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. 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.
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.
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 two or more chemical elements or compounds unite to form a more complex product:
- N2 + 3 H2 → 2 NH3
- 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:
- 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. In the following example of a redox rection, I2 (iodine) is reduced to I- (iodide anion), and S2O32- (thiosulfate anion) is oxidized to S4O62-:
- 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.
- C10H8+ 12 O2 → 10 CO2 + 4 H2O
- CH2S + 6 F2 → CF4 + 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 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.
- Addition reactions, including such reactions as halogenation, hydrohalogenation, and hydration. Based on the mechanism, the main addition reactions are classified as electrophilic, nucleophilic, or radical addition.
- Elimination reactions, including processes such as dehydration.
- Substitution reactions are divided into several types: nucleophilic aliphatic substitution with SN1, SN2 and SNi reaction mechanisms; nucleophilic aromatic substitution; nucleophilic acyl substitution; electrophilic substitution; electrophilic aromatic substitution; and radical substitution.
- Organic redox reactions are redox reactions specific to organic compounds and are very common.
- Rearrangement reactions are divided into 1,2-rearrangements, pericyclic reactions, and metathesis reactions.
- In condensation reactions, two reactants combine, and a small molecule (usually water) is split off. The opposite reaction, when water is consumed in a reaction, is called hydrolysis. Many polymerization reactions are derived from organic reactions. They are divided into addition polymerizations and step-growth polymerizations.
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 electromagnetic radiation, most notably ultra violet, is needed to promote the breaking of bonds to start the reaction. This is particularly true for reactions involving radicals.
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.
- ↑ Chemical reaction IUPAC Gold Book. Retrieved September 24, 2008.
- Chang, Raymond. 2006. Chemistry, 9th ed. New York: McGraw-Hill Science/Engineering/Math. ISBN 0073221031
- Cotton, F. Albert, and Geoffrey Wilkinson. 1980. Advanced Inorganic Chemistry, 4th ed. New York: Wiley. ISBN 0471027758
- McMurry, J., and R.C. Fay. 2004. Chemistry, 4th ed. Upper Saddle River, NJ: Prentice Hall. ISBN 0131402080
- McMurry, John. 2004. Organic Chemistry, 6th ed. Belmont, CA: Brooks/Cole. ISBN 0534420052
- Morrison, Robert T., and Robert N. Boyd. 1992. Organic Chemistry, 6th ed. Englewood Cliffs, NJ: Prentice Hall. ISBN 0-13-643669-2
- Solomons, T.W. Graham, and Craig B. Fryhle. 2004. Organic Chemistry, 8th ed. Hoboken, NJ: John Wiley. ISBN 0471417998
All links retrieved May 10, 2013.
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