Nascent hydrogen

Nascent hydrogen
Nascent hydrogen
CAS number 12385-13-6 YesY
PubChem 5362549
ChemSpider 4515072 YesY
Jmol-3D images Image 1
Molecular formula H
Molar mass 1.01 g mol−1
Exact mass 1.007825032 g mol-1
Solubility in water Reacts
Dipole moment 0 D
Std enthalpy of
217.992-218.004 kJ mol-1
Standard molar
114.715-114.719 J K-1 mol-1
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Atomic hydrogen (or nascent hydrogen)[3] is the species denoted by H (atomic), contrasted with dihydrogen, the usual 'hydrogen' (H2) commonly involved in chemical reactions. It is claimed to exist transiently but long enough to effect 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] Being monoatomic, H atoms are much more reactive and thus a much more effective reducing agent than ordinary diatomic H2, 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] Atomic hydrogen is made of individual hydrogen atoms which are not bound together like ordinary hydrogen into molecules.


Making atomic hydrogen

It takes 4.476 eV to disassociate ordinary H2 hydrogen molecules. When they 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.[4]

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.[5]

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]

See also


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. 

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