Chapter 14 Notes: Part 1 – The Behavior of Gases
Chemistry
(student edition)
Chapter 14 problems - Part 1:
14.1
20 - 23, 31, 33
Gases and Pressure
The KTG:
1. Gases are made up of molecules with lots of
between
2. Particles move quickly is caused by this motion
3. Collisions are completely
4. There are
attractive or repulsive forces between molecules
5. KE =
Properties of Gases resulting from the KTG
1.
2.
3.
4.
5.
6.
- no definite shape or definite volume
- particles glide past each other (gases and liquids are both fluids)
- gases are roughly 1/1000 the density of solids and liquids
- particles can be made much closer (many applications)
- spontaneous mixing of 2 substances due to random molecular motion
- a gas spreads through a small opening between containers
Gas Pressure:
It is the result of simultaneous collisions of billions of rapidly moving particles in
a gas with an object.
P=
F=
SI unit for force is the
A=
SI unit for pressure is the
An apple exerts 1 N and a textbook exerts 15-20 N.
A qualitative description of gases:
P=
T=
P V
when # moles
V=
T V
n=
T P
, P and V
1
STP = standard temperature
pressure P =
T =
Volume:
14.2
kPa,
torr,
mm Hg,
atm
0
K,
C (must use
with gas laws!!!)
takes up
of space (at
)
The Gas Laws
Boyle’s Law: (Irish - 1662) “ The volume of a gas varies inversely with pressure at constant
temperature.”
in a given situation PV =
T is
so…
example - If 150 ml of a sample of O2 at 720 mm Hg has the
pressure increased to 750 mm Hg, what is the new volume?
demo -
draw graph above
Description of what happened in the demonstration:
Charles’ Law (French - late 1700’s) - “The volume of a fixed mass of gas varies directly with
temperature at constant pressure.”
V/T =
P is
so….
( must use
!!!!)
example - 753 ml of nitrogen at 25 C0 is heated to 50 C0 - what
is the new volume?
example -
draw graph above
2
when the graph is extrapolated to 0 K, the volume is 0 - this is impossible as the gas
would have volume of all the particles with no space between them - of course, this is impossible
to test since no gases exist at - 273 C0 - only solids and liquids.
Gay-Lussac’s Law (French) - “The pressure of a fixed mass of gas varies directly with
temperature at constant volume.”
=k
so…
V is
example - If a gas at 3.0 atm and 25 C0 is heated to 52 C0, what is the new pressure?
example Combined Gas Law
laws to create......
draw graph here
- nothing in life is ever constant, so....add together the three previous gas
______________
example - If a sample of gas at 22.0 C0 and 31 kPa occupies 200.0 cm3, what space will it occupy
at STP?
14.4
Diffusion and Effusion
Dalton’s Law of Partial Pressures (English - 1802) The total pressure of a collection of gases in
a mixture is equal to the
of the pressures that each gas would exert by itself in the
same volume.
Collecting gas over water: A chemist runs a reaction where a metal and an acid are mixed.
Hydrogen is collected over water. When the chemist tries to figure out the pressure exerted by
the hydrogen over the water, it is noted that hydrogen is not the only gas above the water…
Zn(s) + H2SO4(aq)  ZnSO4(aq) + H2(g)
H2O(g)
H2O(g)
H2O(g)
H2(g)
H2(g)
H2(g)
Zn(s) + H2SO4(aq)  ZnSO4(aq) + H2(g)
3
Dalton’s Law of Partial Pressures
=
So, for the previous drawing:
=
Ex: A student collects hydrogen gas over water at an atmospheric pressure of 100.0 kPa
and a temperature of 29.0 C0. What is the partial pressure of the hydrogen? (see water vapor
pressure table)
Graham’s Law of Diffusion - (English - 1824) Graham noticed that gases with low densities
diffuse
than gases with higher densities.
Graham’s Law: Under the same conditions of temperature and pressure, gases diffuse at a rate
proportional to the square roots of their
(
).
Formula:
or
rate of gas “A”
rate of gas “B”
density of gas “B”
density of gas “A”
=
velocity of gas “A”
velocity of gas “B”
=
molecular weight of gas “B”
molecular weight of gas “A”
Show derivation of formula from Gas A ½ mv2 = Gas B ½ mv2
Ex: If CO2 molecules travel at 200.0 mph, how fast do H2 molecules go?
Ex: If He atoms travel at 800.0 mph, how fast do nitrogen molecules go?
4
NIB The Kinetic Theory and the Gas Law
In Boyle’s Law…
Pressure = Force/Area. So….
½ the volume means the particles hit the
walls
as often so,
the pressure.
big box
box half as small
In Charle’s Law, temperature is proportional to KE. So....
doubling temperature =
KE
KE =
in order to keep pressure the
pressure (
(
the number of collisions)
), volume must
In Gay-Lussac’s Law, temperature is proportional to KE. So...
doubling temperature =
KE =
volume is constant, so the
KE
pressure (
the # of collisions)
does change
In Dalton’s Law of Partial Pressures, if gas A exerts a pressure of 5 collisions/second and
gas B exerts a pressure of 5 collisions/second, then the total # of collisions/second
should =
collisions/second (
).
5