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I don't know what are the molecular reasons that two substance will form another substance.

Why oxygen molecules can form to ozone molecules? Why hydrogen and oxygen makes water? What are the physical foundations for it? Is it possible to explain it with quantum mechanics?

EDIT: There are models for chemical reactions in terms of continuum mechanics (it is called "Mixture Theory"). But I don't understand Mixture Theory!

I know basics of continuum mechanics, but NO continuum mechanics of mixtures which can react with each other. Can someone explain this pdf:

http://www.mech-wilmanski.de/mixturesrome.pdf

kryomaxim
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  • Do you want a quantum mechanics explanation (if one exists)? Or would any explanation be sufficient? – tpg2114 Sep 23 '16 at 21:59
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    @Gert I disagree -- statistical mechanics and the view of collisions strong enough to excite vibrational modes until they bonds are broken is an entirely appropriate physics viewpoint. And so is QM for that matter. I think it's perfectly fine on topic here, but I was just seeking clarification on the last line -- is that a requirement of the answer (to use QM) or is it just something that would be nice. – tpg2114 Sep 23 '16 at 22:15
  • @tpg2114. The question can be answered here or at Chemistry. As a chemist I've answered several similar questions here. Good chemists are well versed in MOT/VESPR and thermochemistry. – Gert Sep 23 '16 at 22:27
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    http://physics.stackexchange.com/q/35408/ has answers that provide quite a good insight into covalent bonding. – Gert Sep 23 '16 at 22:36
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    Frankly someone asking a question like this shouldn't be thrown into the deep end of the pool. Quantum mechanics is overboard here. // Basically chemical reactions are driven by energy. If the products contain less energy than the reactants then the reaction is spontaneous. If the products contain more energy then some form of energy input is required to drive the reaction. Ozone forms in the upper atmosphere because UV rays from the sun create excited oxygen molecules. In the absence of the UV energy oxygen molecules won't spontaneously create ozone. – MaxW Sep 23 '16 at 22:38
  • @MaxW - And where does the energy come from, MaxW? Frankly, to try and explain chemical bonding without QC in this day and age is folly, IMO. And QC needn't be hugely complicated: it's mainly the breaking/forming of molecular orbitals. – Gert Sep 23 '16 at 22:56
  • @Gert - I'm a professional chemist. I understand at least how to manipulate quantum theory. If I were trying to describe gravity simply I'd say that rocks roll downhill not uphill because of gravity. I wouldn't start off with the metric tensor of spacetime. – MaxW Sep 23 '16 at 23:05
  • And I'd say that's not a great way to teach gravity. Each to their own though. :-) – Gert Sep 23 '16 at 23:14
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    Voted to close - This question now mutated into wanting an explanation of a 124 page pdf file. The paper would need an understanding of physical chemistry before it would make sense. You can't explain p-chem in a couple of paragraphs. – MaxW Sep 24 '16 at 00:26
  • Question which are on-topic on more than one site stay on the one where the OP put it (always assume that's one of the place it is on-topic). This questions is within the range of those considered by working physicists, though it is certainly in the region of heavy overlap with chemistry. – dmckee --- ex-moderator kitten Sep 24 '16 at 03:04

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Basically, forming chemical bonds releases energy. E.g. two atoms join together to make a chemical bond between them and release energy. Thereby, atoms/ molecules join together to get the stability (lesser the energy, higher the stability). You can do a simple calculation, calculate the total energy stored in one mole of Oxygen molecules (bond energy between O-O, per mole), two moles of hydrogen molecules (bond energy between H-H, per two moles), Then find out the energy stored in two moles of water molecules (bond energy between H-O, per two moles). You will see that the reaction of forming water is exothermic.

As a summary

at standard conditions, this reaction releases 285.8 kJ of energy.

H2(g) + 1/2 O2 -> H2O (l), enthalpy = -285.8 kJ per mol

negative value tells that the heat is released

This is why mixing HCl and NaOH releases heat. So overall, the physical foundation is gaining stability!

Kosala
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  • Why heat is in some reaction a lot faster released than in other reactions? – kryomaxim Sep 23 '16 at 22:33
  • @kryomaxim: are you asking why some reactions are faster than others or why are some are more exothermal than others? – Gert Sep 23 '16 at 22:39
  • I am also asking whether some reactions are faster than others. – kryomaxim Sep 23 '16 at 22:41
  • @Kosala. This is why mixing HCl and NaOH releases heat Can you explain why? – Gert Sep 23 '16 at 22:41
  • @kryomaxim - I am also asking whether some reactions are faster than others. Really not easy to explain in a few words. It's hard to summarise the theory of chemical kinetics in a few sentences. It all starts with collision theory. Reactions take place when molecules collide at sufficiently high energies. The more of these reactive collisions per unit of time, the faster the reaction will proceed. – Gert Sep 23 '16 at 22:45
  • @Gerts, the reaction results in water molecules – Kosala Sep 24 '16 at 01:29