1. 1
1
GASES
GASES
SAVE PAPER AND INK!!! When you
print out the notes on PowerPoint,
print "Handouts" instead of "Slides"
in the print setup. Also, turn off the
backgrounds
(Tools>Options>Print>UNcheck
"Background Printing")!
2. 2
2
Importance of Gases
Importance of Gases
• Airbags fill with N
Airbags fill with N2
2 gas in an
gas in an
accident.
accident.
• Gas is generated by the
Gas is generated by the
decomposition of sodium azide,
decomposition of sodium azide,
NaN
NaN3
3.
.
• 2 NaN
2 NaN3
3 ---> 2 Na + 3 N
---> 2 Na + 3 N2
2
3. 3
3
General Properties
General Properties
of Gases
of Gases
• There is a lot of “free” space
There is a lot of “free” space
in a gas.
in a gas.
• Gases can be expanded
Gases can be expanded
infinitely.
infinitely.
• Gases fill containers
Gases fill containers
uniformly and completely.
uniformly and completely.
• Gases diffuse and mix
Gases diffuse and mix
rapidly.
rapidly.
4. 4
4
Properties of Gases
Properties of Gases
Gas properties can be modeled using
Gas properties can be modeled using
math. Model depends on—
math. Model depends on—
• V = volume of the gas (L)
V = volume of the gas (L)
• T = temperature (K)
T = temperature (K)
– ALL temperatures MUST be in
ALL temperatures MUST be in
Kelvin!!! No Exceptions!
Kelvin!!! No Exceptions!
• n = amount (moles)
n = amount (moles)
• P = pressure
P = pressure
(atmospheres)
(atmospheres)
5. 5
5
Pressure
Pressure
Pressure of air is
Pressure of air is
measured with a
measured with a
BAROMETER
BAROMETER
(developed by
(developed by
Torricelli in 1643)
Torricelli in 1643)
Hg rises in tube until force of Hg
Hg rises in tube until force of Hg
(down) balances the force of
(down) balances the force of
atmosphere (pushing up).
atmosphere (pushing up).
(Just like a straw in a soft
(Just like a straw in a soft
drink)
drink)
P of Hg pushing down related to
P of Hg pushing down related to
• Hg density
Hg density
• column height
column height
6. 6
6
Pressure
Pressure
Column height measures Pressure
Column height measures Pressure
of atmosphere
of atmosphere
• 1 standard atmosphere (atm) *
1 standard atmosphere (atm) *
= 760 mm Hg (or torr) *
= 760 mm Hg (or torr) *
= 14.7 pounds/in
= 14.7 pounds/in2
2
(psi)
(psi)
= about 34 feet of water!
= about 34 feet of water!
* Memorize these!
* Memorize these!
7. 7
7
Pressure Conversions
A. What is 475 mm Hg expressed in atm?
1 atm
760 mm Hg
B. The pressure of a tire is measured as 29.4 psi.
What is this pressure in mm Hg?
760 mm Hg
14.7 psi
= 1.52 x 103
mm Hg
= 0.625 atm
475 mm Hg x
29.4 psi x
8. 8
8
Boyle’s Law
Boyle’s Law
P
P1
1V
V1
1 = P
= P2
2 V
V2
2
This means Pressure and
This means Pressure and
Volume are INVERSELY
Volume are INVERSELY
PROPORTIONAL if
PROPORTIONAL if
moles and temperature
moles and temperature
are
are constant
constant (do not
(do not
change). For example, P
change). For example, P
goes up as V goes down.
goes up as V goes down.
Robert Boyle
Robert Boyle
(1627-1691).
(1627-1691).
Son of Earl of
Son of Earl of
Cork, Ireland.
Cork, Ireland.
9. 9
9
Boyle’s Law
Boyle’s Law
A bicycle pump is a good
A bicycle pump is a good
example of Boyle’s law.
example of Boyle’s law.
As the volume of the air
As the volume of the air
trapped in the pump is
trapped in the pump is
reduced, its pressure
reduced, its pressure
goes up, and air is
goes up, and air is
forced into the tire.
forced into the tire.
10. 10
10
Charles’s
Charles’s
Law
Law
If n and P are
If n and P are constant
constant,
,
then
then
V and T are DIRECTLY
V and T are DIRECTLY
proportional.
proportional.
V
V1
1 V
V2
2
=
=
T
T1
1 T
T2
2
• If one temperature goes up, the
If one temperature goes up, the
volume goes up!
volume goes up!
Jacques Charles
Jacques Charles
(1746-1823). Isolated
(1746-1823). Isolated
boron and studied
boron and studied
gases. Balloonist.
gases. Balloonist.
11. 11
11
Charles’s Law
Charles’s Law
Think about what happens to
Think about what happens to
your bike tires in the winter
your bike tires in the winter
As the temperature decreases
As the temperature decreases
the tires deflate
the tires deflate
This also happens if you take
This also happens if you take
a balloon outside on a cold
a balloon outside on a cold
day
day
12. 12
12
Gay-Lussac’s Law
Gay-Lussac’s Law
If n and V are
If n and V are constant
constant,
,
then
then
P and T are DIRECTLY
P and T are DIRECTLY
proportional.
proportional.
P
P1
1 P
P2
2
=
=
T
T1
1 T
T2
2
• If one temperature goes up, the
If one temperature goes up, the
pressure goes up!
pressure goes up!
Joseph Louis Gay-
Joseph Louis Gay-
Lussac (1778-1850)
Lussac (1778-1850)
13. 13
13
Combined Gas Law
• The good news is that you don’t have to
remember all three gas laws! Since they
are all related to each other, we can
combine them into a single equation. BE
SURE YOU KNOW THIS EQUATION!
P1 V1 P2 V2
=
T1 T2
No, it’s not related to R2D2
14. 14
14
Combined Gas Law
If you should only need one of the other gas
laws, you can cover up the item that is
constant and you will get that gas law!
=
P1 V1
T1
P2
V2
T2
Boyle’s Law
Charles’ Law
Gay-Lussac’s
Law
15. 15
15
Practice Problem
A balloon has a volume of 785
mL on a fall day when the
temperature is 21°C. In the
winter, the gas cools to 0°C. What
is the new volume of the balloon?
16. 16
16
Combined Gas Law Problem
A sample of helium gas has a volume of 0.180 L, a
pressure of 0.800 atm and a temperature of 29°C. What is
the new temperature(°C) of the gas at a volume of 90.0
mL and a pressure of 3.20 atm?
Set up Data Table
P1 = 0.800 atm V1 = .180 L T1 = 302 K
P2 = 3.20 atm V2= .090 L T2 = ??
17. 17
17
Calculation
P1 = 0.800 atm V1 = .180 L T1 = 302 K
P2 = 3.20 atm V2= 0.090 L T2 = ??
P1 V1 P2 V2
= P1 V1 T2 = P2 V2 T1
T1 T2
T2 = P2 V2 T1
P1 V1
T2 = 3.20 atm x 0.090 L x 302 K
0.800 atm x .180 L
T2 = 604 K - 273 = 331 °C
= 604 K
18. 18
18
And now, we pause for this
commercial message from STP
OK, so it’s really not THIS kind
of STP…
STP in chemistry stands for
Standard Temperature and
Pressure
Standard Pressure =
1 atm (or an
equivalent)
Standard
Temperature = 0 deg
C (273 K)
STP allows us to
compare amounts of
gases between different
pressures and
temperatures
20. 20
20
Try This One
A sample of neon gas used in a neon sign has a volume of 15
L at STP. What is the volume (L) of the neon gas at 2.0 atm
and –25°C?
21. 21
21
Avogadro’s Hypothesis
Avogadro’s Hypothesis
Equal volumes of gases at the same T
Equal volumes of gases at the same T
and P have the same number of
and P have the same number of
molecules.
molecules.
V and n are directly related.
V and n are directly related.
twice as many
twice as many
molecules
molecules
22. 22
22
IDEAL GAS LAW
IDEAL GAS LAW
Brings together gas
Brings together gas
properties.
properties.
Can be derived from
Can be derived from
experiment and theory.
experiment and theory.
BE SURE YOU KNOW THIS
BE SURE YOU KNOW THIS
EQUATION!
EQUATION!
P V = n R T
P V = n R T
23. 23
23
Using PV=nRT
Using PV=nRT
P = Pressure
P = Pressure
V = Volume
V = Volume
T = Temperature
T = Temperature
N = number of moles
N = number of moles
R is a constant, called the
R is a constant, called the Ideal Gas Constant
Ideal Gas Constant
Instead of learning a different value for R for all the possible
Instead of learning a different value for R for all the possible
unit combinations, we can just
unit combinations, we can just memorize
memorize one
one value and
value and
convert the units to match R.
convert the units to match R.
R = 0.0821
R = 0.0821
L • atm
Mol • K
24. 24
24
Using PV = nRT
Using PV = nRT
How much N
How much N2
2 is required to fill a small room with a
is required to fill a small room with a
volume of 27,000 L to 745 mm Hg at 25
volume of 27,000 L to 745 mm Hg at 25 o
o
C?
C?
Solution
Solution
1. Get all data into proper units
1. Get all data into proper units
V = 27,000 L
V = 27,000 L
T = 25
T = 25 o
o
C + 273 = 298 K
C + 273 = 298 K
P = 745 mm Hg (1 atm/760 mm Hg)
P = 745 mm Hg (1 atm/760 mm Hg)
= 0.98 atm
= 0.98 atm
And we always know R, 0.0821 L atm / mol K
And we always know R, 0.0821 L atm / mol K
25. 25
25
Using PV = nRT
Using PV = nRT
How much N
How much N2
2 is req’d to fill a small room with a volume of 960 cubic
is req’d to fill a small room with a volume of 960 cubic
feet (27,000 L) to P = 745 mm Hg at 25
feet (27,000 L) to P = 745 mm Hg at 25 o
o
C?
C?
Solution
Solution
2. Now plug in those values and solve for the
2. Now plug in those values and solve for the
unknown.
unknown.
PV =
PV = n
nRT
RT
n = 1082 mol
n = 1082 mol
RT RT
RT RT
How many grams of N
How many grams of N2
2 is this?
is this?
1082 mol * (28 g/ 1mol) = 30,296 g!
1082 mol * (28 g/ 1mol) = 30,296 g!
26. 26
26
Learning Check
Dinitrogen monoxide (N2O), laughing
gas, is used by dentists as an
anesthetic. If 2.86 mol of gas occupies a
20.0 L tank at 23°C, what is the
pressurein the tank in the dentist office?
27. 27
27
Learning Check
A 5.0 L cylinder contains oxygen gas at
20.0°C and 735 mm Hg. How many
grams of oxygen are in the cylinder?
28. 28
28
Deviations from
Deviations from
Ideal Gas Law
Ideal Gas Law
• Real molecules have
volume.
The ideal gas consumes the entire
amount of available volume. It
does not account for the volume
of the molecules themselves.
• There are intermolecular
forces.
An ideal gas assumes there are no
attractions between molecules.
Attractions slow down the
molecules and reduce the
amount of collisions.
– Gases behave MOST ideally at
LOW pressure and HIGH
temperature
» (This keeps the gases
moving fast enough not to
stick together and far away
from each other)
29. 29
29
Gases in the Air
The % of gases in air Partial pressure (STP)
78.08% N2 593.4 mm Hg
20.95% O2 159.2 mm Hg
0.94% Ar 7.1 mm Hg
0.03% CO2 0.2 mm Hg
PAIR = PN + PO + PAr + PCO = 760 mm Hg
2 2 2
Total Pressure 760 mm Hg
30. 30
30
Dalton’s Law of Partial Pressures
Dalton’s Law of Partial Pressures
What is the total pressure in the flask?
What is the total pressure in the flask?
P
Ptotal
total in gas mixture = P
in gas mixture = PA
A + P
+ PB
B + ...
+ ...
Therefore,
Therefore,
P
Ptotal
total = P
= PH
H2
2O
O + P
+ PO
O2
2
= 0.48 atm
= 0.48 atm
Dalton’s Law: total P is sum of
Dalton’s Law: total P is sum of
PARTIAL
PARTIAL pressures.
pressures.
2 H
2 H2
2O
O2
2 (l) ---> 2 H
(l) ---> 2 H2
2O (g) + O
O (g) + O2
2 (g)
(g)
0.32 atm
0.32 atm 0.16
0.16
atm
atm
31. 31
31
Health Note
When a scuba diver is several
hundred feet under water, the
high pressures cause N2 from
the tank air to dissolve in the
blood. If the diver rises too
fast, the dissolved N2 will form
bubbles in the blood, a
dangerous and painful
condition called "the bends".
Helium, which is inert, less
dense, and does not dissolve in
the blood, is mixed with O2 in
scuba tanks used for deep
descents.
32. 32
32
Collecting a gas “over water”
• Gases, since they mix with other gases readily, must be
collected in an environment where mixing can not
occur. The easiest way to do this is under water
because water displaces the air. So when a gas is
collected “over water”, that means the container is
filled with water and the gas is bubbled through the
water into the container. Thus, the pressure inside the
container is from the gas AND the water vapor. This is
where Dalton’s Law of Partial Pressures becomes
useful.
34. 34
34
GAS DIFFUSION
GAS DIFFUSION
• Question: When someone near you passes gas, what do
you notice first?
– Then what do you notice?
• Why don’t you smell it immediately?
• What about people across the room?
– This is because the smell spreads out.
This rapid dispersion of particles from high concentration to
low concentration is DIFFUSION in action!
diffusion
diffusion is the gradual mixing of molecules of different gases.
is the gradual mixing of molecules of different gases.
35. 35
35
GAS DIFFUSION
GAS DIFFUSION
• Question: What happens when a nail makes a small puncture
in a tire? Isn’t that diffusion too?
– When this happens, we are looking at EFFUSION, or the
motion of a gas through a small opening
• Since the opening is small, gas particles have to “wait in
line” for other particles to pass through.
– Like a grocery checkout, it’s first come, first served.
• The speed of these molecules comes from Kinetic Energy,
KE = ½ mv2
.
– In this equation mass also affects the speed.
– Lighter particles travel faster and escape more often than
more massive particles.
36. 36
36
GAS DIFFUSION AND
GAS DIFFUSION AND
EFFUSION
EFFUSION
Graham’s law governs effusion
Graham’s law governs effusion
and diffusion of gas
and diffusion of gas
molecules.
molecules.
Thomas Graham, 1805-1869.
Thomas Graham, 1805-1869.
Professor in Glasgow and London.
Professor in Glasgow and London.
Rate of effusion is
Rate of effusion is
inversely proportional
inversely proportional
to its molar mass.
to its molar mass.
M of
M of
Rate fo
Rate fo
37. 37
37
Gas Diffusion
Gas Diffusion
relation of mass to rate of diffusion
relation of mass to rate of diffusion
• HCl and NH3 diffuse
from opposite ends of
tube.
• Gases meet to form
NH4Cl
• HCl heavier than NH3
• Therefore, NH4Cl forms
closer to HCl end of
tube.
QuickTimeª and a
Sorenson Video decompressor
are needed to see this picture.
