En '''cheemsch Reakschoon''' liggt vör, wenn Stoffen wat mitnanner maakt un dorbi wat nieget rutkümmt.

Dat kann sien
* en Stoff fallt utnanner, to’n Bispeel Water in Waterstoff un Suerstoff bi een Elektrolys
*Twee ellers mehr Stoffen geeft een niegen Stoff, t.B.: Natrium un Chlor reagert to dat Solt Natriumchlorid
*Twee ellers mehr Stoffen geeft mehr anner Stoffen, t.B.: Natronlaug un Soltsüür reagert to Natriumchlorid un Water.

Wi schreewt chemische Reaktschonen mit een Piel oder mit een Dubbelpiel. Links steiht, wat insett ward (Edukten), rechts steiht, wat rutkümmt (Produkten).
 Edukten       Produkten
 NaOH + HCl => NaCl + H<sub>2</sub>O

Dat gifft 'n ganzen Barg Oorten vun Reakschonen, t.B.
*Süür - Base - Reakschonen, so as  NaOH + HCl => NaCl + H<sub>2</sub>O
*Redox-Reakschonen, so as 2 Fe + 3 O<sub>2</sub> => 2 Fe<sub>2</sub>O<sub>3</sub> 
*Komplexreakschonen, t.B. Ligandentuusch
*In de Organik ook t.B.
**Additschoonsreakschonen
**Substitutschoonsreakschonen

Bi so en cheemsch Reakschoon geiht dat ook faken üm Energie. De ännert sick. Dorna ünnerscheed wi
*Endotherme Reakschonen, de bruukt Energie
*Exotherme Reakschonen, de sett Energie frie.
Un för dat een Reakschoon mol losgeiht, dor bruukt de Stoffen faken Aktiveerenenergie. Dat heet, dat eersmol wat an Energie tosett warrn mutt, bevör dat losgeiht, ook bi een exotherme Reakschoon. 
As Bispeel kanns dat Füür nehmen. Dor mutts eersmol wat lüttet hebben, wat brennt. Een Füürtüch oder een Rietsticken. Dormit kanns denn dat Papeer ansteken. Un wenn dat brennt, denn fangt ook dat Holt Füür.
De Aktiveerenenergie kann man ok vun de Ümgeven opbracht warrn, wenn dat warm nok is, ellers wann de Aktiveerungsenergie nich so grot is. Denn löpt de Reakschoon all vun alleen af. (t. B. bi de Knallgasreaktschoon)
Un denn könn' bi so ne Reakschoon ook Katalysaters in' speel sin, de mookt, dat nich so veel Aktiveerenenergie nötig is. Dat mookt, dat de Reakschoon loos geiht, ook wenn das nich warm nok is. 

right|180px|thumb|Vapors of [[hydrogen chloride in a beaker and ammonia in a test tube meet to form a cloud of a new substance, ammonium chloride]]
A '''chemical reaction''' is a process that always results in the interconversion of products which are, in general, different from the reactants. Classically, chemical reactions encompass changes that strictly involve the motion of transformations of elementary particles, as well as nuclear reactions.

Different chemical reactions are used in combinations in catalyzed by enzymes form metabolic pathways, by which syntheses and decompositions ordinarily impossible  in conditions within a cell are performed. 
== Reaction types ==
The large diversity of chemical reactions and approaches to their study results in the existence of several concurring, often overlapping, ways of classifying them. Below are examples of widely used terms for describing common kinds of reactions.

*Isomerisation, in which a chemical compound undergoes a structural rearrangement without any change in its net atomic composition; see stereoisomerism
*Direct combination or synthesis, in which 2 or more chemical elements or compounds unite to form a more complex product:
::N<sub>2</sub> + 3 H<sub>2</sub> → 2 NH<sub>3</sub>
*Chemical decomposition or '''analysis''', in which a compound is decomposed into smaller compounds or elements: 
::2 H<sub>2</sub>O → 2 H<sub>2</sub> + O<sub>2</sub>
*Single displacement or substitution, characterized by an element being displaced out of a compound by a more reactive element:
::2 Na(s) + 2 HClNaCl(aq) + H<sub>2</sub>(g)
*Metathesis or '''Double displacement reaction''', in which two compounds exchange ions or bonds to form different compounds:
::NaCl(aq) + AgNO<sub>3</sub>(aq) → NaNO<sub>3</sub>(aq) + AgCl(s)
*base, can have different definitions depending on the acid-base concept employed. Some of the most common are:
:* Arrhenius definition: Acids dissociate in water releasing H<sub>3</sub>O<sup>+</sup> ions; bases dissociate in water releasing OH<sup>-</sup> ions.
:* Brønsted-Lowry definition: Acids are proton (H<sup>+</sup>) donors; bases are proton acceptors. Includes the Arrhenius definition.
:* Lewis definition: Acids are electron-pair acceptors; bases are electron-pair donors. Includes the Brønsted-Lowry definition.
* Redox reactions, in which changes in oxidation numbers of atoms in involved species occur. Those reactions can often be interpreted as transferences of electrons between different molecular sites or species. A typical example of redox rection is:
:2 S<sub>2</sub>O<sub>3</sub><sup>2−</sup>(aq) + I<sub>2</sub>(aq) → S<sub>4</sub>O<sub>6</sub><sup>2−</sup>(aq) + 2 I<sup>−</sup>(aq)
In which I<sub>2</sub> is reduced to I<sup>-</sup> and S<sub>2</sub>O<sub>3</sub><sup>2-</sup> (thiosulfate anion) is oxidized to S<sub>4</sub>O<sub>6</sub><sup>2-</sup>.
*Combustion, a kind of redox reaction in which any combustible substance combines with an oxidizing element, usually oxygen, to generate heat and form oxidized products. The term combustion is usually used for only large-scale oxidation of whole molecules, i.e. a controlled oxidation of a single functional group is not combustion.
::C<sub>10</sub>H<sub>8</sub>+ 12 O<sub>2</sub> → 10 CO<sub>2</sub> + 4 H<sub>2</sub>O
::CH<sub>2</sub>S + 6 F<sub>2</sub> → CF<sub>4</sub> + 2 HF + SF<sub>6</sub>

compounds which have inorganic reactions.
Reactions can also be classified according to their mechanism, some typical examples being:
:*Reactions of ions, e.g. disproportionation of hypochlorite
:*Reactions with reactive ionic intermediates, e.g. reactions of enolates
:*Radical reactions, e.g. combustion at high temperature
:*Reactions of carbenes

== Chemical kinetics ==

The rate of a chemical reaction is a measure of how the concentration or pressure of the involved substances changes with time. Analysis of reaction rates is important for several applications, such as in chemical engineering or in chemical equilibrium study. Rates of reaction depends basically on:
*Reactant concentrations, which usually make the reaction happen at a faster rate if raised through increased collisions per unit time,
*Surface area available for contact between the reactants, in particular solid ones in heterogeneous systems. Larger surface area leads to higher reaction rates.
*Pressure, by increasing the pressure, you decrease the volume between molecules.  This will increase the frequency of collisions of molecules.
*Activation energy, which is defined as the amount of energy required to make the reaction start and carry on spontaneously. Higher activation energy implies that the reactants need more energy to start than a reaction with a lower activation energy.
*Temperature, which hastens reactions if raised, since higher temperature increases the energy of the molecules, creating more collisions per unit time,
*The presence or absence of a catalyst. Catalysts are substances which change the pathway (mechanism) of a reaction which in turn increases the speed of a reaction by lowering the activation energy needed for the reaction to take place. A catalyst is not destroyed or changed during a reaction, so it can be used again.
*For some reactions, the presence of radicals.

Reaction rates are related to the rate law of each reaction.  Note that some reactions have rates that are ''independent'' of reactant concentrations.  These are called zero order reactions.