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In the van der Waals equation of state, 'a' which is the correction value is based on inter-molecular forces of attraction? Then should inter-molecular forces decrease when temperature increases, and thereby cause a change in value of 'a'?

Then why is 'a' independent of temperature?

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The parameter $a$ is independent of temperature because we defined it this way. There is no other reason. Indeed, van der Waals equation is very much not a law of nature. It is just an empirical approximation. There are other approximations, maybe a dozen or more. You may create your own, where $a$ would decrease with temperature. Wait, it's been done already - look at the Berthelot equation!

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  • $\begingroup$ @Karl Point taken. $\endgroup$ – Ivan Neretin Mar 1 '18 at 20:44
  • $\begingroup$ Look at the van der Waals equation. Substitute $a$ with a function of $T$, say, $const\over T$. You've arrived at the Berthelot equation. $\endgroup$ – Ivan Neretin Mar 2 '18 at 4:49

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