then the gases move
  in a straight line until they collide with other
  particles or the sides of the container, which
  causes them to change directions until they
  collide with something else.
• This random bouncing motion, allows for the
  mixing up and spreading of the particles until
  they are uniform throughout the entire container.

17
Gases fill their containers

• 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

18
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.
• Since gases are in constant random motion, they
  are moving and colliding with everything around
  them, and there is so much empty space, the gases
  mix uniformly.

19
Gases diffuse

• 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.
• 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.

20
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.
• The KMT says since they are in constant random
  motion, the particles will colliding with
  anything in their path.
• The definition of pressure is
  the force per unit area so the
  total of all of the tiny
  collisions makes up the
  pressure exer- ted by the
  gas

21
Gases exert pressure

• Its the pressure exerted by the gases that hold
  the walls of a container out
• The pressure of gases is what keeps our tires
  inflated, makes our basketballs bounce, makes
  hairspray come out of the can, helps our lungs
  inflate, allow vacuum cleaners to work, etc.

22
Pressure depends on Temp

• Do you recall the definition of temperature?
• the average kinetic energy of the particles that
  make up an object
• Therefore, the higher the temperature the more
  energy the gas particle has.
• So the collisions are more often and with a
  higher force.
• And since pressure is a function of force, the
  pressure increases inside the container.
• Think about the pressure of a set of tires on a
  car.

23
Pressure depends on Temp
Todays temp 35F
Pressure Gauge
24
Pressure depends on Temp
Todays temp 85F
Pressure Gauge
25
Characteristics of Gases
26
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.

27
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 the gas
2) volume
3) temperature
4) pressure
28
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.
• So amount of a gas, refers
  to how many gas particles
  are in the sample.

29
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.
• - 1000 L 1 m3

30
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
31
Temperature (T)
32
Pressure (P)

• The pressure of a gas is the force exerted on the
  wall of the container, in which a gas is trapped.
• 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)
33
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 a set of
  standard conditions, called STP.
• Standard Temp and Standard Pressure
• Standard Temperature 0C or 273K
• Standard Pressure 1atm or 760mmHg or 101.3kPa
  (depends on the method of measure)

34
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.
• Atmospheric pressure is
  measured with a barometer.

35
Atmospheric Pressure
36
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.

37
Atmospheric Pressure
38
Atmospheric Pressure

• Knowing this atmospheric pressure and predicting
  changes in the atmospheric pressure is how
  forecasters predict the weather.
• Low pressure or dropping pressure indicates a
  change of weather from
  fair to rain.
• High pressure is an
  indication of clear
  skies and sun.

39

Measuring Gases
40
Measuring Gases
41
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.

42
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

43
Boyles Law
44
How does Pressure and Volume of gases relate
graphically?
PV k
Temperature, of particles remain constant
45
Boyles Law
46
Boyles Mathematical Law
If we have a given amount of a gas at a starting
pressure and volume, what would happen to the
pressure if we changed the volume? Or to the
volume if we changed the pressure?
since PV equals a constant
P1V1 P2V2
Eg A gas has a volume of 3.0 L at 2 atm. What
will its volume be at 4 atm?
47
Boyles Mathematical Law

• List the variables or clues given

• P1 2 atm
• V1 3.0 L

• P2 4 atm
• V2 ?

• determine which law is being represented

P1V1 V2 P2
3) Plug in the variables calculate
(2 atm)
(3.0 L)
(4 atm)
(V2)
1.5 L
48
Charless Law

• Jacques Charles studied 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.

49
Charless Law
50
How does Temperature and Volume of gases relate
graphically?
V/T k
Pressure, of particles remain constant
51
Charless Mathematical Law
If we have a given amount of a gas at a starting
volume and temperature, what would happen to the
volume if we changed the temperature? Or to the
temperature if we changed the volume?
since V/T k
Eg A gas has a volume of 3.0 L at 400K. What
is its volume at 500K?
52
Charless Mathematical Law

• List the variables or clues given

• T1 400K
• V1 3.0 L

• T2 500K
• V2 ?

• determine which law is being represented

3) Plug in the variables calculate
3.0L
3.8 L
53
Gay-Lussacs Law

• Old man Lussac studied the relationship between
  temperature and pressure of a gas.
• He measured the pressure of air at different
  temperatures, 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.

54
Think of a tire...
55
Think of a tire...
56
How does Pressure and Temperature of gases relate
graphically?
P/T k
Volume, of particles remain constant
57
Gay-Lussacs Mathematical Law
If we have a given amount of a gas at a starting
temperature and pressure, what would happen to
the pressure if we changed the temperature? Or
to the temp. if we changed the pressure?
since P/T k
Eg A gas has a pressure of 3.0atm
at 400K. What is its pressure at 500K?
58
Gay-Lussacs Mathematical Law

• List the variables or clues given

• T1 400K
• P1 3.0 atm

• T2 500K
• P2 ?

• determine which law is being represented

3) Plug in the variables calculate
3.0atm
3.8 atm
59
Summary of the Named Gas-Laws