Last week the US government announced that it believes it has successfully breached the fuel tank on a dead satellite, effectively destroying the toxic fuel stored on board: 1000 pounds of hydrazine. Hydrazine is a simple nitrogen compound, two NH2 groups joined by a NN single bond. How does such a simple compound power a rocket?
Hydrazine is a hypergolic propellant – one that ignites as soon as it comes into contact with an oxidant (something that will react with it to effectively strip away some electrons from the reactant and force the molecule to bond differently, the changes in the bonds between atoms are what release the energy). Hypergolic is apparently a term coined by the German rocket program from hyper (very) + ergon (Greek for work) + ol (from oleum, the Latin for oil). Hydrazine is that, a liquid (if not particularly oily one) that can be used to push satellites around in orbit – to do work.
Hydrazine is a solid in the satellite’s tanks, and once thawed can be catalytically and rapidly decomposed. Almost any metal will do, though iridium is the usual choice. The reactions produce lots of very hot gases, which you can direct through a thruster:
3 N2H4 → 4 NH3 + N2
N2H4 → N2 + 2 H2
NH3 + N2H4 → 3 N2 + 8 H2
A little thermochemistry can quickly tell you just how much energy you might produce from 1000 pounds of hydrazine. The overall reaction is:
5 N2H4 → 5 N2 + 10 H2
which releases 50,000 Joules of energy per mole of hydrazine. A mole of hydrazine weighs about 32 grams, so you get enough energy to make a cold cup of coffee hot from just over an ounce of hydrazine (do NOT try this at home!). If all the hydrazine in that satellite went up at once, it would release about 8 billion Joules (enough to keep the average US citizen in energy for more than a week).
What does the p in pH stand for?
The term pH has been in use for more than a century. It is a logarithmic measure of the hydrogen ion concentration ([H+]): pH = -log10[H+]. (Technically, there aren’t bare protons (H+) floating around in solutions, but that wasn’t known when pH was introduced!) The original symbol used by Sorensen was pH+.
Theories vary as to the origin of the p – most agree it means power but whether in Latin, French or German, seems in dispute. Thinking it would be either French or Latin as the original paper was published in French, I was surprised to find that it’s neither, though the legend is both old and persistent. By 1920, many authors were assuming that it meant “power”, but Jens Norby returned to the original sources and points out that it was the arbitrary choice of the letters p and q for two variables in the work-up of the experimental data. The variable p eventually ends up in the formula arrived at for the concentration of the hydrogen ion.
The modern form pH was introduced in 1920, “as a matter of typographical convenience”.
For the full explanation, see Jens G. Norby, The origin and the meaning of the little p in pH, Trends in Biochemical Sciences 25, 36-37 (2000). The illustration is a selection from the original paper: Sorensen, Compt. redn. du Lab. de Carlsberg 8 1-168 (1909).
