What is the difference between "atomic hydrogen" and "nascent hydrogen"?

Question

My book (Comprehensive Chemistry by Dr. N . K. Verma, S. K. Khanna, Dr. B. Kapila) mentions two forms of hydrogen — "atomic" & "nascent".

It says that these two forms of hydrogen are more or less the same except that the former is produced by passing $\ce{H2}$ gas through an electric arc struck between two tungsten filaments and the latter is formed during chemical reactions in aqueous solutions. For example, $$\ce{Zn + H2SO4 -> ZnSO4 + 2[H]}$$

The book also said that the reactivity order of these hydrogens is

$$\ce{H2} \lt \ce{[H]} \lt \rm atomic~hydrogen$$

But I think that the reactivity of $\ce{[H]}$ must be same as atomic hydrogen. So please explain the difference between them and also their reactivity order.

Answer 1

Is your book by chance very old? From the Wikipedia entry for "nascent hydrogen":

Nascent hydrogen is purported to consist of a chemically reactive form of hydrogen that is freshly generated, hence nascent. Molecular hydrogen ($\ce{H2}$), 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. Mechanistic understanding of such reactions is now available, and the concept of nascent hydrogen is discounted, even ridiculed.

Then, by example of how this concept came to be:

Reductions of esters to give alcohols using a mixture of sodium and alcohols is called the Bouveault–Blanc reduction. It is an old reaction that has largely been superseded by alternative methods. At the time of popularity, the process caused much puzzlement because esters are unreactive toward hydrogen. It was also known that sodium reacts with alcohols to release $\ce{H2}$. it was concluded that some freshly generated ("nascent") hydrogen was responsible for this remarkable reaction. Subsequent studies have shown that this reaction proceeds via electron-transfer from metallic sodium to the ester substrate followed by protonation of the reduced intermediate. The evolution of hydrogen by the reaction of sodium and alcohol is purely a competitive reaction, the sole benefit being that in the presence of sufficient alkoxide, the sodium/alcohol reaction slows.

So in general, at least toward organic compounds, your relative reactivities of hydrogen species are correct if you take out "nascent" hydrogen so that:

$$\ce{H2} \lt \rm atomic~hydrogen$$

Answer 2

I wonder why, instead of a vague comment that the book is "not old" (whatever that's supposed to mean), you didn't provide its citation.

I'm familiar with the use of a metal surface to catalyze hydrogenation reactions. I'm not familiar with the concept of "nascent hydrogen". First, an arc discharge can reach temperatures of several thousands of degrees. So, I have to assume that we are not talking about atomic hydrogen at such temperatures, but rather in a highly diluted state in, say STP air. Otherwise a comparison of it with the supposed aqueous species [H] wouldn't be very useful. (Hint: atoms are indeed more reactive at higher temperatures.)

There's another problem which is the metal surface. As I said, I'm familiar with the increased reactivity that H₂ shows when absorbed onto a metal surface (also H⁺) but apparently your textbook is claiming that there exists a species [H] in solution. I need the citations to the primary literature to believe that.

The final problem (and I'm not actually sure the distinction is a difference) is the different states and environments of the species H (gas phase, dilute) with [H]aq. At first, chemistry seems to be about atoms and molecules interacting in simple ways in isolation. It doesn't take long at all, to see that environment matters greatly. I don't understand what the context is for your question. In what context will [H] (if it exists as a separate species, and I'm dubious) be compared to atomic H? Was the atomic H dissolved in water so that the environments were comparable?

One last comment: Climbing the steps of knowledge is hard. If you have taken one step farther than anyone before you, it is a major accomplishment. There are few who are able to take more than a few steps up. Even Einstein failed in his rejection of Quantum Mechanics. My point here is that if your textbook is "new", then how old is its author? Is he (she) possibly stuck with some obsolete paradigms?

Answer 3

I think better insights into Hydrogen state can be found here starting about page 13:

Title: The Chemical Elements and Their Compounds, Volume I, Sidgwick, 1950

Text: https://archive.org/stream/in.ernet.dli.2015.8077/2015.8077.The-Chemical-Elementns-And-Their-Compounds-Vol-i-1950_djvu.txt

Image: https://archive.org/details/in.ernet.dli.2015.8077/page/n7/mode/2up

Hydrogen atoms recombine to molecules rapidly and with a large evolution of heat in contact with certain solids, especially metals. Bonhoeffer has shown [20] that the efficiency of different metals in causing the recombination of hydrogen atoms is almost exactly in the reverse order to their overvoltage values, as the following list shows; the metals are in the order of their catalytic efficiency, and the overvoltage stands below each:

 Pt > Pd  >  W  > Fe  > Cr  > Ag  > Cu > Pb > Hg(zero) 

0.000 0.000 0.157 0.175 0.182 0.097 0.19 0.40 0.57

This seems to show that the overvoltage is due to the slowness of recom- bination of the hydrogen atoms after they have neutralized their ionic charges at the cathode, and suggests that the activity of ‘nascent hydro- gen’ is caused by the presence of neutral hydrogen atoms in the liquid.

The heat of recombination of the atoms (51.7 kcals. per gramme) has been ingeniously utilized by Langmuir in his ‘ atomic blowpipe ’ in which a stream of highly atomized hydrogen is directed on the metal to be heated ; this has the further advantage that the hot metal is in a reducing

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Monatomic Hydrogen pg 17

atmosphere [21] ; according to v. Wartenberg [22] the temperature of the flame near the electrode, as judged by the reversal of the spectral lines in com- parison with those of the sun, is 4,600-4,800°. From the heats of reaction (H + H = H2 + 103.4 kcals. ; 2H2 + 02 = 2H20 + 116 kcals.) it follows that for equal volumes atomic hydrogen is 34 per cent, more efficient than ‘Knallgas’, and for equal weights it is 24 times as effective.

In the gas the rate of recombination is much slower. A collision of two hydrogen atoms cannot of itself lead to the formation of a molecule, because the resulting pair cannot get rid of the energy of reaction, and so must separate again ; it is only fruitful when there is a three-body collision

2H + M -> H2M -> H2 + M,

so that the energy can be removed as kinetic energy of the H2 + M (where M of course may be the wall) ; this difficulty arises in all reactions of association or addition. Various determinations of the rate of recombina- tion of hydrogen atoms [23] have confirmed this view, and have shown that the rate is that required by the triple -collision theory ; the half-life of the atoms is about 1 sec. at 0-2 mm. pressure.