Van der waals parameters a and b

[physgirl]

How, if you're given a pair of molecules, would you decide which one might have a higher value for a or b?

As I understand for instance, a's value will be high if two molecules are attracted to each other and b's value will be high if two molecules strongly repel each other... but how would you determine that?

so for something like H2CO2 vs. CH4, in order to determine which has a higher "a", I guessed it was H2CO2 because it can hydrogen bond with each other and so are attracted to each other whereas CH4 aren't... but with something like He and Ne?

and then for deciding which has a higher b value, if you had something like F2 vs. Br2, where would you begin? how would you decide which ones would repel more?

thanks!

 

[Gokul43201]

The volume correction parameter, b is actually indicative of the size of the molecule. A bigger molecule has a larger b value. The Van der Waals model has no mechanism for repulsion other than "hard sphere interaction" (elastic collisions between spheres).