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'''Nascent hydrogen''' is purported to consist of a chemically reactive form of hydrogen that is freshly generated, hence nascent. Molecular hydrogen (H<sub>2</sub>), which is the normal form of this element, is unreactive toward organic compounds, so a special state of hydrogen was once invoked to explain certain kinds of [[hydrogenation]]s. For example reductions of organic compounds with a mixture of sodium and alcohols, the [[Bouveault–Blanc reduction]] does not occur with hydrogen, with the solvent (alcohol) or with sodium, but only with the mixture. Mechanistic understanding of such reactions is now available, and the concept of nascent hydrogen is discounted, even ridiculed.<ref>{{Cite journal |
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'''Atomic hydrogen''' (or '''nascent hydrogen''')<ref>{{Cite journal |
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|bibcode = 2002AcSpe..57..797L }}</ref><ref>V. F�bos, A. K. L. Yuen, A. F. Masters, T. Maschmeyer "Exploring the Myth of Nascent Hydrogen and its Implications for Biomass Conversions" Chem. Asian J. 2012, 7, 2629 – 2637. {{DOI|10.1002/asia.201200557}}[</ref> |
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|bibcode = 2002AcSpe..57..797L }}</ref> consists of individual [[hydrogen]] [[atoms]] that are not bound together like those in ordinary hydrogen [[molecules]]. The species is denoted by ''H (atomic)'', contrasted with the usual ''H<sub>2</sub>'' ([[dihydrogen]] or just 'hydrogen') commonly involved in chemical reactions. It is claimed to exist transiently but long enough to affect chemical reactions. According to one claim, nascent hydrogen is generated ''[[in situ]]'' usually by the reaction of [[zinc]] with an [[acid]], [[aluminium]] ([[Devarda's alloy]]) with [[sodium hydroxide]], or by [[electrolysis]] at the [[cathode]].{{Citation needed|date=April 2008}} Being monoatomic, H atoms are much more reactive and thus a much more effective [[reducing agent]] than ordinary diatomic H<sub>2</sub>, but again the key question is whether H atoms exist in any chemically meaningful way under the conditions claimed. The concept is more popular in engineering and in older literature on catalysis.{{Citation needed|date=April 2008}} |
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==Making atomic hydrogen== |
==Making atomic hydrogen== |
Revision as of 02:18, 18 May 2013
Names | |
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Systematic IUPAC name | |
Identifiers | |
3D model (JSmol)
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ChEBI | |
ChemSpider | |
PubChem CID
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Properties | |
H | |
Molar mass | 1.008 g·mol−1 |
Reacts | |
0 D | |
Thermochemistry | |
Std molar
entropy (S⦵298) |
114.715-114.719 J K-1 mol-1 |
Std enthalpy of
formation (ΔfH⦵298) |
217.992-218.004 kJ mol-1 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Nascent hydrogen is purported to consist of a chemically reactive form of hydrogen that is freshly generated, hence nascent. Molecular hydrogen (H2), which is the normal form of this element, is unreactive toward organic compounds, so a special state of hydrogen was once invoked to explain certain kinds of hydrogenations. For example reductions of organic compounds with a mixture of sodium and alcohols, the Bouveault–Blanc reduction does not occur with hydrogen, with the solvent (alcohol) or with sodium, but only with the mixture. Mechanistic understanding of such reactions is now available, and the concept of nascent hydrogen is discounted, even ridiculed.[3][4]
Making atomic hydrogen
It takes 4.476 electron volts to disassociate an ordinary H2 hydrogen molecule. When the atoms recombine, they liberate this energy. An electric arc or ultraviolet photon can generate atomic hydrogen.
Atomic hydrogen can be formed under vacuum at temperatures high enough (> 2000 K) [citation needed] to thermally dissociate the molecule, or equivalent excitation in an electric discharge. Also, electromagnetic radiation above about 11 eV [citation needed] can be absorbed by H2 and lead to its dissociation.
Uses of atomic hydrogen
The atomic hydrogen torch uses it to generate very high temperatures near 4,000°C for welding. Hydrogen is a powerful reducing agent which eliminates the need for flux to prevent oxidation of the weld.
Atomic hydrogen determines the frequency of hydrogen masers which are used as precise frequency standards. They operate at the 1420 MHz frequency corresponding to an absorption line in atomic hydrogen.
NASA has investigated the use of atomic hydrogen as a rocket propellant. It could be stored in liquid helium to prevent it from recombining into molecular hydrogen. When the helium is vaporized, the atomic hydrogen would be released and combine back to molecular hydrogen. The result would be an intensely hot stream of hydrogen and helium gas. The liftoff weight of rockets could be reduced by 50% by this method.[5]
Nascent hydrogen is claimed to reduce nitrites to ammonia, or arsenic to arsine even under mild conditions[citation needed]. Detailed scrutiny of such claims usually points to alternative pathways, not H atoms.
In nature
Most interstellar hydrogen is in the form of atomic hydrogen because the atoms can seldom collide and combine. They are the source of the important 21 cm hydrogen line in astronomy at 1420 MHz.[6]
Another meaning
Occasionally, hydrogen chemisorbed on metal surfaces is referred to as "nascent", although this terminology is fading with time.[citation needed] Other views hold that such chemisorbed hydrogen is "a bit less reactive than nascent hydrogen because of the bonds provided by the catalyst metal surface".[citation needed] Also, such catalyst provided atoms are not called nascent hydrogen, because they do not need to be captured and reacted in their instantaneous, temporary, "just generated" state, because the catalyst is able to reversibly generate them from the hydrogen gas supply at any time.[citation needed]Nascent hydrogen is represented as [H] because it is neither reactant nor product of the reaction rather a transition form which is expressed in [] to differentiate it from reactants and products.
See also
- Nascent state (chemistry)
- Cold fusion
- Devarda's alloy
- Marsh test
- Lithium aluminium hydride
- Lithium borohydride
- Sodium borohydride
References
- ^ "Hydrogen atom - PubChem Public Chemical Database". The PubChem Project. USA: National Center for Biotechnology Information.
- ^ "hydrogen(.) (CHEBI:29235)". Chemical Entities of Biological Interest (ChEBI). UK: European Bioinformatics Institute.
- ^ Laborda, F. (2002). "Hydride generation in analytical chemistry and nascent hydrogen: when is it going to be over?". Spectrochimica Acta Part B: Atomic Spectroscopy. 57 (4): 797–802. Bibcode:2002AcSpe..57..797L. doi:10.1016/S0584-8547(02)00010-1. ISSN 0584-8547. Retrieved 2009-05-01.
{{cite journal}}
: Unknown parameter|coauthors=
ignored (|author=
suggested) (help) - ^ V. F�bos, A. K. L. Yuen, A. F. Masters, T. Maschmeyer "Exploring the Myth of Nascent Hydrogen and its Implications for Biomass Conversions" Chem. Asian J. 2012, 7, 2629 – 2637. doi:10.1002/asia.201200557[
- ^ NASA/TM—2002-211915 : Solid Hydrogen Experiments for Atomic Propellants
- ^ 21 cm Line
Further reading
- Tommasi, D. (1897). "Comment on the Note of R. Franchot entitled "Nascent Hydrogen"". The Journal of Physical Chemistry. 1 (9): 555. doi:10.1021/j150591a004. ISSN 1618-2642.
- Meija, Juris (2008). "Nascent hydrogen challenge". Analytical and Bioanalytical Chemistry. 391 (5): 1475–6. doi:10.1007/s00216-008-2143-4. ISSN 1618-2642. PMID 18488209.
{{cite journal}}
: Unknown parameter|coauthors=
ignored (|author=
suggested) (help)
- Meija, Juris (2008). "Solution to nascent hydrogen challenge". Analytical and Bioanalytical Chemistry. 392 (5): 771–772. doi:10.1007/s00216-008-2356-6. ISSN 1618-2642. PMID 18795271.
{{cite journal}}
: Unknown parameter|coauthors=
ignored (|author=
suggested) (help)