At what temperature water decomposes into hydrogen and oxigen?

[SAZAR]

At what temperature water decomposes into hydrogen and oxigen?

 

[Bystander]

You might want to pin this down a little more tightly --- water decomposes at any T greater than absolute zero --- it's also been identified in solar spectra at something like 8000 K (somewhere between surface and coronal temps). You're looking at a temperature and pressure dependent equilibrium state, delG(T,P) = -RTlnK, and there is no one (T,P) state at which water abruptly ceases to be stable.

 

[GCT]

If you asking when the reaction is spontaneous or favorable rather, then this is something you can figure out using the Gibbs equations and the relevant parameters, you need to show your approach first, as this seems to be a homework question.

 

[SAZAR]

OK

(This is not a homework, I'm just asking a question...)

The question is clear:
At what temperature water decomposes into hydrogen and oxigen?

The circumstances are:
- Sea level atmospheric pressure (on Earth (this Earth, this widely aproximate time of its history)).
- Sealed container with water in it (only half of it is full, and container is solid and unmeltable, thight sealed, can withstand enormous pressures - (the way you will freely consider perfect in this context)).
- You heat it up with an apropriate source of heat for this context.

What temperature would cause water in it to decompose and at a very short amount of time produce maximum amount of hydrogen and oxygen?

 

[Bystander]

"Maximum amount of hydrogen and oxygen?" Infinite temperature, or whatever theoretical limit to temperature there might be.

 

[GCT]

Now you're referring to the rate of the reaction, note that the combination of hydrogen and oxygen to become water is exothermic, what you're referring to is some type of an "reverse" explosion situation and I don't believe that a discussion based on this notion would be too fruitful. If you want to get some kind of an understanding of what is being mentioned here, study up on some of the common mechanisms of explosions...you're asking that such a mechanism apply to an already thermodynamically unfavorable reaction, thus it isn't so much of a theoretical issue to begin with.

 

[Gokul43201]

I've come across numbers like 3000K for some industrial processes. Seems to me that that would give pretty low yields considering the bond energies are each a few hundred kJ/mol.

 

[SAZAR]

2726.85 °C?
Gee, that's a mighty high temperature...

 

[Cesium]

IIRC, one of the reasons why it is very dangerous to try to attempt to extinguish a thermite reaction with water is because the reaction reaches a high enough temperature that the water splits into hydrogen and oxygen. The hydrogen then ignites, further invigorating the thermite. I think thermite reactions can reach temperatures around 2500C, which corresponds nicely to the figure given by Gokul.

But ya, the main point that everyone else has made is true...there is no set temperature/pressure point at which water decomposes.

 

[SAZAR]

Of course, but I wasn't asking a theoretical question; I was asking a pure practical question.

 

[Bystander]

Of course, but I wasn't asking a theoretical question; I was asking a pure practical question.

"Practical" is when you apply constraints, "pin it down," and state that you want to achieve 90% thermolysis, or 99, or 80, or whatever.

 

[SAZAR]

OK. My fault, I haven't pointed-out at the beginning of the thread that I want a practical answer.
('practical' = data about how people do it here in normal conditions (e.g. If I ask at what temperature water evaporates, the practical answer would be: "At 100 Celsius degrees." - nice and simple (dissregarding the fact that atmospheric pressure changes the evaporation point, and a fact that water "dissapears" at any temperature if you leave it long enough...)))

 

[Astronuc]

Of course, but I wasn't asking a theoretical question; I was asking a pure practical question.

Practical at 2726.85 °C?

There is a reason why people try to perfect catalysts to crack chemical compounds, namely to reduce the energy required to do so.

 

[Bystander]

(snip)There is a reason why people try to perfect catalysts to crack chemical compounds, namely to reduce the energy required to do so.

You wanta run that through the first and second laws again and restate it?

 

[Astronuc]

You wanta run that through the first and second laws again and restate it?

I wasn't necessarily reflecting on the bond energy as much as the fact that one usually adds much more heat (energy) than is required to break all the bonds. This is a consequence of recombination, loss of heat by conduction and convection to the surrounding structure, and a consequence of Maxwell Boltzmann distribution in thermal systems. Catalysts are used to reduce the activation energy of a reaction. Lowering temperature is always desirable.

http://www.schoolscience.co.uk/content/5/chemistry/catalysis/catsch2pg2.html

Water is pretty stable, and perhaps there is not an effective catalyst that effectively reduces the energy required to break the bonds.

On the other hand, the sulfur iodide process is being considered with operation around 850°C - a lot less than 2700°C. I'll have to dig a bit for the energy/unit mass of H₂ produced.

 

[Bystander]

Catalysts do not change energy of reaction, they do not change "position" of equilibrium; they may reduce activation energy, they may allow reactions to proceed by different mechanisms. Use of Rube Goldberg sequences of reactions is not catalysis. Energy losses are more a function of process and plant design than chemistry.

 

[Bystander]

From Wiki:
2 H₂SO₄ → 2 SO₂ + 2 H₂O + O₂ (830°C)
I₂ + SO₂ + 2 H₂O → 2 HI + H₂SO₄ (120°C)
2 HI → I₂ + H₂ (320°C)

Step 1, quench the product, take off O₂ plus garbage to a clean-up stream, send the SO₂ stream to step 2. Step 2, add I₂, take off H₂ plus HI to step 3, send H₂SO₄ to step 1. Step 3, quench and separate H₂ and I₂, send H₂ to clean-up, send I₂ to step 2.

Bit of a materials challenge for heat exchangers, plant cost and maintenance-wise far as beating electrolysis, but, might be workable. Materials science is more likely to boost operating temperatures and power plant efficiencies first. This has the look of "oil shale" technology to it, if other technologies remain frozen, it'll be economically viable in another five years.