Editing Reaction rate constant (section)

== Elementary steps ==
For an [[Reaction step|elementary step]], there ''is'' a relationship between stoichiometry and rate law, as determined by the [[law of mass action]].  Almost all elementary steps are either unimolecular or bimolecular.  For a unimolecular step

{{block indent|em=1.5|text=A → P}}

the reaction rate is described by <math>r = k_1[\mathrm{A}]</math>, where <math>k_1</math> is a unimolecular rate constant. Since a reaction requires a change in molecular geometry, unimolecular rate constants cannot be larger than the frequency of a molecular vibration.  Thus, in general, a unimolecular rate constant has an upper limit of ''k''<sub>1</sub> ≤ ~10<sup>13</sup> s<sup>−1</sup>.  

For a bimolecular step

{{block indent|em=1.5|text=A + B → P}}

the reaction rate is described by <math>r=k_2[\mathrm{A}][\mathrm{B}]</math>, where <math>k_2</math> is a bimolecular rate constant. Bimolecular rate constants have an upper limit that is determined by how frequently molecules can collide, and the fastest such processes are limited by [[diffusion]].  Thus, in general, a bimolecular rate constant has an upper limit of ''k''<sub>2</sub> ≤ ~10<sup>10</sup> M<sup>−1</sup>s<sup>−1</sup>.  

For a termolecular step

{{block indent|em=1.5|text=A + B + C → P}}

the reaction rate is described by <math>r=k_3[\mathrm{A}][\mathrm{B}][\mathrm{C}]</math>, where <math>k_3</math> is a termolecular rate constant.

There are few examples of elementary steps that are termolecular or higher order, due to the low probability of three or more molecules colliding in their reactive conformations and in the right orientation relative to each other to reach a particular transition state.<ref>{{Cite book|url=https://archive.org/details/LowryT.H.RichardsonK.S.MechanismAndTheoryInOrganicChemistry3RdEd1091PagsHarpercollins1987| title=Mechanism and theory in organic chemistry|last=Lowry|first=Thomas H.|date=1987|publisher=Harper & Row|others=Richardson, Kathleen Schueller| isbn=978-0060440848 | edition= 3rd | location=New York|oclc=14214254}}</ref>  There are, however, some termolecular examples in the gas phase. Most involve the recombination of two atoms or small radicals or molecules in the presence of an inert third body which carries off excess energy, such as O + {{chem|O|2}} + {{chem|N|2}} → {{chem|O|3}} + {{chem|N|2}}.  One well-established example is the termolecular step 2 I + {{chem|H|2}} → 2 HI in the [[Hydrogen iodide#Synthesis|hydrogen-iodine reaction]].<ref>{{Cite book| title=Kinetics and Mechanism|last1=Moore|first1=John W.|last2=Pearson|first2=Ralph G. |edition=3rd |date=1981|publisher=John Wiley| isbn=978-0-471-03558-9|pages=226–7}}</ref><ref>The reactions of nitric oxide with the diatomic molecules {{chem|Cl|2}}, {{chem|Br|2}} or {{chem|O|2}} (e.g., 2 NO + {{chem|Cl|2}} → 2 NOCl, etc.) have also been suggested as examples of termolecular elementary processes.  However, other authors favor a two-step process, each of which is bimolecular: (NO + {{chem|Cl|2}} ⇄ {{chem|NOCl|2}}, {{chem|NOCl|2}} + NO → 2 NOCl). See: {{cite book |editor1-last=Compton |editor1-first=R.G. |editor2-last=Bamford |editor2-first=C. H. |editor3-last=Tipper |editor3-first=C.F.H. |chapter=5. Reactions of the Oxides of Nitrogen §5.5 Reactions with Chlorine |chapter-url={{GBurl|GwhMyI_tZO4C|p=174}} |title=Reactions of Non-metallic Inorganic Compounds |publisher=Elsevier |series=Comprehensive Chemical Kinetics |volume=6 |date=2014 |orig-date=1972 |isbn=978-0-08-086801-1 |pages=174 |url=}}</ref><ref>{{Cite journal| last=Sullivan|first=John H.|date=1967-01-01|title=Mechanism of the ''Bimolecular'' Hydrogen—Iodine Reaction|journal=The Journal of Chemical Physics|volume=46|issue=1|pages=73–78|doi=10.1063/1.1840433|bibcode=1967JChPh..46...73S |issn=0021-9606}}</ref> In cases where a termolecular step might plausibly be proposed, one of the reactants is generally present in high concentration (e.g., as a solvent or diluent gas).<ref>{{Cite book|title=Chemistry & chemical reactivity|last=Kotz|first=John C.|date=2009|publisher=Thomson Brooks/ Cole| others=Treichel, Paul., Townsend, John R.|isbn=9780495387039|edition=7th|location=Belmont, Calif.|pages=703|oclc=220756597}}</ref>