I was reading here about how the ideal gas law assumes point masses and non-interaction. Is it fair to say that all chemistry arises from failures of that?

Of course, such a sweeping generalization will be strictly false, but is it on the right track? (I'm reminded of Feynman, "All mass is interaction," although that seems to be getting at something deeper.)

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    $\begingroup$ I would personally find that kinda backwards. There is no fundamental reason to think of the ideal gas as the basis against which everything is judged. Rather the ideal gas is the unusual situation in which the relationships between pressure, volume, temperature and internal energy are easy to understand. We teach it first because it is comprehensible not because it is privileged. $\endgroup$ – dmckee --- ex-moderator kitten Apr 30 '13 at 19:34
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    $\begingroup$ This is kind of a cute way of looking at it. However, if you have a system of point masses that does interact, the interaction could be through any of the fundamental forces: electromagnetic, nuclear, or gravitational. Only in the case of the electromagnetic interaction do you get chemistry. $\endgroup$ – user4552 Apr 30 '13 at 19:43
  • $\begingroup$ Right, I don't mean to give a human model priority; just trying to "organize our ignorance" & understand how that model relates to some of our other modeling. If the statement is "only" backwards, then I'd say it's in good shape, since we can turn it around easily enough. Is "arises" problematic? How about: "Chemistry happens where the ideal gas law fails." (Also, I only mean logical implication, not physical causation.) $\endgroup$ – Toph May 1 '13 at 1:29

I think the reasonable answer to your question is no.

For example hydrogen and oxygen, and mixtures thereof, are close to ideal gases at STP. By close to ideal I mean that they obey the ideal gas law to a close approximation. however, apply a bit of activation energy and they behave in a distinctly non-ideal gas way!

The problem is that when dealing with ideal gases we almost invariably assume the system is at equilibrium i.e. in a state of minimum free energy. A mixture of hydrogen and oxygen at STP is only in a local minimum, and within this it does behave in a close to ideal way. However add enough energy (i.e. the activation energy) to take it out of the local minimum and you see non-ideal behaviour as it moves to a global minimum. After the reaction has completed, the reaction products (water molecules) again behave as an approximately ideal gas.

  • $\begingroup$ I don't see the relevance of this example. There is a time during which the hydrogen-oxygen mixture is behaving as an ideal gas, and during this time there is no chemistry. There is a later time during which the mixture is reacting explosively. At this later time, it's not acting as an ideal gas and there is chemistry going on. AFAICT your example supports the proposed definition. $\endgroup$ – user4552 May 1 '13 at 15:10
  • $\begingroup$ If by non-ideal you mean everything except ideal gases then yes I agree, but the point is then a rather facile one. To me non-ideal is to ideal much as PPN is to Newtonian gravity i.e. different but recognisably derived from. $\endgroup$ – John Rennie May 1 '13 at 16:51

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