Any chemical reaction in which electrons are transferred. Addition of hydrogen or electrons is reduction, and removal of hydrogen or electrons is oxidation (originally applied to combination with oxygen but now including transfer of hydrogen or electrons). The processes always occur simultaneously: one substance is oxidized by the other, which it reduces. The conditions of the substances before and after are called oxidation states, to which numbers are given and with which calculations can be made. (Valence is a similar but not identical concept.) The chemical equation that describes the electron transfer can be written as two separate half reactions that can in theory be carried out in separate compartments of an electrolytic cell (see electrolysis), with electrons flowing through a wire connecting the two. Strong oxidizing agents include fluorine, ozone, and oxygen itself; strong reducing agents include alkali metals such as sodium and lithium.
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Amount of heat that must be added or removed during a chemical reaction to keep all substances involved at the same temperature. If it is positive (heat must be added), the reaction is endothermic; if it is negative (heat is given off), the reaction is exothermic. Accurate heat of reaction values are needed for proper design of equipment used in chemical processes; they are usually estimated from compiled tables of thermodynamics data (heats of formation and heats of combustion of many known materials). The activation energy is unrelated to the heat of reaction.
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Speed at which a chemical reaction proceeds, in terms of amount of product formed or amount of reactant consumed per unit of time. The reaction rate depends on the nature of the reacting substances and the type of chemical change, as well as temperature and pressure, especially if gases are involved. In general, the reactions of ions occur very rapidly, but those in which covalent bonds are formed or broken are slower. Catalysts usually accelerate reaction rates. The prediction, measurement, and interpretation of reaction rates are subjects of the branch of chemistry known as chemical kinetics. Seealso mass action, law of.
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Any chemical process in which substances are changed into different ones, with different properties, as distinct from changing position or form (phase). Chemical reactions involve the rupture or rearrangement of the bonds holding atoms together (see bonding), never atomic nuclei. The total mass and number of atoms of all reactants equals those of all products, and energy is almost always consumed or liberated (see heat of reaction). The speed of reactions varies (see reaction rate). Understanding their mechanisms lets chemists alter reaction conditions to optimize the rate or the amount of a given product; the reversibility of the reaction and the presence of competing reactions and intermediate products complicate these studies. Reactions can be syntheses, decompositions, or rearrangements, or they can be additions, eliminations, or substitutions. Examples include oxidation-reduction, polymerization, ionization (see ion), combustion (burning), hydrolysis, and acid-base reactions.
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Laboratory technique used to make numerous copies of specific DNA segments quickly and accurately. These are needed for various experiments and procedures in molecular biology, forensic analysis (DNA fingerprinting), evolutionary biology (to amplify DNA fragments found in ancient specimens), and medicine (to diagnose genetic disease or detect low viral counts). Invented by Kary Mullis, PCR requires a DNA template (as little as one molecule) to copy, nucleotides to build the copies, and the enzyme DNA polymerase to catalyze the formation of bonds between the nucleotide monomers. Each three-step cycle (separating the two strands of the DNA double helix, marking the ends of the segment to be copied, and catalyzing the formation of bonds), which takes only minutes to complete, doubles the number of DNA strands present in the reaction medium. Repetition of this cycle many times results in an exponential increase in the amount of DNA.
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Chemical reaction initiated by absorption of energy in the form of visible (light), ultraviolet, or infrared radiation. Primary photochemical processes occur as an immediate result, and secondary processes may follow. The most important example is photosynthesis. Vision depends on photochemical reactions that occur in the eye (see retina; rhodopsin). In photographic film and paper, light activates their silver salts to a state that is easy to reduce to metallic silver grains during development. Bleaching of laundry, tanning of human skin, formation of Earth's ozone layer, and many industrial reactions are also photochemical. Certain air pollutants (see smog) become more reactive and form noxious compounds in photochemical reactions.
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Process yielding products that initiate further processes of the same kind. Nuclear chain reactions are a series of nuclear fissions initiated by neutrons produced in a preceding fission. A critical mass, large enough to allow more than one fission-produced neutron to be captured, is necessary for the chain reaction to be self-sustaining. Uncontrolled chain reactions, as in an atomic bomb, occur when large numbers of neutrons are present and the reactions multiply very quickly. Nuclear reactors control their reactions through the careful distribution of the fissionable material and insertion of neutron-absorbing materials.
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Consider the reaction below:
The two elements involved, iron and chlorine, each change oxidation state; iron from 2+ to 3+, chlorine from 0 to 1−. There are then effectively two half-reactions occurring. These changes can be represented in formulas by inserting appropriate electrons into each half-reaction:
In the same way given two half-reactions it is possible, with knowledge of appropriate electrode potentials, to arrive at the full (original) reaction.