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| − | [[Image:Chemical decomposition.gif|right|thumb]]
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| − | '''Chemical decomposition''' is the separation (or breakdown) of a [[chemical compound]] into smaller compounds or [[chemical element|elements]]. It is sometimes defined as the opposite of [[chemical synthesis]]. Chemical decomposition is often an undesired [[chemical reaction]]. The stability that a chemical compound ordinarily has is eventually limited when exposed to extreme environmental conditions like [[heat]], [[radiation]], [[humidity]] or the [[acidity]] of a [[solvent]]. The details of decomposition processes are generally not well defined, as a [[molecule]] may break up into a host of smaller fragments. Chemical decomposition is exploited in several analytical techniques, notably [[mass spectrometry]], traditional [[gravimetric analysis]], and [[thermogravimetric analysis]].
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| − | A broader definition of the term '''decomposition''' also includes the breakdown of one phase into two or more phases.<ref name="Gold">{{GoldBookRef|title=decomposition|url=http://goldbook.iupac.org/D01547.html}}</ref>
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| − | <p> There are broadly 3 types of decomposition reactions:
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| − | [[Heat|thermal]], [[electrolysis|electrolytic]], and [[catalysis|catalytic]].</p>
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| − | ==Reaction formulas==
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| − | The generalized reaction formula for chemical decomposition is:
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| − | : AB → A + B
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| − | with a specific example being the [[electrolysis]] of [[water (molecule)|water]] to gaseous [[hydrogen]] and [[oxygen]]:
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| − | : 2H<sub>2</sub>O → 2H<sub>2</sub> + O<sub>2</sub>
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| − | ===Additional examples===
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| − | An example of spontaneous decomposition is that of [[hydrogen peroxide]], which will slowly decompose into water and oxygen:
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| − | : 2H<sub>2</sub>O<sub>2</sub> → 2H<sub>2</sub>O + O<sub>2</sub>
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| − | [[Carbonate]]s will decompose when heated, a notable exception being that of [[carbonic acid]], H<sub>2</sub>CO<sub>3</sub>. Carbonic acid, the "fizz" in sodas, pop cans and other carbonated beverages, will decompose over time (spontaneously) into [[carbon dioxide]] and water
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| − | : H<sub>2</sub>CO<sub>3</sub> → H<sub>2</sub>O + CO<sub>2</sub>
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| − | Other carbonates will decompose when heated producing the corresponding [[metal]] [[oxide]] and carbon dioxide. In the following equation ''M'' represents a metal:
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| − | : MCO<sub>3</sub> → MO + CO<sub>2</sub>
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| − | A specific example of this involving [[calcium carbonate]]:
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| − | : CaCO<sub>3</sub> → CaO + CO<sub>2</sub>
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| − | Metal [[chlorate]]s also decompose when heated. A metal [[chloride]] and oxygen gas are the products.
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| − | : MClO<sub>3</sub> → MCl + O<sub>2</sub>
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| − | A common decomposition of a chlorate to evolve oxygen utilizes [[potassium chlorate]] as follows:
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| − | : 2KClO<sub>3</sub> → 2KCl + 3O<sub>2</sub>
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| − | == Thermal decomposition ==
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| − | '''Thermal decomposition''', also called '''thermolysis''', is defined as a [[chemical reaction]] whereby a [[chemical substance]] breaks up into at least two chemical substances when heated. The reaction is usually [[endothermic]] as heat is required to break [[chemical bond]]s in the compound undergoing decomposition. The ''decomposition temperature'' of a substance is the [[temperature]] at which the substance [[Chemical decomposition|decomposes]] into smaller substances or into its constituent [[atoms]].
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| − | For example, [[calcium carbonate]] decomposes into [[calcium oxide]] and [[carbon dioxide]]. Some compounds, on the other hand, simply decompose into their constituent elements. [[Water]], when heated to well over 2000 degrees [[Celsius]], breaks up into its components - [[hydrogen]] and [[oxygen]].
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| − | A common example is the decomposition of [[copper carbonate]] into [[copper oxide]] and [[carbon dioxide]], seen here:
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| − | :[[copper(II) carbonate|CuCO<sub>3</sub>]] → [[copper(II) oxide|CuO]] + [[carbon dioxide|CO<sub>2</sub>]]
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| − | The [[copper carbonate]] turns from a green powder into a black [[copper oxide]], and [[carbon dioxide]] is released in a gaseous state.
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| − | Decomposition may be aided by the presence of a catalyst. For example, hydrogen peroxide decomposes more quickly with the use of manganese(IV) oxide:
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| − | :2H<sub>2</sub>O<sub>2</sub>(aq) → 2H<sub>2</sub>O(l) + O<sub>2</sub>(g)
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| − | High temperatures can also induce [[polymerization]], which produces larger molecules, possibly also causing thermal decomposition and evaporation of smaller molecules in the process. Such reactions are called [[pyrolysis]] reactions. A common example is [[coking]], which is the formation of an amorphous carbon structure along with the evaporation of hydrogen and other pyrolysis gases.
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| − | If thermal decomposition of a substance is significantly exothermic, then the substance is thermodynamically unstable. If initiated, its decomposition forms a positive feedback loop and undergoes [[thermal runaway]] up to the point of causing an explosion.
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| − | This process can be seen in almost every office as a coffee pot is left on the hot plate. When examined, one can see an oily substance on the top that is the organic components of the coffee coming out of solution due to over or re-heating.
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| − | ==See also==
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| − | * [[Analytical chemistry]]
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| − | * [[Carbonate]]
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| − | * [[Catalysis]]
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| − | * [[Chemical reaction]]
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| − | * [[Combustion]]
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| − | * [[Electrolysis]]
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| − | * [[Heat]]
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| − | == Notes ==
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| − | <references/>
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| − | ==References==
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| − | * McMurry, John. 2004. ''Organic Chemistry'', 6th ed. Belmont, CA: Brooks/Cole. ISBN 0534420052
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| − | * Solomons, T.W. Graham, and Craig B. Fryhle. 2004. ''Organic Chemistry'', 8th ed. Hoboken, NJ: John Wiley. ISBN 0471417998
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| − | * Zumdahl, Steven S. 2005. ''Chemical Principles''. New York, NY: Houghton Mifflin. ISBN 0618372067
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| − | ==External links==
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| − | *[http://umbbd.msi.umn.edu Biodegradation database]
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| − | [[Category:Physical sciences]]
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| − | [[Category:Chemistry]]
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| − | [[Category:Inorganic chemistry]]
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| − | [[Category:Organic chemistry]]
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| − | {{credits|Chemical_decomposition|250471842|Thermal_decomposition|255461722}}
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