Why is the pressure correction constant 'a' independent of temperature?

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?




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](https://en.wikipedia.org/wiki/Real_gas#Berthelot_and_modified_Berthelot_model)!


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