Gases
Kinetic-Molecular Theory of Gases
Notes#5
 All particles are in constant motion.
 As temperature increases kinetic energy
?
increases
 As gas particles move apart the volume ? increases
Point Masses
 Gas particles are treated as a point with no
volume and no mutual attraction
-this is because they’re so small
compared to the distances between them.
Ideal Gases
 A theoretical gas with no volume and no
attraction.
 A series of theories will be studied about ideal
gases
-standard pressure of 101.32 kPa
-standard temperature of 0°C or 273K
-standard conditions are abbreviated
STP
Therefore:
 Kinetic theory explains properties of gases
based on a molecular view.
 The assumptions are:




The molecules are in continuous, random motion.
A molecule has negligible volume.
The forces between molecules are negligible.
The average kinetic energy depends on the
temperature.
Four Gas Law Variables Are:





V = volume
P = pressure
T = temperature
n = number of particles
(Case is important)
Behavior of Gases
 Compression
 Expansion
 Diffusion – movement of material from
high to low concentration
- lighter particles diffuse faster than heavier
particles
 Effusion- gas escapes through a tiny
opening
Gas Pressure
 Pressure = force/area
 Snowshoes in the snow – force is spread out over
a larger area
 Gas particles exert pressure as they collide with
the walls of their container
 More particles in a given space, greater pressure
 Barometer is tool used to measure atmospheric
pressure – mercury rises or falls
Units of pressure
 SI unit is Pascal (Pa)
 1Pa = 1 N/m2 derived from force
 1atm = 760mm Hg = 760 torr = 101.3 kPa
=14.7 psi
 1torr = 1 mm Hg
Dalton’s Law of Partial Pressure
 Total pressure of a mixture of gases is equal
to the sum of all the pressures of the
individual gases
 Pg. 392 practice problems
Boyle’s Law
 Relates volume and pressure
-gas exerts pressure on its container’s walls
-pressure depends on
*number of molecules
*average kinetic energy of the molecules
 Pressure = P
.. .. ..
.
.. .
Boyle’s Law
 Relates volume and pressure
-gas exerts pressure on its container’s walls
-pressure depends on
*number of molecules
*average kinetic energy of the molecules
 Plunger applies pressure (now 2P).
-As pressure doubles, volume
becomes ½.
-(note the same number of particles
now occupying ½ the space)
.. ... .
.. ......
Boyle’s Law
 Boyle’s Law PV=k
*P=pressure
*V= volume
*k=constant
 Experiments happen at room temperature(about
25ºC.) We need to convert the results to STP.
Boyle’s Law
 Since
k  V1P1
and
 then (substituting for k)
k  V2 P2
V1P1  V2 P2
Boyle’s Law
 Since
and
k  V1P1
 then (substituting for k)
or
V1P1
P2
k  V2 P2
V1P1  V2 P2
 V2
Boyle’s Law
 Since
and
k  V1P1
 then (substituting for k)
or
V1P1
V2 
P2
 Units


pressure - kPa
volume - cm3
k  V2 P2
V1P1  V2 P2
Problem: a gas is collected in a 242 cm3
container at 87.6 kPa. What is its volume at
STP?
V1= 242 cm3 P1= 87.6 kPa V2= ? P2= 101.325 kPa
Think: pressure goes from 87.6 kPa to 101.325 kPa
so Volume should be________ than 242 cm3
Math:
V1P1  V2 P2
V1P1
V2 
P2
V1P1
V2 
P2
242cm  87.6kPa
V2 
101.325kPa
3
 so
V2  209cm
3
 Note: 209 < 242cm3
 Now do CMC 358: 1, 2, 3(a,c,e) and 4(a,c,e).