Deviation from the Ideal Gas Law
Introduced| English |
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| real gas/rɪəl ɡæs/ |
Where the perfect model breaks
- The simple gas equation works beautifully -- most of the time.
- But squeeze a gas hard or chill it, and it misbehaves.
- Real particles take up space and do attract each other.
- Knowing when the model fails is as useful as the model itself.
Two broken assumptions
- The ideal model assumes zero particle volume and no attractions.
- At high pressure, the particles' own volume starts to matter.
- At low temperature, their attractions pull them together.
Select both reasons real gases deviate from the ideal law.
Finite molecular volume and intermolecular attractions cause the deviations.
High pressure, low temperature
- A real gas 真实气体 deviates most at high pressure and low temperature.
- There the particles are crowded and slow.
- At low pressure and high temperature, gases act nearly ideal.
Real gases deviate most from ideal behaviour at...
Crowded (high P) and slow (low T) is where volume and attractions matter.
At low pressure and high temperature, gases behave nearly ideally.
Spread out and fast-moving, molecules barely feel volume or attractions.
Which gases stray most
- Big molecules, with more volume, deviate more.
- Polar molecules, with stronger attractions, deviate more.
- Small, nonpolar gases like helium stay closest to ideal.
Which gas behaves most ideally?
Small, nonpolar helium has tiny volume and weak attractions.
Polar molecules deviate more from ideal behaviour than nonpolar ones.
Stronger attractions between polar molecules increase the deviation.
Why does a real gas push with less pressure than $PV = nRT$ predicts at high pressure?
- Attractions pull the molecules inward, softening their wall collisions.
- So the measured pressure is a little lower than ideal.
When gases stop being ideal
Sort each condition by whether a real gas behaves nearly ideally or deviates.
Attractions between real molecules make the measured pressure...
Inward pulls soften wall collisions, lowering the pressure.
A real gas deviates most exactly where it is crowded (high $P$) and slow (low $T$) -- the opposite of ideal conditions. Two corrections matter: the finite volume of molecules and the attractions between them. And small, light, nonpolar gases behave most ideally.
A real gas deviates from $PV = nRT$ because its molecules take up space and attract each other. Deviations are largest at high pressure and low temperature, and worst for big or polar molecules. At low pressure and high temperature, or for small nonpolar gases, the ideal law works well.