Chemical Reaction

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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 leads to the interconversion of chemical substances.[1] The substances initially involved in a chemical reaction are called the reactants, and the substances produced by the reaction are called the products. Given that chemical reactions are usually characterized by a chemical change, they yield one or more products that are, in general, different from the reactants.

Classically, chemical reactions encompass changes that strictly involve the motion of electrons in the forming and breaking of chemical bonds. However, the general concept of a chemical reaction, in particular the notion of a chemical equation, is applicable to transformations of elementary particles and nuclear reactions.

A series of different chemical reactions may be performed to synthesize a desired product. In biochemistry, sets of chemical reactions catalyzed by enzymes make up metabolic pathways, in which syntheses and decompositions ordinarily impossible under conditions within a cell are performed.

Chemical reactions take place within each living organism, allowing the organism to survive, grow, and reproduce. In addition, researchers and chemical engineers utilize chemical reactions to produce a vast array of materials, including petrochemicals, agrochemicals, ceramics, polymers and rubber (elastomers), oleochemicals (oils, fats, and waxes), explosives, fragrances, and flavors. Modern society is highly dependent on these products.

Chemical equations

A chemical reaction is symbolically represented by a chemical equation, wherein one set of substances, called the reactants, is converted into another set of substances, called the products. The reactants and products are shown using their chemical formulas, and an arrow is used to indicate the direction of the reaction. The reactants are usually placed to the left of the arrow, and the products are placed to the right. If the reaction is irreversible, a single arrow is used; if the reaction is reversible, a double arrow (pointing in opposite directions) is used.

For example, the combustion of methane in oxygen may be represented by the following equation:

CH4 + 2 O2 → CO2 + 2 H2O

This equation represents an irreversible reaction in which one molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water.

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.

N2 + 3 H2 → 2 NH3
2 H2O → 2 H2 + O2
2 Na(s) + 2 HCl(aq) → 2 NaCl(aq) + H2(g)
NaCl(aq) + AgNO3(aq) → NaNO3(aq) + AgCl(s)
  • 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.
2 S2O32−(aq) + I2(aq) → S4O62−(aq) + 2 I(aq)
C10H8+ 12 O2 → 10 CO2 + 4 H2O
CH2S + 6 F2CF4 + 2 HF + SF6

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

Organic reactions

Organic reactions encompass a wide assortment of reactions involving compounds that 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.

There is no limit to the number of possible organic reactions and mechanisms. However, certain general patterns are observed that can be used to describe many common or useful reactions. Each reaction has a stepwise reaction mechanism that explains how it happens. Organic reactions can be organized into several basic types, with some reactions fitting into more than one category. Some of the basic types of organic chemical reactions are noted below.

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:

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.

See also

Notes

  1. Chemical reaction IUPAC Gold Book. Retrieved September 24, 2008.

References
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External links

All links retrieved February 8, 2017.

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