Editing Chemical reaction (section)

==Catalysis==
{{main|Catalysis}}
{{Further|Reaction Progress Kinetic Analysis}}
[[File:Activation energy.svg|thumb|right|Schematic potential energy diagram showing the effect of a catalyst in an endothermic chemical reaction. The presence of a catalyst opens a different reaction pathway (in red) with lower activation energy. The final result and the overall thermodynamics are the same.]]
[[File:Pot catalytique vue de la structure.jpg|thumb|Solid heterogeneous catalysts are plated on meshes in ceramic [[catalytic converter]]s in order to maximize their surface area. This exhaust converter is from a [[Peugeot 106]] S2 1100]]

In [[catalysis]], the reaction does not proceed directly, but through a reaction with a third substance known as [[catalyst]]. Although the catalyst takes part in the reaction, forming weak bonds with reactants or intermediates, it is returned to its original state by the end of the reaction and so is not consumed. However, it can be inhibited, deactivated or destroyed by secondary processes. Catalysts can be used in a different phase ([[heterogeneous catalysis|heterogeneous]]) or in the same phase ([[homogeneous catalysis|homogeneous]]) as the reactants. In heterogeneous catalysis, typical secondary processes include [[coking]] where the catalyst becomes covered by [[polymer]]ic side products. Additionally, heterogeneous catalysts can dissolve into the solution in a solid-liquid system or evaporate in a solid–gas system. Catalysts can only speed up the reaction – chemicals that slow down the reaction are called inhibitors.<ref>{{GoldBookRef|title=catalyst|file=C00876}}</ref><ref>{{GoldBookRef|title=inhibitor|file=I03035}}</ref> Substances that increase the activity of catalysts are called promoters, and substances that deactivate catalysts are called catalytic poisons. With a catalyst, a reaction that is kinetically inhibited by high activation energy can take place in the circumvention of this activation energy.

Heterogeneous catalysts are usually solids, powdered in order to maximize their surface area. Of particular importance in heterogeneous catalysis are the [[platinum group]] metals and other transition metals, which are used in [[hydrogenation]]s, [[catalytic reforming]] and in the synthesis of commodity chemicals such as [[nitric acid]] and [[ammonia]]. Acids are an example of a homogeneous catalyst, they increase the nucleophilicity of [[carbonyl]]s, allowing a reaction that would not otherwise proceed with electrophiles. The advantage of homogeneous catalysts is the ease of mixing them with the reactants, but they may also be difficult to separate from the products. Therefore, heterogeneous catalysts are preferred in many industrial processes.<ref>{{cite book | author = Elschenbroich, Christoph | title = Organometallchemie | edition = 6th | publisher = [[Vieweg+Teubner Verlag]] | location = Wiesbaden | year = 2008 | isbn = 978-3-8351-0167-8 | page = 263}}</ref>