The Nature of Gases

1
The Nature of Gases
2
The Nature of Gases

• All gases exhibit remarkably similar physical
  behavior
• Pure gases and mixtures behave the same!
• A mole of methane (CH4) in a balloon behaves just
  like a mole of air in a balloon!
• Because all gases behave the same, regardless of
  their chemical identities, we refer to them all
  as particles.
• Particles include single atoms (ex He), diatomic
  molecules (ex O2), and polyatomic molecules (ex
  CO2 ).

3
Properties and Characteristics of Gases

• 1. Gases have mass.
• 2. Gases are easy to compress.
• 3. Gases fill their containers completely.

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Properties and Characteristics of Gases

• 4. Gases move through other gases easily and
  rapidly.
• 5. Gases exert pressure.
• 6. Gas pressure depends on temperature.

5
Kinetic Molecular Theoryesp. gasses!

• gases (and other matter) are made up of tiny
  particles (atoms and molecules) that have mass
• the distance between particles is very large
• the particles are in constant, random motion
• collisions between particles and with container
  walls are elastic (no energy is lost to friction)
• kinetic energy of a gas depends on the
  temperature
• gas particles dont exert forces on each other
  (there is no attraction no IMF)

6

• https//twigcarolina.com/experiment/frozen-balloon
  -4210/

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Measuring Gases

• To completely describe a gas and make predictions
  about behavior when conditions are changed, four
  variables must be examined
• amount usually measured and described in terms
  of moles
• relate mass to moles using molar mass relate
  particles to moles using Avogadros number
• From the periodic table! 1 mole 6.02x1023
  particles
• volume usually measured in liters volume of
  gas is simply the volume of its container

8
remember metric prefixes and conversions

• 1000 L 1 kL
• 100 L 1 hL
• 10 L 1 daL
• 1 L 1 L
• 1 L 10 dL
• 1 L 100 cL
• 1 L 1000 mL
• Others on the front of your data book ?

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Measuring Gases

• temperature the average kinetic energy of the
  gas measured in lab on a Celsius thermometer,
  then converted to the Kelvin scale (recall that
  absolute zero is the temperature at which all
  molecular motion ceases)
• K C 273
• 25C 298 K 100C 373 K 225 K -48 C
• pressure from the collision of particles with
  the container walls each collision results in an
  outward push

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Pressure air and atmospheric pressure

• Atmospheric pressure is a result of the fact that
  gases are pulled on by gravity and the gas
  particles are constantly crashing into each other
  and you (and everything else).
• There is higher pressure at lower elevation
  because the column of air above you is longer
  (and therefore heavier!).

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Pressure

• Pressure is measured by a barometer (the height
  of the mercury column is balanced by the
  atmospheric pressure.)

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Calculating Pressure

• Pressure Force
• Area
• Common units pascals, kilopascals, atmospheres,
  millimeters of mercury (mm Hg), bars, torrs
• Conversions
• 1 N/m2 1 Pa (N/m2 Newton per square meter)
• 1000 Pa 1 kPa
• 1 atm 101.3 kPa
• 1 atm 101,325 Pa
• 1 atm 760 mm Hg

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Complete the following conversions using factor
labeling

• a. 2.2 atm ________kPa
• b. 103,442 Pa _________kPa
• c. 778 mm Hg _________ atm

2.2 atm x 101.3 kPa 222.86 kPa 220 kPa
1 atm
103442 Pa x 1 kPa 103.442 kPa
1000 Pa
778 mm Hg x 1 atm 1.02368 atm
1.02 atm 760 mm Hg
14

• https//twigcarolina.com/film/gas-laws-3612/

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Practice

• A researcher checks a barometer to find that
  atmospheric pressure is 1.05 atm at the beginning
  of an experiment involving gases. What is the
  pressure in millimeters of mercury?

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Enclosed Gases

• Open container gases can move freely, they will
  expand in response to added energy such that the
  pressure stays the same inside and out.
• Closed container gases cannot move freely, in
  response to added energy the pressure inside the
  container will increase relative to the outside.
• How would you describe the pressure of a gas in
  an expandable container like a balloon?

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Standard Temperature and Pressure

• Referred to as STP
• Temperature 0C or 273 K
• Pressure 1 atm or 760 mm Hg or 101.3 kPa
• What do you already know about gas at STP?

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Avogadros Law

• Equal volumes of gases at the same temperature
  and pressure contain an equal number of
  particles.
• all gases show the same physical behavior
• a larger volume is due to a larger number of
  particles (assuming P and T are constant)
• 1 mole of gas at STP 6.02 x 1023 particles
  22.4 L
• Because of this.

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Daltons Law of Partial Pressures

• The sum of the partial pressures of all the
  components in a gas mixture is equal to the total
  pressure of the gas mixture.
• PT Pa Pb Pc

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Example Problems

• 1. What is the atmospheric pressure if the
  partial pressures of nitrogen, oxygen, and argon
  are 604.5 mm Hg, 162.8 mm Hg, and 0.5 mm Hg,
  respectively? (remember, PT Pa Pb Pc)
• PT 604.5 mm Hg 162.8 mm Hg 0.5 mm Hg
  767.8 mm Hg
• 2. A mixture of gases contains 3 moles of Neon
  and 5 moles of Argon. If the total pressure is
  2.5 atm, what is the partial pressure of each
  component? (Hint neon contributes 3/8 of the
  total pressure)
• 2.5 atm is created by 8 moles of gas, so if Ne is
  3/8 and Ar is 5/8
• PNe (3/8) x 2.5 atm 0.94 atm
• PAr (5/8) x 2.5 atm 1.56 atm

PT 0.94 atm 1.56 atm 2.5 atm
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Collecting a gas over water

• Because gases mix readily with other gases, they
  must be collected in an environment where mixing
  cannot occur.
• The easiest way to do this is with water because
  water displaces the air in, say, a test tube.
• So when a gas is collected over water, that
  means the container (test tube) is filled with
  water and the gas is bubbled through the water
  into the container.
• However, water vapor will exist above the surface
  of the water water vapor
• Thus, the pressure inside the container is from
  the collected gas and the water vapor.
• This is where Daltons Law of Partial Pressures
  becomes useful.

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Collecting a gas over water
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Practice

• Hydrogen is collected over water in a collecting
  bottle. If the partial pressure of water vapor
  is 85 mm Hg when the total pressure of the gas is
  665 mm Hg, what is the partial pressure of the
  hydrogen gas in the bottle? (remember, PT Pa
  Pb Pc)

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Collecting a gas over water

• The vapor pressure from water depends on the
  temperature of the water. You can look it up if
  necessary.
• At room temperature (about 21C) how much
  pressure would be due to water vapor?