Editing Collision theory (section)

{{Short description|Chemistry principle}}
[[Image:Molecular-collisions.jpg|thumb|325px|[[Reaction rate]] tends to increase with [[concentration]] phenomenon explained by '''collision theory''']]
'''Collision theory''' is a principle of [[chemistry]] used to predict the rates of [[chemical reaction]]s. It states that when suitable particles of the [[Reagent|reactant]]  hit each other with the correct orientation, only a certain amount of collisions result in a perceptible or notable change; these successful changes are called successful collisions. The successful collisions must have enough energy, also known as [[activation energy]], at the moment of impact to break the pre-existing bonds and form all new bonds. This results in the products of the reaction. The [[activation energy]] is often predicted using the [[transition state theory]]. Increasing the concentration of the reactant brings about more collisions and hence more successful collisions. Increasing the temperature increases the average kinetic energy of the molecules in a solution, increasing the number of collisions that have enough energy. Collision theory was proposed independently by [[Max Trautz]] in 1916<ref>Trautz, Max. [https://doi.org/10.1002/zaac.19160960102 ''Das Gesetz der Reaktionsgeschwindigkeit und der Gleichgewichte in Gasen. Bestätigung der Additivität von C<sub>v</sub> − 3/2 R. Neue Bestimmung der Integrationskonstanten und der Moleküldurchmesser''], Zeitschrift für anorganische und allgemeine Chemie, Volume 96, Issue 1, Pages 1–28, (1916).</ref> and [[William Lewis (physical chemist)|William Lewis]] in 1918.<ref>{{GoldBookRef | file = C01170 | title = collision theory}}</ref><ref> William Cudmore McCullagh Lewis, [https://doi.org/10.1039/CT9181300471 ''XLI.—Studies in catalysis. Part IX. The calculation in absolute measure of velocity constants and equilibrium constants in gaseous systems''], J. Chem. Soc., Trans., 1918, 113, 471-492.   </ref>

When a catalyst is involved in the collision between the reactant molecules, less energy is required for the chemical change to take place, and hence more collisions have sufficient energy for the reaction to occur. The reaction rate therefore increases.

Collision theory is closely related to [[chemical kinetics]]. 

Collision theory was initially developed for the gas reaction system with no dilution. But most reactions involve solutions, for example, gas reactions in a carrying inert gas, and almost all reactions in solutions. The collision frequency of the solute molecules in these solutions is now controlled by [[diffusion]] or [[Brownian motion]] of individual molecules. The flux of the diffusive molecules follows [[Fick's laws of diffusion]]. For particles in a solution, an example model to calculate the collision frequency and associated coagulation rate is the [[Smoluchowski coagulation equation]] proposed by [[Marian Smoluchowski]] in a seminal 1916 publication.<ref name=Smoluchowski1916>{{cite journal
|last=Smoluchowski
|first=Marian
|title=Drei Vorträge über Diffusion, Brownsche Molekularbewegung und Koagulation von Kolloidteilchen
|journal=Phys. Z.
|year=1916
|volume=17
|pages=557–571, 585–599
|language=German
|bibcode=1916ZPhy...17..557S}}</ref> In this model, Fick's flux at the infinite time limit is used to mimic the particle speed of the collision theory.