Gases and Atmospheric Chemistry

1
Gases and AtmosphericChemistry

• SCH 3U

2
Kinetic Molecular Theory (KMT)

• Attempts to explain why gases behave the way
  they do.
• 1) Gases are mostly empty space the volume of
• particles is negligible.
• 2) Gas particles are in constant random motion.
• 3) Gas particles neither attract nor repel each
• other.
• 4) Pressure is due to collisions of gas particles
• with container walls.
• 5) The average kinetic energy of a gas sample is
• proportional to the Kelvin Temperature

3
Pressure and Volume

• Boyles Law - At a constant temperature, the
  volume of a given mass of gas is inversely
  proportional to the pressure.
• V ? 1/P
• P1 V1 P2 V2
• where, P1 and V1 are the original pressure and
  volume and
• P2 and V2 are the new pressure and
  volume

4
Pressure

• The force exerted on an object per unit of
  surface area.
• The pressure of a gas is determined by the
  kinetic motion of its molecules. The molecules
  collide with the walls of the container.
• The collective number of collisions as well as
  the strength of the force determine the overall
  gas pressure.

5
Units of Pressure

• 760 mm Hg
• 760 torr
• 1 atm
• 101.3 kPa
• 14.7 lb/in2 (p.s.i.)
• Use equivalent fractions to convert between units
  of pressure

6
Volume and Temperature

• Charles Law - At a constant pressure, the
  volume of a given mass of gas is directly
  proportional to its absolute temperature.
• V ? T
• V1
  V2
• T1
  T2
• T1 and V1 are the initial temperature and
  volume and
• T2 and V2 are the final temperature
  and volume

7
Kelvin Temperature

• Temperature is a measure of the average kinetic
  energy of the particles within a substance.
• Absolute zero (-273?C) the temperature at which
  the average kinetic energy of all molecules is
  equal to zero. All molecular motion ceases.
• Kelvin scale a temperature scale beginning at
  absolute zero ( 0 K -273?C ) where each Kelvin
  unit is equal to 1?C.

8
Converting Temperature

• There are no degree signs used in the Kelvin
  scale and no negative values.
• TK ?C 273.15
• Try These

Temperature ?C Temperature K
Freezing point of water O
Life threatening fever 314
Oil temperature for French fries 162
Hottest day on record 331
Coldest day on record -88
9
Pressure and Temperature

• Gay-Lussacs Law - The pressure of a fixed
  amount of gas, at a constant volume, is directly
  proportional to its Kelvin temperature.
• P ? T
• P1 P2
• T1
  T2
• T1 and P1 are the initial temperature and
  pressure and
• T2 and P2 are the final temperature and
  pressure

10

• So far we have seen three gas laws

Jacques Charles
Robert Boyle
Joseph Louis Gay-Lussac
These are all subsets of a more encompassing law
the combined gas law
11
Time to Practice

• A 350 mL sample of helium gas is collected at
  22.0 oC and 99.3 kPa.  What volume would this gas
  occupy at STP?

12
The Ideal Gas Law

• PV nRT
• P Pressure (in kPa) V Volume (in L)
• T Temperature (in K) n moles

R is constant. If we are given three of P, V, n,
or T, we can solve for the unknown value. This
law allows us to involve the quantity of a gas
into our analysis.
13
Sample problems
How many moles of H2 is in a 3.1 L sample of H2
measured at 300 kPa and 20C?
PV nRT
P 300 kPa, V 3.1 L, T 293 K
(300 kPa)(3.1 L) n (8.31 kPaL/Kmol)(293 K)
n 0.38 mol
How many grams of O2 are in a 315 mL container
that has a pressure of 1215.9 kPa at 25C?
PV nRT
P 1215.9 kPa, V 0.315 L, T 298 K
n 0.1547 mol
0.1547 mol x 32 g/mol 4.95 g
14
Simulation

• http//www.chem.ufl.edu/itl/2045/MH_sims/gas_sim.
  html