5 ms·

> per volume ammonia holds more hydrogen than does liquid hydrogen How is this possible? Suspect the answer would be way over my head!

tempestn 8y ago
I would expect this is due to the fact that each ammonia molecule contains three hydrogen atoms, whereas a hydrogen molecule contains only two. So liquid ammonia would be expected to contain more hydrogen per volume than liquid hydrogen.
- bayesian_horse 8y ago
  That would be true of their gases.
  - tempestn 8y ago
    Wouldn't it be true of both liquid and gas?
maga 8y ago
Ammonia is 10x more dense than hydrogen gas at the same temperature/pressure, so you can pack more of it into the same volume.
- gonvaled 8y ago
  "liquid hydrogen"
  - maga 8y ago
    It applies to liquids as well.
Shoh3pif 8y ago
Armchair chemistry ahead: Molecular hydrogen is unpolarized while ammonia molecules can establish hydrogen bonds, thus packing them more densely under the same conditions. For really dense hydrogen we would need its metallic form but that requires pressures you might find in the core of jupiter. The electromagnetic force is more convenient than gravity.
masklinn 8y ago
Intramolecular bonds are quite a bit smaller than intermolecular distances, so if you pack atoms into longer molecules you get a denser result. I'm guessing very long molecules become problematic (because they don't pack well), but ammonia is a small molecule so way below that. Liquid ammonia has a density of ~690 kg/m3 and a molar mass of 17 g/mol so ~40k mol / m3, as NH3 that's ~120k atoms of hydrogen per cubic meter. Meanwhile liquid hydrogen has a density of 71g/L and a molar mass of 2.02 g/mol so a very similar ~35k mol/m3 but as H2 that's only ~70k atoms of hydrogen per cubic meter. And liquid hydrogen aside from being extremely flammable, can't exist above 30K and degrades storage material (https://en.wikipedia.org/wiki/Hydrogen_embrittlement https://en.wikipedia.org/wiki/Hydrogen_embrittlement), ammonia is much more forgiving and liquid at ambient temperature above 1MPa (10 times atmospheric pressure), not innocuous by any means but way easier to transport and store (storage requirements are similar to propane).
- meta_AU 8y ago
  Probably also because ammonia is polar, three positive protons and an electron pair in a tetrahedral arrangement. Whereas H2 is two protons hiding an electron pair in a linear arrangement.
  - falsedan 8y ago
    spot on
- mdturnerphys 8y ago
  ~120k mol/m^3 and ~70k mol/m^3

tempestn 8y ago

I would expect this is due to the fact that each ammonia molecule contains three hydrogen atoms, whereas a hydrogen molecule contains only two. So liquid ammonia would be expected to contain more hydrogen per volume than liquid hydrogen.

bayesian_horse 8y ago
That would be true of their gases.
- tempestn 8y ago
  Wouldn't it be true of both liquid and gas?

maga 8y ago

Ammonia is 10x more dense than hydrogen gas at the same temperature/pressure, so you can pack more of it into the same volume.

gonvaled 8y ago
"liquid hydrogen"
- maga 8y ago
  It applies to liquids as well.

Shoh3pif 8y ago

Armchair chemistry ahead: Molecular hydrogen is unpolarized while ammonia molecules can establish hydrogen bonds, thus packing them more densely under the same conditions. For really dense hydrogen we would need its metallic form but that requires pressures you might find in the core of jupiter. The electromagnetic force is more convenient than gravity.

masklinn 8y ago

Intramolecular bonds are quite a bit smaller than intermolecular distances, so if you pack atoms into longer molecules you get a denser result. I'm guessing very long molecules become problematic (because they don't pack well), but ammonia is a small molecule so way below that. Liquid ammonia has a density of ~690 kg/m3 and a molar mass of 17 g/mol so ~40k mol / m3, as NH3 that's ~120k atoms of hydrogen per cubic meter. Meanwhile liquid hydrogen has a density of 71g/L and a molar mass of 2.02 g/mol so a very similar ~35k mol/m3 but as H2 that's only ~70k atoms of hydrogen per cubic meter. And liquid hydrogen aside from being extremely flammable, can't exist above 30K and degrades storage material (https://en.wikipedia.org/wiki/Hydrogen_embrittlement https://en.wikipedia.org/wiki/Hydrogen_embrittlement), ammonia is much more forgiving and liquid at ambient temperature above 1MPa (10 times atmospheric pressure), not innocuous by any means but way easier to transport and store (storage requirements are similar to propane).

meta_AU 8y ago
Probably also because ammonia is polar, three positive protons and an electron pair in a tetrahedral arrangement. Whereas H2 is two protons hiding an electron pair in a linear arrangement.
- falsedan 8y ago
  spot on
mdturnerphys 8y ago
~120k mol/m^3 and ~70k mol/m^3