real gases and van der Waal equation

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From: Sally Seebode (sseebode@earthlink.net)
Date: Fri Oct 10 2003 - 10:22:07 PDT 

Message-ID: <004001c38f53$07e7dfc0$3105060a@smuhsd.k12.ca.us>
From: "Sally Seebode" <sseebode@earthlink.net>
Subject: real gases and van der Waal equation
Date: Fri, 10 Oct 2003 10:22:07 -0700

I have a discrepancy between my understanding of real gases and the van der
Waal equation. The equation is (P + na^2/V^2)(V - nb) = nRT. My
understanding is that real gases do not follow the ideal gas law due to a
break down of the kinetic theory at low temperatures and high pressures.

At low temperatures, the molecules are moving more slowly and therefore do
experience attractive forces. This causes molecules headed for the container
wall to experience a slight pull back into the container and thus the force
with which they hit the wall is slightly lower than we expect. If the
experimental pressure is lower than that calculated from ideal gas law, why
does the equation add the value na^2/V^2 to correct this error?

Likewise, at high pressure, the molecules do take up some space, so the
calculated volume is less than the experimentally measured volume. Why do we
subtract the value nb to correct this?

Any corrections to my logic would be appreciated.

sally

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