Intro to gases and gas laws

1. EQ:
How do we use the Kinetic
Molecular Theory to explain
the behavior of gases?
Topic #32: Introduction
to Gases

2. States of Matter
 2 main factors determine state:
• The forces (inter/intramolecular)
holding particles together
• The kinetic energy present (the
energy an object possesses due to its
motion of the particles)
• KE tends to ‘pull’ particles apart

3. Kinetic Energy , States of Matter &
Temperature
 Gases have a higher kinetic energy
because their particles move a lot more
than in a solid or a liquid
 As the temperature increases, there gas
particles move faster, and thus kinetic
energy increases.

4. Characteristics of Gases
Gases expand to fill any container.
• random motion, no attraction
Gases are fluids (like liquids).
• no attraction
Gases have very low densities.
• no volume = lots of empty space

5. Characteristics of Gases
Gases can be compressed.
• no volume = lots of empty space
Gases undergo diffusion & effusion
(across a barrier with small holes).
• random motion

6. Kinetic Molecular Theory of ‘Ideal’
Gases
 Particles in an ideal gas…
• have no volume.
• have elastic collisions (ie. billiard
ball particles exchange energy with
eachother, but total KE is conserved
• are in constant, random, straight-line
motion.
• don’t attract or repel each other.
• have an avg. KE directly related to
temperature ( temp= motion= KE)

7. Real Gases
Particles in a REAL gas…
• have their own volume
• attract each other (intermolecular
forces)
Gas behavior is most ideal…
• at low pressures
• at high temperatures
Why???

8. Real Gases
 At STP, molecules of gas are moving fast and
are very far apart, making their intermolecular
forces and volumes insignificant, so
assumptions of an ideal gas are valid under
normal temp/pressure conditions. BUT…
• at high pressures: gas molecules are
pushed closer together, and their
interactions with each other become more
significant due to volume
• at low temperatures: gas molecules move
slower due to KE and intermolecular
forces are no longer negligible

9. Pressure
area
force
pressure 
Which shoes create the most pressure?

10. Atmospheric Pressure
 The gas molecules in the atmosphere are
pulled toward Earth due to gravity, exerting
pressure
 Why do your ears ‘pop’ in an airplane?

11. Pressure
Barometer
• measures atmospheric pressure
Mercury Barometer

12. Units of Pressure
2
m
N
kPa 
 At Standard Atmospheric Pressure
(SAP)
101.325 kPa (kilopascal)
1 atm (atmosphere)
760 mm Hg
(millimeter Hg)
760 torr
14.7 psi (pounds per square inch)

13. Standard Temperature & Pressure
Standard Temperature & Pressure
0°C 273 K
1 atm 101.325 kPa
-OR-
STP

14. Temperature: The Kelvin Scale
ºC
K
-273 0 100
0 273 373
273


 K
C K = ºC + 273
Always use absolute temperature
(Kelvin) when working with gases.

15. Kelvin and Absolute Zero
Scottish physicist Lord Kelvin
suggested that -273oC (0K) was the
temperature at which the motion
particles within a gas approaches
zero.. And thus, so does volume)
 Absolute Zero:
http://www.youtube.com/watch?v=JHXxPnmyDbk
 Comparing the Celsius and Kelvin Scale:
http://www.youtube.com/watch?v=-G9FdNqUVBQ

16. Why Use the Kelvin Scale?
Not everything freezes at 0oC, but
for ALL substances, motion stops
at 0K.
It eliminates the use of negative
values for temperature! Makes
mathematic calculations possible
(to calculate the temp. twice
warmer than -5oC we can’t use 2x(-
5oC) because we would get -10oC!)

17. Kelvin Scale vs Celsius Scale

18. Converting between Kelvin and Celsius
a) 0oC =_____K
b) 100oC= _____K
c) 25oC =______K
d) -12oC = ______K
e) -273K = ______oC
f) 23.5K = ______oC
g) 373.2K= ______oC
273


 K
C K = ºC + 273

19. Learning Goal:
I will be able to understand
what kinetic energy is and
how it relates to gases and
temperature, describe the
properties of a real and ideal
gas and understand what
Absolute Zero is and how to
convert between the Kelvin
and Celsius temperature
scales.
How Did We Do So Far?

20. Part B: The Gas Laws
Part B:
Learning Goals
I will be able to describe
Boyle’s, Charles’ and
Gay-Lussac’s Laws
relating T, P and/or V
and be able to calculate
unknown values using
the equations derived
from these laws, as well
as the combined gas
law.

21. 1. Intro to Boyle’s Law
 Imagine that you hold the tip of a
syringe on the tip of your finger so
no gas can escape. Now push
down on the plunger of the syringe.
What happens to the volume in the
syringe?
What happens to the pressure the
gas is exerting in the syringe?

22. 1. Boyle’s Law

23. 1. Boyle’s Law
 The pressure and volume of a gas
are inversely proportional (as one
increases, the other decreases,
and vice versa
• at constant mass & temp
P
V

24. 1. Boyle’s Law
Boyle’s Law leads to the mathematical
expression: *Assuming temp is constant
P1V1=P2V2
Where P1 represents the initial pressure
V1 represents the initial volume,
And P2 represents the final pressure
V2 represents the final volume

25. Example Problem:
A weather balloon with a volume of 2000L at a pressure of 96.3
kPa rises to an altitude of 1000m, where the atmospheric pressure
is measured to be 60.8kPa. Assuming there is no change in the
temperature or the amount of gas, calculate the weather balloon’s
final volume.

26. You Try:
Atmospheric pressure on the peak of Kilimanjaro can be as low as
0.20 atm. If the volume of an oxygen tank is 10.0L, at what
pressure must the tank be filled so the gas inside would occupy a
volume of 1.2 x 103L at this pressure?

27. 2. Intro to Charles’ Law
Imagine that you put a
balloon filled with gas in
liquid nitrogen
What is happening to the
temperature of the gas in
the balloon?
What will happen to the
volume of the balloon?

28. 2. Charles’ Law

29. V
T
2. Charles’ Law
The volume and absolute
temperature (K) of a gas
are directly proportional (an
increase in temp leads to an
increase in volume)
• at constant mass &
pressure

30. 2. Charles’ Law

31. 2. Charles’ Law
 Charles’ Law leads to the
mathematical expression:
*Assuming pressure remains constant

32. Example Problem:
A birthday balloon is filled to a volume of 1.5L of helium gas in an
air-conditioned room at 293K. The balloon is taken outdoors on a
warm day where the volume expands to 1.55L. Assuming the
pressure and the amount of gas remain constant, what is the air
temperature outside in Celsius?

33. You Try:
A beach ball is inflated to a volume of 25L of air at 15oC. During
the afternoon, the volume increases by 1L. What is the new
temperature outside?

34. 3. Intro to Gay-Lussac’s Law
 Imagine you have a balloon
inside a container that ensures
it has a fixed volume. You heat
the balloon.
What is happening to the temp of
the gas inside the balloon?
What will happen to the pressure
the gas is exerting on the
balloon?

35. P
T
3. Gay-Lussac’s Law
The pressure and absolute
temperature (K) of a gas
are directly proportional (as
temperature rises, so does
pressure)
• at constant mass &
volume

36. 2. Gay-Lussac’s Law
 Gay-Lussac’s Law leads to the
mathematical expression:
*Assuming volume remains constant
Egg in a bottle to show Gay-Lussac's Law:
T & P relationship:
http://www.youtube.com/watch?v=r_JnUBk1JPQ

37. Example Problem:
The pressure of the oxygen gas inside a canister with a fixed
volume is 5.0atm at 15oC. What is the pressure of the oxygen gas
inside the canister if the temperature changes to 263K? Assume
the amount of gas remains constant.

38. You Try:
The pressure of a gas in a sealed canister is 350.0kPa at a room
temperature of 15oC. The canister is placed in a refrigerator that
drops the temperature of the gas by 20K. What is the new
pressure in the canister?

39. 4. Combined Gas Law
P1V1
T1
=
P2V2
T2
By combining Boyle’s, Charles’ and Gay
Lussac’s Laws, the following equation is
derived:

40. Example Problem:
A gas occupies 7.84 cm3 at 71.8 kPa & 25°C. Find
its volume at STP.

41. Any Combination Questions 
a) A gas occupies 473 cm3 at 36°C. Find its volume at 94°C
b) A gas’ pressure is 765 torr at 23°C. At what temperature will the
pressure be 560. torr

42. How Did You Do?
Part B:
Learning Goals
I will be able to describe
Boyle’s, Charles’ and
Gay-Lussac’s Laws
relating T, P and/or V
and be able to calculate
unknown values using
the equations derived
from these laws, as well
as the combined gas
law.