In other words, do the atoms in a hydrogen molecule stay together when they combine with O? If I react O2 with a 50:50 mix of H2 and D2, and if the H and D atoms behaved independently, I would expect to get 25% H2O, 25% D2O, and 50% HDO. If the D and H reacted with O entirely as molecules, all of the water would be H2O and D2O. Obviously many intermediate conditions are possible.
I know that this probably is a very simple question with a well known answer, but I can't come up the right search terms to find it.
If you mean an isotope composition of formed liquid water, then you have to keep in mind a fast H/D exchange in liquid water. The distribution of H2O, HDO and D2O will be simply statistical.
I was thinking of water vapor, before it condenses, although I neglected to specify that. What would be the distribution immediately after formation, before any exchange?
Or, to ask the essentially same question in another way, will the formation of water create DH molecules in the unreacted hydrogen, or will it remain all H2 or D2?
OK, you have a gas phase reaction. Is it catalyzed by Pt? If no, what are the conditions?
Simply burning a mixture of H2 and O2 to produce water vapor, as in a torch, for example. No catalyst.
The question is how can you analyze he isotopic distribution inside the flame?
I'm not sure that you can, but what would you expect to find if you could? And what would you find in the water vapor coming out of the flame if you analyzed it without ever condensing it?
There is no reason to discuss something which can't be verified by experiment.
I'm not sure if it's possible, but maybe the isotopic composition of the flame could be measured spectroscopically, considering the red shift of absorption lines from molecules with heavier isotopes.
That said, I am following this question because it is a relatively simple premise, yet I do not know the answer. I do believe that it could be verified by experiment, however, given a clever enough experimental set up.
Considering the high energy states involved in a reaction like this, I suspect there would be no large isotopic fractionations and therefore there would be a number of H2O, D2O, and DOH controlled by their initial abundances in the gas (perhaps stochastic). The exchange between D2 and H2 molecules to form DH, prior to water formation, may be negligible (?) But would probably increase with temperature... but not sure if this would affect the result.
Of course, i'm not sure about any of these hypotheses, and will keep following the question for any potential insight...
Michael A. Antonelli Thanks for the reply. Isotope fractionation probably would be very small (despite the large mass difference) but what I'm wondering is if H2 or D2 react in pairs or as single atoms. When O takes on two H/D's, are these more likely to originate from the same diatomic molecule than from different molecules? It's a matter (I think) of when and how the O-O and H-H (D-D) bonds are broken and new bonds form, and how fast this is. O2 has a higher ionization energy than H2, which I (perhaps naively) take it to mean that H-H bonds will break before O-O bonds, but the ionization energy of D2 is higher than H2, so it things get more complicated.
This is a classical branched radical reaction with tons of publications on reaction mechanism. Free radicals/atoms such as H, HO, HO2 (D, DO. D2O) are involved. The kinetic isotopic effects are small due to a high energy. The reaction requires an ignition to start and may proceed as an explosion in a wide range of H2/O2 ratio, and dependent on inert gas (N2, Ar, CO2) present in the mixture. In practice a ratio G2/O2 between 4:1 or 5:1 is needed to avoid an oxidizing flame. The final isotopic distribution might strongly depend on position of measurements in the flame. There is a gradient of concentrations within the flame.
- Badges
- Science topic
- Similar topics
- Geoscience
- Atmospheric Sciences
- Climate Science
- Climate Change