I am writing an essay on the chemistry of rocket fuel, and I was wondering what, if any, would be a commonly accepted value for the actual yield of the reaction between hydrogen, $\ce{H2},$ and oxygen, $\ce{O2}.$ I would also love it if you could cite some sources so I can find more information on this exothermic reaction:

$$\ce{H2 + O2 -> H2O + heat}$$

  • 1
    $\begingroup$ I would guess about 100% of theoretical yield under ambient conditions, based on the Gibbs energy of reaction. Thermodynamic tables in any General Chemistry textbook give the Gibbs energy of formation for water. $\endgroup$ – Karsten Theis Nov 24 '19 at 3:24
  • $\begingroup$ if you people would stop deleting information from others' questions, they wouldn't be so unclear. Just sayin $\endgroup$ – AndyCandy02 Dec 4 '19 at 16:15
  • $\begingroup$ The title as written by the OP was "Oxygen and Hydrogen actual yield", and now it is "Typical reaction yield for combustion of hydrogen". The actual yield depends on temperature and pressure. I don't think the editing made it less clear. Also, it looks like the OP appreciated the answer. $\endgroup$ – Karsten Theis Dec 4 '19 at 16:52

The equilibrium constant for the reaction $\ce{2H2 + O2 <=> 2H2O}$ is colossal, equal to $\mathrm{2.4 \times 10^{47}}$ at $\mathrm{500\ K}$ (source). This would ensure virtually 100% yield for the combustion of a stoichiometric mixture of oxygen and hydrogen in equilibrium conditions. However, a rocket engine is a reaction vessel that is far from equilibrium, and therefore oxygen could remain unreacted. This is in fact a problem, as it could damage the engine at its operating temperature. This, among other reasons, is why oxygen/hydrogen mixtures for rocket engines are actually very hydrogen-rich, containing up to twice the stoichiometric amount of hydrogen required.

So in practice, taking everything into account, rocket engines running on oxyhydrogen fuel are designed to have a practically 100% combustion yield based on the limiting reagent, oxygen, while possessing a substantial excess of hydrogen fuel. There is a strong engineering pressure to maximise energy output by ensuring the oxygen is completely consumed; unreacted oxygen is a waste of lift capacity, storage space, materials, cost, etc.


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