Gases

1
Gases

• Properties of Gases

2
Properties of Gases

• Model used to described the reasons why a gas
  behaves the way is does
• Kinetic Molecular Theory of Gases
• (1) A gas consists of tiny particles, either
  atoms or molecules, moving about at random
• (2) Attractive forces b/w the particles of gas
  can be neglected
• (3) Volume occupied by gas molecules is small
• (4) Avg. kinetic energy of gas molecules is
  proportional to Kelvin Temp.
• (5) Gas particles in constant motion
• derive quantitatively the ideal gas law.

3
Properties of Gas

• Described by four properties pressure, volume,
  temperature, and the amount of gas

4
Gas Pressure

• Gas particles hitting the walls of a container,
  exert Pressure
• Force acting on a certain area
• Pressure (P) force/area
• Atmospheric Pressure mass of air particles,
  collide with the surface of the Earth
• Can be measured using a barometer/ atmospheric
  pressure is 760 mm Hg (unit atmosphere atm)
• 1 atm can be expressed as 760 torr
• Torr and mm Hg is used interchangeably
• 1 atm 760 mm Hg 760 torr
• 1 mm Hg 1 torr
• SI units for pressure is measured in Pascals
  (Pa) 1 atm 101, 325 Pa (due to it being a
  small unit, pressure is normally reported in
  kilopascals

5
Units for Measuring Pressure

• Atm pressure changes with weather and altitude
• Sunny day, air is more dense, which is more
  pressure on the mercury surface etc.

6
Units of Pressure

• Study Check What is pressure in atmospheres for
  a gas that has a pressure of 655 torr????

7
Boyles Law

• Analogy Pressure inside a bicycle pump (air
  particles compressed)
• Witness a Inverse Relationship- a change in one
  property a (volume) causes a change in the
  direction of another property (pressure
• Relationship of pressure and volume
• P1V1 P2V2
• States that volume (V) of a sample gas changes
  inversely with the pressure (P) of the gas as
  long as there was no change in temperature (T) or
  the amount of gas (n)

8
Calculating Pressure When Volume Changes

• Study Check
• A sample of helium gas has a volume of 150mL at
  750 torr. If the volume expands to 450mL at
  constant temperature, what is the new pressure in
  torr??????

9
Charles Law

• Analogy Hot-air balloon (air is heated,
  temperature rises, air particles move, causing
  the volume of the balloon to increase
• Jacques Charles proposed that the volume (V) of
  gas is related to temperature (T) when there is
  no change in pressure (P) or the amount (n) of
  gas
• Charles Law V1/T1 V2/T2

10
Calculating Volume When Temperature Changes

• Study ChecK
• A mountain climber inhales 486 mL of air at a
  temperature of -8 degrees Celsius. What volume
  will the air occupy in the lungs if the climbers
  body temperature is 37 degrees Celsius???????

11
Gay-Lussacs Law

• -Watching the molecules of a gas as temperature
  rises, realize how quickly they move
• Gay-Lussacs Law- relates temperature and
  pressure states that the pressure of gas is
  related to its Kelvin temperature
• Equation P1/T1 P2/T2
• Meaning- increase in temp increases the pressure
  of a gas, and a decrease in temp decreases the
  pressure of gas
• The ratio of pressure (P) to temperature (T) is
  the same under all conditions as long as volume
  (V) and the amount of gas (n) do not change

12
Calculating Pressure When Temp. Changes

• Study Check Homework
• In a storage area where the temperature has
  reached 55 degrees Celsius, the pressure of
  oxygen gas in a 15.0-L steel cylinder is 965
  torr. To what temperature would the gas have to
  cooled to reduce the pressure to 850 torr?
• ?????

13
Combined Gas Law

• Pressure-volume-temperature relationships can be
  combined into one single relationship known as
  combined gas law
• Equation P1V1/T1 P2V2/T2
• HOMEWORK
• Study Check A weather balloon is filled with
  15.0 L of helium at a temperature of 25 degrees
  Celsius and a pressure of 685 mm Hg. What is the
  pressure of the helium in the balloon in the
  upper atmosphere when the temperature is -35
  degree Celsius and the volume becomes 34.0
  L?????????

14
Avogadros Law

• Volume and moles
• Considering how the properties of gas change when
  there is a change in the number of moles or grams
• States that the volume of gas is related to the
  number of moles of a gas when the temperature and
  pressure do not change
• Equation V1/n1 V2/n2

15
Calculating Volume for a Change in Moles

• HOMEWORK!!!!
• Study Check At a certain temperature and
  pressure, 8.00g oxygen gas has a volume of
  5.00-L. What is the volume after 4.00g oxygen gas
  is added to the balloon???

16
STP and Molar Volume

• Using Avogadros number, two gases will have
  equal volumes if they contain the same number of
  moles of gas at the same temp. and pressure
• Compare different gases we use a condition called
  standard temp. (273K) and pressure (1 atm) STP
• STP Conditions
• Standard Temp 0 degrees (273 K)
• Standard Pressure is 1 atm (760 mm Hg)
• 1 mole of any gas has a volume of 22.4 L which is
  called Molar Volume
• Molar Volume Volume of 1 mole of gas at STP
  22.4 L

17
Conversion Factors

• Study Check How many moles of nitrogen (N2) are
  present in 5.6 L of the gas at STP

18
Partial Pressure (Daltons Law)

• Mixtures of Gas such as the air we breathe
• All gas particles behave in the same way
• Total Pressure of gases in a mixture results from
  collisions of the gas particles
• Daltons Law states that the total pressure of
  gas is the sum of the Partial Pressures of the
  gases in the mixture
• Ptotal P1 P2 P3
• Partial Pressure- in a gas mixture, each gas
  exerts pressure, which is the pressure it would
  exert if it were the only gas in the container

19
(No Transcript)