Sciencemadness Discussion Board

Is "nascent hydrogen" actually a free radical?

Random - 4-8-2013 at 15:48

Which would explain the reduction with metal/acid.
Quote:

t sufficiently low temperatures, free protons will bind to electrons. However, the character of such bound protons does not change, and they remain protons. A fast proton moving through matter will slow by interactions with electrons and nuclei, until it is captured by the electron cloud of an atom. The result is a protonated atom, which is a chemical compound of hydrogen. In vacuum, when free electrons are present, a sufficiently slow proton may pick up a single free electron, becoming a neutral hydrogen atom, which is chemically a free radical. Such "free hydrogen atoms" tend to react chemically with many other types of atoms at sufficiently low energies. When free hydrogen atoms react with each other, they form neutral hydrogen molecules (H2), which are the most common molecular component of molecular clouds in interstellar space.


Now this isn't in vacuum, but imagine one molecule of HCl reacting with one atom of iron, wouldn't that release the free radical, which is actually atomic hydrogen?

Bot0nist - 4-8-2013 at 15:55

My understanding after reading the lashings Nicodem unleashed at AJkoer in the potassium threads is that "nascent hydrogen" is complete hogwash, and not just in relation to that topic. I admit though, I have done little to no research for my self on the subject. I am sure AJ or Anders (same person?) can post tomes about it here though, as they are really the only members I recall summoning the term...

franklyn - 4-8-2013 at 21:18

Hydrogen protons particularly of acid species are very mobile , detaching from
it's electron which remains behind , to roam about on it's parent molecule.
Compounds that contain both acidic and basic centers tautomerize to this
Zwitterionic form. Solid Sulfamic acid exists as a Zwitterion , which I find quite
remarkable without benefit of a solvent.

Monoatomic hydrogen exists within a metal lattice which hosts it to form an
interstitial hydride. What I find very interesting about this type of material is
that when an electric field ( voltage ) is applied to a wire hydride , the contained
hydrogen diffuses from ( + ) to ( - ) exactly opposed to the electron current
which migrates from ( - ) to ( + ). Practically nothing is available regarding the
study of this. For example is the magnetic field produced more intense ? Since
in effect the charge carriers are effectively doubled.

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bfesser - 5-8-2013 at 07:08

<strong>Bot0nist</strong> is correct. <a href="http://en.wikipedia.org/wiki/Nascent_hydrogen" target="_blank">Nascent hydrogen</a> <img src="../scipics/_wiki.png" /> is antiquated pseudo-scientific bullshit. You had better hope that any discussion about it dies down before <strong>Nicodem</strong> gets back! :o

AndersHoveland - 5-8-2013 at 15:50

I do not have any references, but I remember reading in a book somewhere stating that, although the reducing power of zinc with hydrochloric acid was originally believed to be due to "nascent hydrogen", this was later proved false. Many early chemical publications refer to "nascent hydrogen", and are usually referring to using zinc with hydrochloric acid.

Real nascent hydrogen is produced in an atomic hydrogen torch, sometimes used for welding.

I am not aware of any chemical reactions that can produce hydrogen radicals at room temperature. I am sure some must exist, but they are very rare, since hydrogen radicals are very reactive.

[Edited on 5-8-2013 by AndersHoveland]

bfesser - 5-8-2013 at 16:11

<strong>AndersHoveland</strong>, that is not "<em>real</em> nascent hydrogen," that is atomic hydrogen. If you had bothered to read the Wikipedia article I linked to, you would see that this is discussed there. Also, as a rule of thumb which particularly applies to you, any post starting with "I do not have any references, but..." is likely garbage that shouldn't have been posted in the first place.

[Just removed similar garbage from the Wikipedia page which has been without reference since 2008!]

[Edited on 6.8.13 by bfesser]

12AX7 - 8-8-2013 at 21:21

If you talk about pure gas phase, HCl reacting with an Fe(0) crystal surface, plenty of complicated things could be going on. Probably, the molecule adsorbs to the surface (meaning, disperson forces), then orbitals overlap, a reaction occurs as electrons move around, and a Cl- and H+ are left stuck in place. The H+ should pick up charge quickly (either as a hydride, hanging off the metal, or an adsorbed radical). Iron is known to have catalytic properties in various reactions, so I don't find it hard to imagine these atoms sticking to the surface and sliding along according to electron densities. H radicals, of course, will find each other quickly (by diffusing over the surface, desorbing into the gas phase, or diffusing into the bulk metal itself), forming H2 molecules. Some H2 will remain adsorbed, but as reactions continue, more and more will return to the gas phase.

If you talk about HCl in aqueous solution, the electrochemical story will explain the reactions nicely. It's no simpler than the gaseous case -- much more complicated, given the peculiar properties of water -- but it's also practical and well studied. It's noteworthy that electrochemical reactions are well known to undergo radical processes; whether or not this actually involves atomic H radicals doesn't matter to the end product (for example, electrochemical decarboxylation -- I forget the name of the reaction -- turns R-COOH into R-H, R-R, and CO2, through a radical mechanism).

If you're talking double gaseous (i.e., literally one molecule of HCl and one lone atom of Fe in the ground state), that's a more fundamental, less well studied, and much less practical question -- they'll probably react, but the lone Fe atom (roughly a triple radical!) is easily more reactive than molecular HCl; they may very well stick together as an H-Fe-Cl radical.

It's an excellent question just how you got that lone Fe atom. It could be at very low density, preventing it from finding more of its kind to clump with (ultimately forming an aerosol of metal particles, or condensation on the nearest surface). This, incidentally, is how they start making extremely cold gasses (Bose-Einstein condensates) of atoms like rubidium: below the vapor pressure of solid metal, there's still enough atoms to play with, yet it won't condense into deposits (indeed, the deposits would evaporate).

Alternately, an extraordinarily high temperature could be used, enough to atomize the metal. Now, merely vaporized Fe is probably diatomic (like most metal gasses); a quick search didn't turn up what form it's actually in, so that's my assumption. It's noteworthy that a welder's arc at 20,000 K or so doesn't vaporize much metal off the surface (not much smoke is produced), and at temperatures that high, ionization is sure to occur, so you'd be looking at a plasma containing Fe2 (or whatever molecular form it takes), and ground and excited states of atomic and ionic Fe; reactions with such a soup would only rarely involve only monatomic, ground state Fe.

Tim

franklyn - 22-9-2013 at 19:22

Hydrogen liberated during chemical reactions is always in atomic state , known as nascent hydrogen. If another atom is not available to bond to at that instant it combines in pairs as molecular hydrogen. There is more to this than rudimentary chemistry alone. It is acknowledged and the object of continued investigation that quantum states in the electronic structure of both bound and lone atoms determines whether any given reaction can proceed at all. It is responsible for what results and yields are obtained in any given process.
See _
www.sciencemadness.org/talk/viewthread.php?tid=25538&pag...

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Bezaleel - 25-9-2013 at 04:19

From an earlier thread on this subject, I remember that one of the convincing arguments that nascent hydrogen is NOT the the reason for the reaction to occur, is that in that case you could Mg as well as you could use Zn to create H-radicals. In many reactions involving nascent hydrogen this seems not to be the case. They work with Zn, but not with Mg, ceteris paribus.

Nicodem - 25-9-2013 at 07:37

The term "nascent hydrogen" was used in a more than a century old hypothesis that in metal dissolving reductions, some unknown excited form of hydrogen forms which is able to react with the substrate (they did not know much about electrochemical properties of different metals, so they did not yet understood the reduction mechanism). The hypothesis about the nascent state of matter was put forward because it was by then known that hydrogen itself is unable to perform such reductions, so they needed some magic explanation. For this reason they came up with the idea that hydrogen is in some short lasting, but a more reactive state, for a short time after its formation (hence the "nascent"). Some authors popularized the term, before the hypothesis was experimentally demonstrated wrong. It was found that the reduction has nothing to do with hydrogen. It is now known, that the metal dissolving reductions occur via the classical SET mechanism, where hydrogen is obviously not involved (though it can be formed in a parasitic side reaction).

"Nascent hydrogen" as then known has nothing to do with atomic hydrogen, or quantum states, or whatever else. The hypothesis even predates the Bohr's model of the atom, let alone the modern atom model or any of the quantum level phenomena like electron or nuclear spins and similar. So, there is no way the hypothesis ever referred to any of these things - it simply could not and it never really tried to explain what this nascent state was supposed to be on the atomic/molecular level as that would be too ambitious for those times. In short, there is no such thing as nascent hydrogen. It is a historical term that died when the reaction mechanism was explained by a better hypothesis later confirmed experimentally.

Bezaleel - 26-9-2013 at 04:52

Nicodem, do you have a link to the description of the SET mechanism you mention? (This is new to me.)

watson.fawkes - 26-9-2013 at 05:37

Quote: Originally posted by Bezaleel  
Nicodem, do you have a link to the description of the SET mechanism you mention?
I had the same question yesterday, and used Google. It helps to have figured out that SET stands for single electron transfer.

http://www.metallacycle.com/chemistry/set/

Nicodem - 26-9-2013 at 06:53

The topic about the mechanism of the dissolving metal reductions was already discussed in another thread. Besides March's book, there are plenty of other tutorials to the theory. One such example are the two chapters from Comprehensive orgnic synthesis, vol. 8:

J. W. Huffman, Reduction of C=X to CHXH by Dissolving Metals and Related Methods (pp 107-128).

S. Yamamura and S. Nishiyama, Reduction of C=X to CH2 by dissolving metals and related methods (307-326)

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