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).
A) 1. Enthalpy ( delta H) is the amount of heat content.
1. Heat content is accounted for by a change in “heat flow” or enthalpy of the reaction system.
1. Endothermic reaction: delta H > 0
(i.e., H products >H reactants).
Heat absorbed goes to increase the enthalpy of the reaction system.
2. Exothermic reaction: delta H < 0
(i.e., H products < H reactants).
Heat is evolved at the expense of the reaction system.
2. Thermochemical Equation: specify delta H in kilojoules/mole.
1. CH4(g) + 2O2(g) --> CO2(g) + 2H2O(l) + 890.3 kJ
delta H = -890 kJ
6.00kJ + H2O(s) –> H2O(l)
delta H = +6.00kJ
! In some textbooks delta H is written as a product or reactant !
The preceding is based upon the Law of Conservation of Energy (James Joule, 1818-1889, Joule also developed the First Law of Thermodynamics): energy is neither created nor destroyed in ordinary chemical or physical changes.
2. Quantitative delta H
delta H = qreaction mixture (at constant temperature only)
q = (m)( delta t)(Cp)
q = heat absorbed by the water in joules (J)
m = mass of substance
delta t = tfinal – tinitial
Cp = specific heat of water = 4.184 J/g oC
When using moles, molar heat capacity is used. The units are kJ/mol K
1 cal = 4.184 J
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