Dr. Joshi’s AP Chemistry
Real Gases
Kinetic-Molecular Theory (ideal gases):
• Gas molecules are always moving. They move in a straight line until collision. (Random motion.)
• VOLUME OF A GAS MOLECULE IS ZERO.
• INTERMOLECULAR INTERACTIONS ARE NEGLIGIBLE.
• Collisions are elastic. (Energy is not lost upon collision.)
• Average kinetic energy is proportional to temperature.
PART A
1. Why does the line for (PV)/(nRT) = 1 represent an ideal gas?
2. At which pressure do real gases behave like ideal gases?
3. The behavior of real gases is closest to ideal at ________________ pressures.
4. Do gas molecules interact with each other more or less at high pressures? Explain.
All graphs and tables are from Chemistry, 8th ed by Zumdahl and Zumdahl.
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Dr. Joshi’s AP Chemistry
Real Gases
PART B
5. A real gas behaves most like an ideal gas at _________________ temperatures.
6. Do gas molecules interact with each other more or less at high temperatures? Explain. (Hint:
Temperature is proportional to kinetic energy.)
All graphs and tables are from Chemistry, 8th ed by Zumdahl and Zumdahl.
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Dr. Joshi’s AP Chemistry
Real Gases
PART C
The volume available for real gases to move around in is actually less than the volume of the container.
This is because the gas molecules themselves have volume.
Adjusting the volume term in the ideal gas law…
7. Which gas has the largest value for b? Why is the value for b largest for this gas?
All graphs and tables are from Chemistry, 8th ed by Zumdahl and Zumdahl.
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Dr. Joshi’s AP Chemistry
Real Gases
In reality, gas molecules are attracted to each other when they get close enough to “feel” each other.
Correcting for attraction…
8. Why is the value for the variable a (in Table 5.3) so large for water? Explain in terms of
intermolecular interactions.
9. Why is the a value so small for He? Explain in terms of intermolecular forces/interactions.
10. Do you see a trend in the values for a in the noble gases? Explain.
All graphs and tables are from Chemistry, 8th ed by Zumdahl and Zumdahl.
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Dr. Joshi’s AP Chemistry
Real Gases
Putting it all together to get the final equation for real gases (van der Waals eqn)…
All graphs and tables are from Chemistry, 8th ed by Zumdahl and Zumdahl.
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