1. Physical Characteristics of Gases
Chapter 10

2. Kinetic-molecular theory
Particles of matter are always in motion
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3. Ideal gas
An imaginary gas that perfectly fits all the assumptions of the kinetic-molecular theory.
We can often treat real gases as ideal gases and still get good results.
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4. Assumptions of KMT of gases
Large numbers of tiny particles that are far apart compared to their size
Low density
Easily compressed
Elastic collisions
No kinetic energy is lost when gas particles collide with each other or their container
It can be transferred between particles, but the total kinetic energy remains the same
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5. Gas particles are in continuous, rapid, random motion
There are no attractive or repulsive forces between gas particles
When they hit, they don’t stick together
The average kinetic energy of gas particles depends on the temperature of the gas
Direct relationship
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6. Expansion Gases have indefinite shape and volume.
They completely fill any container they are in.
They also take the shape of that container.
Because:
they move rapidly in all directions and don’t stick together
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7. Fluidity Gas particles slide past each other. They can flow
Fluid: something that can flow (can be gas or liquid)
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8. Diffusion
Spontaneous mixing of the particles of two substances caused by their random motion.
Gas particles spread out to fill their new container
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9. Effusion When gas particles pass through a small opening
Particles leak out of the container
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10. Real gases Do not completely follow kinetic-molecular theory
Especially deviant at high pressures and low temperatures
Noble gases are closest to ideal
Very polar gases are farthest from ideal
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11. Discuss
Describe the conditions under which a real gas is most likely to behave ideally.
Explain the following properties of gases using the kinetic-molecular theory: expansion, fluidity, low density, compressibility, and diffusion.
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12. Describing gases Needed: They are mathematically related. Volume
Temperature
Number of molecules
Pressure
They are mathematically related.
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13. Pressure balloon The force per unit area on a surface.
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14. Force A push or a pull Measured in newtons (N).
At the Earth’s surface, 1 kg of mass exerts 9.8 N of force due to gravity.
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15. Chemistry Chapter 10

16. Pressure of gases
Gases exert pressure on any surface with which they collide.
Depends on volume, temperature, and number of molecules
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17. Atmospheric pressure
Air around Earth exerts a pressure on it’s surface and everything on it.
Like the weight of all the molecules pressing down.
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18. Barometer Used to measure atmospheric pressure.
Height of liquid (usually mercury) in tube can be used to express atmospheric pressure.
At sea level, the average is 760 mm Hg. h
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19. Manometer Used to measure the pressure of gases.
The height difference between the two arms is the pressure.
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20. Pressure Units
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21. STP Standard temperature and pressure. 0 °C and 1 atm
Used to compare volumes of gases.
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22. Example
A weather report gives a current atmospheric pressure of mm Hg. Convert this to
Atmospheres
atm
Torr
745.8 torr
Kilopascals
99.43 kPa
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23. Discuss Define pressure What is STP? Convert 151.98 kPa to atmospheres
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24. Boyle’s Law Fixed: mass and temperature
The volume varies inversely with pressure
Less volume, means the particles hit the walls more often.
This increases the pressure
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25. Boyle’s Law Mathematically: Each sample of gas has its own k.
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26. Example
A helium-filled balloon contains 125 mL of gas at a pressure of atm. What volume will the gas occupy at standard pressure, assuming constant temperature?
122 mL
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27. You try
A weather balloon with a volume of L is released from Earth’s surface at sea level. What volume will the balloon occupy at an altitude of 20.0 km, where the air pressure is 10.0 kPa, assuming constant temperature?
13.9 L
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28. Charles’s Law Fixed: mass and pressure
Volume varies directly with temperature.
As temperature goes up, the particles have more energy, so they hit the walls more often and with more force
This pushes the walls outward.
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29. Charles’s Law
Mathematically
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30. Kelvin Scale
Charles’s law works more elegantly on the Kelvin Scale than the Celsius Scale.
If you double the temperature, the volume doubles.
Not true with Celsius
We must use Kelvin for Charles’s Law.
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31. Kelvin Scale Absolute zero: lowest possible temperature
All particle motion stops
0 K, °C
Often rounded to 273
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32. Example
A balloon filled with oxygen gas occupies a volume of 5.5 L at 25 °C. What volume will the gas occupy at 100. °C, assuming constant pressure?
6.9 L
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33. You try
A sample of nitrogen gas is contained in a piston with a freely moving cylinder. At 0.0 °C, the volume of the gas is 375 mL. To what temperature must the gas be heated to occupy a volume of 500. mL, assuming constant pressure?
91 °C
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34. Gay-Lussac’s Law Fixed: mass and volume
Pressure varies directly with temperature (in Kelvin)
As temperature goes up, energy of particles goes up.
They go faster and hit the walls harder.
If the walls can’t move, the pressure goes up.
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35. Gay-Lussac’s Law
Mathematically:
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36. You try
The temperature within an automobile tire at the beginning of a long trip is 25 °C. At the conclusion of the trip, the tire has a pressure of 1.80 atm. What is the final Celsius temperature within the tire if its original pressure was 1.75 atm? Assume constant volume.
34 °C
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37. Combined gas law
Expresses the relationship between pressure, volume, and temperature of a fixed amount of gas.
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38. You try
The volume of a gas at 27.0 °C and atm is 80.0 mL. What volume will the same gas sample occupy at standard conditions?
14.6 mL
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39. Dalton’s Law
The total pressure in a container is the sum of the partial pressures of all the gases in the container.
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40. Application We can collect gases by displacing water. When we do this,
Read Patm from the barometer. Look up Pwater in table A-8 in the appendix.
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41. Example
A student has stored mL of neon gas over water on a day when the temperature is 27.0 °C. If the barometer in the room reads mm Hg, what is the pressure of the neon gas in its container?
716.6 mm Hg
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42. You try
A sample of nitrogen gas is collected over water at a temperature of °C. What is the pressure of the nitrogen gas if atmospheric pressure is 785 mm Hg?
764 mm Hg
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