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| In [[chemistry]], a '''chemical transport reaction''' describes a process for purification and [[crystallization]] of non-[[volatility (chemistry)|volatile]] [[solid]]s.<ref>Michael Binnewies, Robert Glaum, Marcus Schmidt, Peer Schmidt "Chemical Vapor Transport Reactions – A Historical Review" Zeitschrift für anorganische und allgemeine Chemie 2013, Volume 639, pages 219–229. {{DOI|10.1002/zaac.201300048}}</ref> The process is also responsible for certain aspects of mineral growth from the effluent of [[volcano]]es. The technique is distinct from [[chemical vapor deposition]], which usually entails decomposition of molecular precursors (e.g. SiH<sub>4</sub> → Si + 2H<sub>2</sub>) and which gives conformal coatings.
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| The technique, which was popularized by Schäfer,<ref>{{cite journal
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| | author = Günther Rienäcker, Josef Goubeau
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| | title = Professor Harald Schäfer
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| | journal = [[Zeitschrift für anorganische und allgemeine Chemie]]
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| | year = 1973
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| | volume = 395
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| | issue = 2-3
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| | pages = 129–133
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| | doi = 10.1002/zaac.19733950202}}</ref> entails the reversible conversion of nonvolatile [[Chemical element|element]]s and [[chemical compound]]s into volatile derivatives.<ref>Schäfer, H. "Chemical Transport Reactions" Academic Press, New York, 1963.</ref> The volatile derivative migrates throughout a sealed reactor, typically a sealed and evacuated glass tube heated in a [[tube furnace]]. Because the tube is under a temperature gradient, the [[volatility (chemistry)|volatile]] derivative reverts to the parent solid and the transport agent is released at the end opposite to which it originated (see next section). The transport agent is thus [[catalyst|catalytic]]. The technique requires that the two ends of the tube (which contains the sample to be crystallized) be maintained at different temperatures. So-called two-zone tube furnaces are employed for this purpose. The method derives from the [[crystal bar process|Van Arkel de Boer process]]<ref>{{cite journal | doi = 10.1002/zaac.19251480133 | title = Darstellung von reinem Titanium-, Zirkonium-, Hafnium- und Thoriummetall | first = A. E. | last = van Arkel | coauthors = de Boer, J. H. | volume = 148 | issue = 1 | pages =345–350 | year = 1925 | journal = Zeitschrift für anorganische und allgemeine Chemie | language = German}}</ref> which was used for the purification of titanium and vanadium and uses iodine as the transport agent.
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| [[Image:Titan-crystal_bar.JPG|thumb|left|240px|Crystals of [[Titanium]] grown using the Van Arkel de Boer process with I<sub>2</sub> as the transport agent.]]
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| ==Cases of the exothermic and endothermic reactions of the transporting agent==
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| Transport reactions are classified according to the [[thermodynamic]]s of the reaction between the solid and the transporting agent. When the reaction is [[exothermic]], then the solid of interest is transported from the cooler end (which can be quite hot) of the reactor to a hot end, where the equilibrium constant is less favorable and the crystals grow. The reaction of [[molybdenum dioxide]] with the transporting agent [[iodine]] is an exothermic process, thus the MoO<sub>2</sub> migrates from the cooler end (700 °C) to the hotter end (900 °C):
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| :MoO<sub>2</sub> + I<sub>2</sub> {{eqm}} MoO<sub>2</sub>I<sub>2</sub> ΔH<sub>rxn</sub> < 0 (exothermic)
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| Using 10 milligrams of iodine for 4 grams of the solid, the process requires several days.
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| Alternatively, when the reaction of the solid and the transport agent is endothermic, the solid is transported from a hot zone to a cooler one. For example:
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| :[[Iron(III) oxide|Fe<sub>2</sub>O<sub>3</sub>]] + 6 [[hydrogen chloride|HCl]] {{eqm}} [[Iron(III) chloride|Fe<sub>2</sub>Cl<sub>6</sub>]]+ 3 H<sub>2</sub>O ΔH<sub>rxn</sub> > 0 (endothermic)
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| The sample of iron(III) oxide is maintained at 1000 °C, and the product is grown at 750 °C. HCl is the transport agent. Crystals of [[hematite]] are reportedly observed at the mouths of volcanoes because of chemical transport reactions whereby volcanic hydrogen chloride volatilizes iron(III) oxides.<ref>{{cite journal
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| | author = P. Kleinert, D. Schmidt
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| | title = Beiträge zum chemischen Transport oxidischer Metallverbindungen. I. Der Transport von α-Fe<sub>2</sub>O<sub>3</sub> über dimeres Eisen(III)-chlorid
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| | journal = [[Zeitschrift für anorganische und allgemeine Chemie]]
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| | year = 1966
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| | volume = 348
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| | issue = 3-4
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| | pages = 142–150
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| | doi = 10.1002/zaac.19663480305}}</ref>
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| ==Halogen lamp==
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| A similar reaction like that of MoO<sub>2</sub> is used in [[halogen lamp]]s. The tungsten is evaporated from the tungsten filament and converted with traces of oxygen and iodine into the WO<sub>2</sub>I<sub>2</sub>, at the high temperatures near the filament the compound decomposes back to tungsten oxygen and iodine. <ref>{{cite journal
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| | author = J. H. Dettingmeijer, B. Meinders
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| | title = Zum system W/O/J. I: das Gleichgewicht WO<sub>2</sub>, f + J<sub>2</sub>, g = WO<sub>2</sub>J<sub>2</sub>,g
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| | journal = [[Zeitschrift für anorganische und allgemeine Chemie]]
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| | year = 1968
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| | volume = 357
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| | issue = 1-2
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| | pages = 1–10
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| | doi = 10.1002/zaac.19683570101}}</ref>
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| :WO<sub>2</sub> + I<sub>2</sub> <math>\overrightarrow{\leftarrow}</math> WO<sub>2</sub>I<sub>2</sub> ΔH<sub>rxn</sub> < 0 (exothermic)
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| ==References==
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| {{reflist}}
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| [[Category:Inorganic chemistry]]
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| [[Category:Solid-state chemistry]]
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