catalysis (Greek κατάλυσις, katálysis - the dissolution, abolishment, abolition) is called the change (acceleration, change of the selectivity) of a chemical reaction by participation of a catalyst. The catalyst comes out from the Gesamtreaktion again and can thus several catalysis cycles go through. A reaction, which is thermodynamically not permitted, is not made possible also by a catalyst, because the principle of the energy conservation permits only the decrease to the activation energy of the reaction, not however the change of reaction energy.
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in Catalyst is a material in a chemical reaction is involved without thereby to be used. The catalyst accelerates a reaction, and/or. the necessary activation energy lowers , so that a reaction e.g. at lower temperatures takes place. Many reactions are at allonly by a catalyst possible, because without catalyst the reaction would run off extremely slowly (e.g.) in some millions years or at the temperature necessary for the reaction without catalyst the thermodynamic equilibrium on the side of the basic materials is.
a simple chemical reaction A + B -> OFF knows z. B. as follows by a catalyst “K” to be affected.
A + K -> AK AK + B -> + K hereby
is also avowed OFF that against the assumption from the early history of chemistry not only the presence of a catalyst thoseReaction affects, but the catalyst is actually involved in the reaction, even if it is present afterwards invariably.
it is differentiated fundamentally between heterogeneous catalysis and homogeneous catalysis. A special form represents the phase transfer catalysis. Becomes partial alsothe bio catalysis as special form regards.
Heterogeneous catalysis the reactions are called, with which the catalyst and the basic materials of the reaction are in different phases. (E.G. in oil refineries, if the heated oil in the gas condition with firm catalysts so-called. Contactsreacted or in the vehicle catalyst with that the gaseous Verbrennungsprodukte at a solid to less poisonous exhaust gases react).
With the homogeneous catalysis both the basic materials and the catalyst are in the same phase. Often it acts thereby overReactions with liquid basic materials, with those z. B. a catalytically working acid as liquid is in addition-given. Also the Olefinmetathese is catalyzed by homogeneous transition metal catalysts.
in the heterogeneous catalysis with
monomolecular reactions e.g. disintegrates to mechanisms. simply the Eduktat the catalyst surface. With bimolecular reactions three mechanisms are conceivable:
1. Langmuir Hinshelwood mechanism
with the Langmuir Hinshelwood mechanism first both Edukte are adsorbed by the catalyst surface.
A g -> A ads B g -> B ads
following react the adsorbedEdukte on the catalyst surface:
A ads + B ads -> C ads
in the last step desorbs the product
C ads -> C g
2. Eley Rideal mechanism
with the Eley Rideal mechanism is adsorbed first only a Edukt by the catalyst surface.
A g -> A ads
following react the adsorbed Edukte with the still free Edukt:
A ads + B g -> C ads
in the last step desorbs again the product.
C ads -> C g
3. Mars van Krevelen mechanism
The Edukt A is adsorbed first from the gaseous phase on the catalyst surface.ASSI
A g -> A ads
oxygen is adsorbed on another surface of the solid body, dissociated there and diffused to the surface (close.: surface), at A adsorbsis:
O 2, g -> 2 O ads -> 2 O surf
then the reaction with A takes place to BO and C and it follows the desorption of the products into the gaseous phase.
A ads + O surf -> BO ads + C ads -> BO g + C g
of metabolic procedures in organisms are catalyzed by enzymes. These reactions are generally characterised by extremely high efficiency and selectivity and run at mild temperatures and in aqueousMillieu off. Reactive species, which would react with water, become shielded by hydrophobe „bags “. Many biocatalysts are proteins or contain protein components.
phase transfer catalysis
mediating a catalyst the contact of two reactants, those in different phases (usually aqueous and organicPhase) are present, then one speaks of phase transfer catalysis. The catalyst of such a reaction makes possible depresses the reactants through the phase boundary.
For example Kronenether make the solution possible of alkali metal ions in organic solvents, quart acres of ammonium salts with lipohilen alkyl residues improve the solubility of nonpolar moleculesin water.
some typical examples of catalysis procedures
- the catalytic fragmentation (Cracking) from oil to gasoline products in oil refineries.
- Reduction of nitrogen oxides from the engine exhaust gases in platinum - catalyst of motor vehicles
- the ammonia synthesis
- oxidation of hydrogen and oxygen to water in gas cells
- burn of sugar by cigarette ash (a popular charm trick)
- almost all biochemical procedures by enzymes are catalyzed.
Kripylo, P.; Wendlandt, K. - P.; Vogt, F.:. Heterogeneous catalysis in the chemical technology. GermanTo publishing house for basic industry GmbH, Leipzig, 1993
Web on the left of
- http://www.uni-bayreuth.de/departments/didaktikchemie/umat/katalyse2/katalyse2.htm - lecture principles of the catalysis
- http://reaktionskinetik.de/seite31.html - descriptive explanation of the catalysis
- chemistryin the everyday life: Catalysis - which is that?