How do you calculate the temp at which a H2O molecule splits into single atoms?

achilles89

Hi

I want to figure out how to calculate the temperature at which more energy is required to keep hydrogen and oxygen together (as in the case of H2O) than to separate them...in other words the temperature at which hydrogen and oxygen will no longer combine due to extreme temperatures. How would I do this? I am not a math mozart so I would appreciate elaborate description and possibly external links too :)

Dr_Morbius

http://en.wikipedia.org/wiki/Thermal_decomposition

achilles89

...really? I've been scowering the web for over a week now..I too know how to use Google. I was hoping someone knew more about it.

Dr_Morbius

The only way to measure the temperature of decomposition is to heat the substance and check for decomposition products. There may be a way to do it using quantum mechanics but I have no clue how to do that.

Borek

It should be possible to use thermodynamic data - calculate equilibrium constant as a function of temperature. It still requires experimental data.

achilles89

Thank you. I will just have to conduct some tests. Is it possible to predict such things with quantum mechanics?

Ygggdrasil

In theory, if you know the enthalpy change (ΔH) and entropy change (ΔS) associated with the reaction, you can calculate the temperature at which transforming breaking apart water is as favorable as forming water. This occurs when the change in free energy (ΔG) of the reaction is equal to zero, and because ΔG = ΔH - TΔS, it is easy to solve for this temperature.

However, this approach relies on the assumption that ΔH and ΔS do not change with temperature. While this assumption is good over fairly small ranges of temperature, I'm not sure how well they hold over larger ranges.

Edit: it is also possible to calculate equilibrium constants as well as some of the thermodynamic data from quantum mechanics. Statistical mechanics textbooks will have information on how to do that.

achilles89

What I am looking to do is determine the relative thermal point at which hydrogen and oxygen gases can exist together and not recombine into water. The experiment I am planning to perform involves thermal disassociation of H2O...however, in order to prevent an explosion due to additional water input, I wish to determine the minimum temperature above their autocombustion temperature at which they will not recombine due to thermal conditions.
Any thoughts?

epenguin

Only that dissociation at given temperature will also be favoured by low pressure, concentration, by rarefaction.