1. Gases and Atmospheric Chemistry
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
Boyle’s 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 it’s 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
T 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
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-Lussac’s Law - The pressure of a fixed amount of gas, at a constant volume, is directly proportional to its Kelvin temperature.
P  T
P1 = P2
T T2
T1 and P1 are the initial temperature and pressure and
T2 and P2 are the final temperature and pressure

10. These are all subsets of a more encompassing law: the combined gas law
So far we have seen three gas laws:
Robert Boyle
Jacques Charles
Joseph Louis Gay-Lussac V1 T1 = V2 T2 P1 T1 = P2 T2
P1V1 =
P2V2
These are all subsets of a more encompassing law: the combined gas law
P1V1 P2V2
T T2 =

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 = 8.31 kPa • L
K • mol
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 20°C?
PV = nRT
P = 300 kPa, V = 3.1 L, T = 293 K
(300 kPa)(3.1 L) = n (8.31 kPa•L/K•mol)(293 K)
(8.31 kPa•L/K•mol)(293 K)
(300 kPa)(3.1 L)
= n = 0.38 mol
How many grams of O2 are in a 315 mL container that has a pressure of 12 atm at 25°C?
PV = nRT
P= kPa, V= L, T= 298 K
(8.31 kPa•L/K•mol)(298 K)
( kPa)(0.315 L)
= n = mol
mol x 32 g/mol = 4.95 g

14. Simulation