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Re: Monatomic hydrogen as fuel - need a low bond energy to single hydrog

Subject: Re: Monatomic hydrogen as fuel - need a low bond energy to single hydrogen atoms.
From: "tadchem"
Date: 22 Mar 2006 18:06:22 -0800
Newsgroups: sci.astro,, sci.physics, sci.chem
Robert Clark wrote:
> Atomic hydrogen or monatomic (or monoatomic) hydrogen fuel isn't a
> type of nuclear engine. The terminology just means the hydrogen occurs
> as single atoms rather than the usual H2 seen in hydrogen gas. The
> specific impulse (ISP) of this fuel could be as high as 1500 s. The big
> problem with this fuel is keeping it stable against it's recombining
> into H2 when stored in its monatomic form. This still hasn't been
> solved:

The recombination of H atoms into H2 is exothermic at temeratures belor
those of the surface of a star.  Inhibiting the recombination will
require complicating the system, adding weight and reducing the GLOW
advantage of utilizing hydrogen in the first place.

> Use of Atomic Fuels for Rocket-Powered Launch Vehicles Analyzed.

This has the look of a 'back-of-the-envelope' calculation that ignores
a *lot* of factors such as the weight of the cryogenic carrier fluid
and its container.  For example, the lightest cryogenic fluid that can
be used is liquid helium, but this will be *superfluid* at the
temperatures involved, and will likely provide a very feeble barrier to
recombination of H atoms.  Also, the weight of the helium is almost
negligible compared to the weight of the container needed to maintain
its liquid state, even in large amounts.  LHe is also VERY expensive.

>  Current research is focused in storing it as individual atoms within
> cryogenic liquid helium.
>  However, arcjet engines or arcjet torches that use hydrogen as the
> fuel can produce monatomic hydrogen by high electrical power. High
> power or temperature is needed to break the hydrogen bond of H2, at a
> bond energy of about 104 kcal/mol.
>  Are there molecules with bonds of hydrogen at much lower bond
> energies?

Try this table:

Methane looks *real* good.

>  I thought of molecules that specifically had the type of bonds known
> in biochemistry as "hydrogen bonds". These are, in most cases, weaker
> than covalent bonds. However, the only cases I've seen had an H atom
> attached to another atom by a hydrogen bond but that H atom was also
> attached to a second atom by a covalent bond. "Hydrogen bonds" always
> have the H between two other atoms but is it possible for both of these
> bonds to be of the "hydrogen bond" type?

No.  "Hydrogen bonding" is peculiar to hydrogen (because of the small
size of the hydrogen atom *once it has already bonded chemically* to a
more electronegative atom such as a halogen (fluorine family),
chalcogen (oxygen family), or pnictogen (nitrogen family).  Unlike
other atoms, hydrogen does not have a 'hard nut' of a filled 1S

>  According to this page hydrogen iodide, HI, with a bond energy of 71
> kcal/mole can be decomposed into H and I by uv light at a wavelength of
> 266 nm:
> Chem 32 Virtual Manual.
>  Are there other gases where a single hydrogen atom can be dissociated
> at a lower energy?

Google "hydride photodissociation"

Tom Davidson
Richmond, VA

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