Talk:Molecularity

Termolecular reactions

The aticle previously stated: "It is still discused if termolecular processes in fact exist, and they are in any case, very rare". This is wrong. In atmospheric chemistry there are large numbers of reactions which are termolecular. See for example the NASA JPL "Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies" section 2. There are pages of the things there. A simple example will suffice to explain. Take the reaction which oxidises NO2 in the atmosphere:

NO2 + OH → HNO3

It is impossible for a reaction such as this to simultaneous conserve momentum and kinetic energy. What happens normally is that an excited version of the nitric acid is formed to allow conservation of energy. However this energy can be enough to cause the reverse reaction to occur. The nitric acid only remains intact if it collides with another entity (labeled as M by convention) which can then colisionally stablise the excited nitric acid. This explains why many of these reactions have been observed to have a reactuion reate which increases with pressure. If the pressure is too low the excited molecule falls apart before the colisional stabilisation can take place. I hope this is clear. (and if anyone wants to correct any details please do - this is from my memory of lectures about a decade ago...)--NHSavage 18:12, 12 December 2006 (UTC)

Well I guess i didn't catch the concept propperly. You know more about this than i do, so i'm just asking, to check if i've got it right now:

I see it this way: that termolecular reaction, and all those in the NASA link, although obviously termolecular (the reference says clearly that they are) are not that 3 molecules actually collide at the same time, but rather, two collide, something is formed and the THAT collides with a third molecule that takes the excess energy and deactivates thermally or whatever (it reminds me of all the kinetic theories for unimolecular reactions). So it fulfills the definition of termolecular, but not in the "sense" of 3 molecules simultaneously colliding. Or it DOES mean the three of them collide at the same time?? Should we add a line explaining that it is not that 3 molecules actually meet in the same place at the same time? Knights who say ni 19:08, 12 December 2006 (UTC)

As far as I remember the science you are correct - it is perhaps best represented as:

NO2 + OH → HNO3* (where HNO3* means an excited version of HNO3)

HNO3* + M → HNO3 + M

The question to me is, how do we explain this clearly and more important accuractly. Can I think about this?--NHSavage 20:43, 12 December 2006 (UTC)

Now I read the actual definition carefully as well by the strictist application this reaction is not a termolecular reaction but 2 bimolecular ones with the first reaction being reversible. This may be one where the common use of the term "termolecular" in gas kinetics is not strictly speaking the same as its definition here. What we really need is a real physical chemist not a physicist who pick some gas kinetics up while doing atmospheric chemistry....--NHSavage 20:52, 12 December 2006 (UTC)

This reference seems to suggest that the term termolecular should be "termolecular, that is, reactions involving three molecules interacting simultaneously" in contrast to the way I have seen it being used.

how about the following

---

Molecularity in chemistry is the number of colliding molecular entities that are involved in a single reaction step. While the order of a reaction is derived experimentally, the molecularity is a theoretical concept and can only be applied to elementary reactions. In elementary reactions, the reaction order, the molecularity and the stoichiometric coefficient are the same, although only numerically, because they are different concepts.

• A reaction involving one molecular entity is called unimolecular.
• A reaction involving two molecular entities is called bimolecular.
• A reaction involving three molecular entities is called termolecular. Termolecular reactions in homogenous solutions or gas mixtures are very rare [1]. However the term Termolecular is also used to refer to three body association reactions of the type:

$A+B{\overset {M}{\to }}C$

Where the M over the arrow denotes that to conserve energy and momentum a second reaction with a third body is required after the initial colision of A and B.[2] These reactions frequently have a pressure and temperature dependence region of transition between second and third order kinetics.[3]

--NHSavage 21:30, 12 December 2006 (UTC)

I went on and implemented it with some more text an changing links to footnotes. It looks like we had some ambiguity Knights who say ni 23:48, 12 December 2006 (UTC)

Nice job.--NHSavage 19:19, 13 December 2006 (UTC)

Applicability for elementary reactions

I don't think that the concept of molecularity is applicable only for elementary reactions.It can be extended to any complex reaction provided we take the no. of molecule colliding in RDS.I have read this concept at various place.Editors please take a note of this.Prav001 (talk) 14:33, 25 December 2011 (UTC)

Ozone reaction is poor example

The last section comparing molecularity and order now uses the reaction 2 O₃ → 3 O₂ as an example. This is a poor choice because the rate law is v = - k[O₃]² / [O₂], with order (-1) for O₂. When one partial order is negative, the overall order is usually considered as undefined, as noted in the article on order of reaction. Instead of dealing with the complication of negative partial orders in this article, I suppose to substitute a clearer example which only involves positive partial orders. Dirac66 (talk) 00:16, 19 September 2016 (UTC)