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23_Vd_Waals

# 23_Vd_Waals - INDIANA UNIVERSITY DEPARTMENT OF PHYSICS P309...

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Unformatted text preview: INDIANA UNIVERSITY, DEPARTMENT OF PHYSICS, P309 LABORATORY Laboratory #23: Real Gases Goal: How does the behavior of a real gas differ from the ideal gas laws? Study of the liquid-gas interface, the Van der Waals equation of state, and the Clausius-Clapeyron equation. Equipment: Critical point apparatus (CENCO 32019), bottles with sulfurhexafluoride (SF 6 ) and ethane (CH 3 CH 3 ). (A) Physics: One needs three parameters to describe the state of a given amount by mass (e.g., 1 kgmol) of a gas. These parameters are the volume per kgmol, V M (m 3 /kg), the pressure p (Pa) and the temperature T (K). These ‘state’ parameters are not independent, but are rather related by the the state equation. For an ‘ideal’ gas, the state equation is pV=RT , where R =8314.3 J/K/kgmol is the gas constant. This relation follows from kinetic theory, where one models a gas as a collection of point-like masses that collide elastically with the walls of the container. But real gases consist of real molecules. These are not point-like, but occupy some volume b . Therefore, in the real state equation V should be replaced by V&b . Also, real molecules attract each other (cohesive forces). This leads to an ‘internal’ pressure p int because molecules on the surface have neighbors only on one side. This pressure is proportional to the square of the number density of molecules (can you see why?), and thus to V M-2 . This leads to a more realistic state equation, called the Van-der ....
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23_Vd_Waals - INDIANA UNIVERSITY DEPARTMENT OF PHYSICS P309...

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