The advantage of hydrazine as a
rocket fuel is its high energy per
unit weight available from its
decomposition. The decomposition
products are
nitrogen, and a relatively large amount of the desired
hydrogen. Also, since hydrazine is a
liquid at
STP, it is preferred
over the storage of gaseous or (
cryogenic) liquid
hydrogen.
Hydrazine is a bifunctional base, forming hydrazinium salts when reacted with water:
N2H4(aq) + H2O =
N2H5+ + OH-
N2H5+(aq) +
H2O = N2H62+ + OH-
Anhydrous hydrazine is a remarkably stable chemical compound, but
its combustion in air generates an enormous amount of heat:
approximately 622 kJ.mol-1
As m_turner writes, the synthesis of hydrazine is done by the reaction of ammonia with sodium hypochlorite:
NH3 + NaOCl → NaOH + NH2Cl (Fast
reaction step)
NH3 + NH2Cl +NaOH →
N2H4 + NaCl + H2O
However the problem is that there is a fast competing reaction:
2 NH2Cl + N2H4 →
2 NH4Cl + N2
In order to increase the hydrazine yield, gelatin is added to
sequester trace amounts of heavy metal ions that catalyze the
competing reaction.
ONE BIG WORD OF WARNING: Hydrazine is a seriously toxic
compound. This is the description on the label (and that's just the
diluted, aqueous form of hydrazine):
Highly toxic! Sensitizer! Corrosive! Mutagen!
Cancer suspect agent! Readily absorbed through skin! Target organ:
liver, kidneys! Target organ: blood, nerves!
(with exclamation marks and all). Trust me, that information is not
exaggerated in any way. If you ever have to work with this
compound, read the MSDS, wear all the proper safety gear, and work in a
fume hood. Hydrazine is immediately dangerous to life and health at
concentrations over 80 ppm.
The health danger is another reason why NASA wants to replace
hydrazine as rocket fuel