A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, often coming about only after the input of some type of energy, viz. heat, light or electricity. Classically, chemical reactions encompass changes that strictly involve the motion of electrons in the forming and breaking of chemical bonds, although the general concept of a chemical reaction, in particular the notion of a chemical equation, is applicable to transformations of elementary particles, as well as nuclear reactions.
The substance/substances initially involved in a chemical reaction are called reactants. Chemical reactions are usually characterized by a chemical change, and they yield one or more products, which usually have properties different from the reactants.
Different chemical reactions are used in combination in chemical synthesis in order to get a desired product. In biochemistry, series of chemical reactions catalyzed by enzymes form metabolic pathways, by which syntheses and decompositions ordinarily impossible in conditions within a cell are performed.
Some common kinds of reactions are listed below. Note that it is perfectly possible for a single reaction to fall under more than one category:
* Isomerisation, in which a chemical compound undergoes a structural rearrangement without any change in its net atomic composition;
* Chemical decomposition or analysis, in which a compound is decomposed into smaller compounds or elements:
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 reaction, 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:
2 S2O32−(aq) + I2(aq) → S4O62−(aq) + 2 I−(aq)
* 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.
* Disproportionation a redox reaction in which 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.
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,
Reaction rates are related to the concentrations of substances involved in reactions, as quantified by the rate law of each reaction. Note that some reactions have rates that are independent of reactant concentrations. These are called zero order reactions.
Reactions and Energy
Chemical energy is part of all chemical reactions. Energy is needed to break chemical bonds in the starting substances. As new bonds form in the final substances, energy is released. By comparing the chemical energy of the original substances with the chemical energy of the final substances, you can decide if energy is released or absorbed in the overall reaction.
A chemical reaction in which energy is released is called an exothermic reaction. Exo means "go out" or "exit." Thermic means "heat" or "energy." Exothermic reactions can give off energy in several forms. If heat is released in an exothermic reaction, the nearby matter will become warmer. The nearby matter absorbs the heat released by the reaction. The reaction between gasoline and oxygen in a car's engine is an exothermic reaction.
* List of reactions
* Is This Reaction a Substitution, Oxidation-Reduction, or Transfer? / N.S.Imyanitov. J. Chem. Educ. 1993, 70(1), 14 – 16.
1. ^ International Union of Pure and Applied Chemistry. "chemical reaction". Compendium of Chemical Terminology Internet edition.