Why Does the Van der Waals Equation Adjust Pressure Up and Volume Down?

AI Thread Summary
The Van der Waals equation modifies pressure and volume to account for real gas behavior, differing from the ideal gas law. The adjustment of volume down reflects the finite size of gas molecules, which occupy space. Conversely, pressure is adjusted up to account for intermolecular attractions that effectively reduce the pressure exerted by the gas. When comparing measurements with a = 0 and a > 0, the presence of attractive forces increases the observed pressure, necessitating the upward adjustment in the equation. Understanding these adjustments is crucial for accurately modeling real gas behavior.
Qube
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Homework Statement



Why does the Van der Waals equation adjust pressure up and volume down?

Homework Equations



(P + a)(V-b) = nRT (overly simplified, but it gets the point across).

The Attempt at a Solution



Okay. Volume refers to total free volume in a container in the context of the gas law. I can understand why volume is adjusted down relative to the ideal gas law equation (PV = nRT). Gases aren't point masses and actually occupy some volume.

However, why is pressure adjusted up? Gas molecules may attract and participate in inelastic collisions. This would seem to decrease the actual pressure. Why then would pressure need to be adjusted up?
 
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Qube said:
Gas molecules may attract and participate in inelastic collisions. This would seem to decrease the actual pressure.
If you were to measure the pressure of a gas with ##a = 0##, and then, with everything else the same, with ##a > 0##, how would the measurements differ?
 
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