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A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. The one or more substances present at the start of a reaction are called reactants and the one or more substances present at the end of the reaction are called products. The study of chemical reactions is part of the field of science called chemistry.

Chemical reactions can result in molecules attaching to each other to form larger molecules, molecules breaking apart to form two or more smaller molecules, or rearrangement of atoms within or across molecules. Chemical reactions usually involve the making or breaking of chemical bonds.

Chemical reactions can be either spontaneous and require no input of energy, or non-spontaneous which often require the input of some type of energy such as heat, light or electricity. Classically, chemical reactions are strictly transformations that involve the movement of electrons in the forming and breaking of chemical bonds. A more general concept of a chemical reaction would include nuclear reactions and elementary particle reactions.

Reaction types

The common kinds of classical chemical reactions include:

• Isomerization, 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:

N₂ + 3 H₂ → 2 NH₃

• Chemical decomposition in which a compound is decomposed into elements or smaller compounds:

2 H₂O → 2 H₂ + O₂

• Single displacement or substitution, characterized by an element being displaced out of a compound by a more reactive element:

2 Na(s) + 2 HCl(aq) → 2 NaCl(aq) + H₂(g)

• Metathesis or double displacement, in which two compounds exchange ions or bonds to form different compounds:

NaCl(aq) + AgNO₃(aq) → NaNO₃(aq) + AgCl(s)

• Acid-base reactions, broadly characterized as reactions between an acid and a 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 H₃O⁺ ions; bases dissociate in water releasing OH^- ions.
  • Brønsted-Lowry definition: Acids are proton (H⁺) 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. An example of a redox reaction is:

2 S₂O₃²⁻(aq) + I₂(aq) → S₄O₆²⁻(aq) + 2 I⁻(aq)

In which I₂ is reduced to I^- and S₂O₃^2- (thiosulfate anion) is oxidized to S₄O₆^2-.

• 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.

C₁₀H₈+ 12 O₂ → 10 CO₂ + 4 H₂O

CH₂S + 6 F₂ → CF₄ + 2 HF + SF₆

• Disproportionation with one reactant forming two distinct products varying in oxidation state.

2 Sn²⁺ → Sn + Sn⁴⁺

Organic reactions encompass a wide assortment of reactions involving compounds which have carbon as the main element in their molecular structure. The reactions in which an organic compound may take part are largely defined by its functional groups.