Honors Chemistry
Chapter 11 notes--Gases
I. Nature of Gases
A. Kinetic description (Hill and p. 339-342)
1. Gases have particles of unique sizes and masses
2. Gas particles collide constantly changing speeds and directions
3. Gas particles are separated by great distances
4. Gas particles do not interact except during momentary collisions; however, they are colliding
about 5 x 109 times every second
5. Gas collisions are elastic. That is they conserve energy, the sum of their energies remains
unchanged.
B. Physical properties (Hill)
1. Low density (ice—.917 g/ml, liquid water—1 g/ml, steam—.000561 g/ml)
2. Diffusibility—ability to spread out
3. Permeability—the ability of a gas to mingle with a porous substance
4. Compressibility—the ability of a gas to be squished
5. Expansibility—gases can expand without limit
C. Gas Pressure (Hill and p. 370-373)
1. Pressure is the average force exerted per unit area when molecules collide with a boundary.
2. More particles (moles) in a given area will increase the pressure (more collisions with the
boundary)
3. Units of pressure (Hill and p. 376-379)
a. psi—pounds per square inch (14.7 psi)
b. atm—atmosphere (1 atm)
c. mm Hg—millimeters of Hg (760 mm Hg)
d. torr—named after Torricelli, equals a mm Hg (760 torrs)
e. Pa—pascal, named after Pascal, (101,325 Pa), SI unit, frequently measured as 101.3 kPa
f. conversions 1atm=760 torr= 760 mm Hg=14.7 psi=101,325 Pa
4. Devices to measure pressure (p. 374-375)
a. Barometer
1) Invented by Torricelli
2) originally used a column of mercury in a dish of mercury
3) column of mercury is in a sealed tube; therefore, all pressure must come from the dish
of mercury
4) 1 atmosphere (1 atm) supports a column of 760 mm Hg
b. Pressure Gauge
1) measures pressure in something
2) normally is pushing against a spring (see p.375)
D. Pressure, Volume, and Temperature (Hill)
1. The volume of a gas depends not only on the number of molecules (moles), but also on the
temperature and the pressure of the gas.
2. Volume
a. the SI unit is the liter (L or l or l)
b. for gases we normally use the liter
3. Temperature
a. defined—the average amount of energy in an object
b. short definition—molecular speedometer
c. the SI unit is the Kelvin
E. Temperature scales (Hill and p. 346-348)
1. Fahrenheit—(-460o absolute zero, 32o freezing pt. water, 212o boiling pt. water)
a. American system
b. Mr. Hill’s viewpoint—stupid system based on the definition of temperature
c. Not a scale that is mathematically correct
2. Celsius—(-273o absolute zero, 0o freezing pt. water, 100o boiling pt. water)
a. European system and most of the known world
b. Mr. Hill’s viewpoint—acceptable system, but still not right
c. Not a scale that is mathematically correct
3. Kelvin—(0 absolute zero, 273 freezing pt. water, 373 boiling pt. water)
a. Scientific system
b. Mr. Hill’s viewpoint—only accurate system, but not practical for everyday use
c. Only scale that is mathematically correct
II. Gas Laws
A. Boyle’s Law: Pressure and Volume (p. 380-385)
1. P1V1 = P2V2
2. Pressure and volume are inversely proportional (as one goes up, the other one goes down)
3. Graphing—curve going down left to right (p. 381)
4. Application
a. Weather balloons
b. Ear discomfort as you ascend in a plane or driving
5. Kinetic explanation
a. If temperature remains the same and the volume decreases, the pressure must increase
(more collisions with the boundary)
b. If temperature remains the same and the volume increases, the pressure must decrease
(fewer collisions with the boundary)
6. Problems
B. Charles’ Law: Temperature and Volume (p. 389-391)
1. V1/T1 = V2/T2
2. Temperature and volume are directly proportional (as one goes up, the other one goes up)
3. The Kelvin temperature MUST be used
4. Graphing—straight line going up left to right (p. 390)
5. Kinetic explanation
a. If pressure remains the same and the volume decreases, the temperature must decrease
b. If pressure remains the same and the volume increases, the temperature must increase
6. Problems
C. Gay-Lussac’s Law: Temperature and Pressure (Hill)
1. P1/T1 = P2/T2
2. Temperature and pressure are directly proportional (as one goes up, the other one goes up)
3. The Kelvin temperature MUST be used
4. Graphing—straight line like Charles’ law
5. Kinetic explanation
a. If volume remains the same and the temperature increase, the pressure must increase
b. If volume remains the same and the temperature decreases, the pressure must decrease
6. Application
a. hot air balloons
b. pressure cookers
7. Problems
D. Combined Gas Law: Volume, Pressure and Temperature (p. 392-393)
1. P1V1/T1 = P2V2/T2
2. The Kelvin temperature MUST be used
3. Standard temperature and pressure (STP)
a. Standard temperature is 0oC or 273 K
b. Standard pressure is 1 atm or 101 kPa
4. Problems
E. Dalton’s Law of Partial Pressures (Hill)
1. The law stated—the total pressure of a mixture of gases equals the sum of the partial
pressures
2. This is based on the kinetic model that gas particles do not interact with each other; therefore,
a. the one gas hits the boundary as much as it normally would
b. even when another gas is present
3. Example: dry gas, 78% nitrogen, 21 % oxygen, 1% argon
a. If you have a sample that has a pressure of 760 torr, then
b. 592.8 torr comes from nitrogen
c. 159.6 torr comes from oxygen
d. 7.6 torr comes from argon
4. Gases are frequently collected by bubbling it in water
a. Called collecting a gas over water or collection by water displacement
b. Water is placed in a test tube and inverted in a container of water
c. The gas is then bubbled into the test tube
d. The gas replaces the water in the test tube, pushing the water out
e. some of the water evaporates with the gas, thereby causing additional pressure + vapor
pressure Pgas = Pcollected gas + Pwater
f. vapor pressure is dependent on its temperature, this is provided in tables
Vapor Pressure of Water at Various Temperatures
Temperature (oC)
Vapor Pressure (torr)
Vapor Pressure (kPa)
0.0
4.579
0.6105
5.0
6.543
0.8723
10.0
9.209
1.228
15.0
12.788
1.705
20.0
17.535
2.338
25.0
23.756
3.167
30.0
31.824
4.243
35.0
42.175
5.623
40.0
55.324
7.376
45.0
71.88
9.583
50.0
92.51
12.33
55.0
118.04
15.74
60.0
149.38
19.92
65.0
187.54
25.00
70.0
233.7
31.16
75.0
289.1
38.54
80.0
355.1
47.34
85.0
433.6
57.81
90.0
525.76
70.10
95.0
633.90
84.51
100.0
760.00
101.3
III. Gases and the Mole
A. Law of Combining Gas Volumes (Hill and p. 394-396)
1. Gay-Lussac
a. Gases always react in ratios of small whole numbers—Gay-Lussac.
b. 1 l H2 plus 1 l Cl2 will produce 2 l HCL (volume ratio)
c. This correlates with the coefficients from a balanced equation.
d. H2 + Cl2  2HCl (molar ratio)
2. Avogadro
a. under equivalent conditions, equal volumes of gases contain the same number of
molecules
b. Values were experimentally determined after Avogadro’s death
c. At STP, 1 mole of a gas contains 6.023 x 1023 molecules and will take up 22.4 l of volume—
this is known as molar volume
B. Ideal Gas Law (p. 418-419)
1. PV = nRT
2. Describes the behavior of an ideal gas (one that behaves as expected) in terms of pressure, P,
volume, V, temperature, T, and the number of moles, n.
3. Ideal gas—most gases behave normally, but change as the approach low temperatures or high
pressures, because they start reacting with each other
4. R = 8.31 kPa L/mol K or .0821 atm L/ mol K or 62.36 mm Hg L/mol K
5. At STP one mole of a gas occupies 22.4 liters
6. Problems