[JoGo]

I have posted a thread on the decomposition of molecules, now I am curious about a similar topic. My reply to this subject was that molecules only decompose when they have received enough energy. But someone else replied that even the most stabilized particles will decay over time, such as protons.
Now, if we look at this with a theoretical view point: we take ONE water molecule and put it in a perfect vacuum space, will this molecule decompose over a certain amount of time (assuming for some reason that the hydrogen and oxygen molecules will never decay themselves).

[Arkcon]

Your question is a little esoteric, I have to warn you.  It is very rare in chemistry to think about possible reactions of a single molecule in a "perfect" vacuum, because it is very rarely that we conduct experiments under those conditions.  We prefer to experiment with things in bulk, and speak of probabilities in bulk. 

I also don't see what advantage your thought experiment provides:  Why does it have to be just one?  If just one, among a group of trillions of other molecules in a droplet of water decomposes ... isn't that the same?  Why does it have to be in a vacuum?  If it happens in a droplet, it is no more or less easy to see than isolated alone in a vacuum.  With the added difficulty of isolating it alone added to the difficulty of detecting its decomposition alone.

Considering a beaker of water, on a lab bench, located somewhere on Earth: Will some of the molecules decompose, so you mean, break into hydrogen and oxygen?  Not without significant energy input -- electric current, for example will decompose a large bulk of it, and we'll be able to see the hydrogen and oxygen gasses.  That's much simpler to understand.  And yes, theoretically, we assume that a single molecule, subjected to enough energy, will break the bonds holding it together, and stop being water, and start being a pair of hydrogen ions and an oxygen ion. (Because you've given it no other molecule to make a full oxygen molecule.)

[JoGo]

Ok, thanks for the help but can someone please answer my question in a more direct sense?

[Borek]

Not to be rude, but if your not going to answer the question, please don't reply because your answer was a waste of my time.

Arkcon tried to explain to you why the question as posted is either difficult to answer or makes not much sense. It is not that he is wasting your time, you are wasting his time ignoring his effort.

[vex]

Ok, without ganging up on the OP...

The way we usually think about chemical reactions is with statistical thermodynamics, which comes from Boltzmann statistics and involves a lot of ensemble averaging. Therefore, we think of reaction mixtures as a statistical population for which we can calculate means and probabilistic distributions. Obviously, for a single molecule in a vacuum, we can't do this, but let's start from there.

Let's think about H₂ and O₂. According to how we define standard enthalpy, these molecules have ΔH° = 0 kcal/mol. If you put H₂ and O₂ together, there is an extremely vigorous reaction resulting in the production of H₂O. This demonstrates that the energy of the system is drastically reduced by reacting the two gases to form water. Conclude: water is considerably stabilized relative to H₂ and O₂.

Now, what you're proposing is the opposite: can we spontaneously go backwards to H₂ + 1/2 O₂ with no energetic input? If we had an ensemble, we would calculate the Boltzmann distribution and would likely find that some absurdly small fraction, i.e. one in trillions of trillions, of water molecules will spontaneously undergo the reaction you propose. Let's call that a 1e-40 probability. Now, reduce the size of the system to 1 molecule, which has a 1e-40 chance of ever decomposing into its constituent elements spontaneously. At this point, we can safely say that the atoms composing the water molecule will decay radioactively or by whatever other path before the water molecule lyses itself.

Maybe someone with a more advanced physics knowledge will come correct me. 

[JoGo]

Sorry I am having a very difficult time of understanding why my question seems so difficult to answer? Just needs a bit of imagination I guess...

Anyways, your answer is quite help full, yet ignoring my details in the original question, you say that the oxygen and hydrogen molecules will decay before the molecule lyses (which I'm not sure what that means). Now, what do you mean when you state that the molecule will "lyse"?

P.S. Once again, this is a very theoretical question, its not supposed to make much physical sense, I'm just trying to figure out the behavior of a singled out molecule in a "made-up" space.

[DrCMS]

Sorry I am having a very difficult time of understanding why my question seems so difficult to answer?

Because

Once again, this is a very theoretical question, its not supposed to make much physical sense,

as chemist most of us only work in the physical world and try to make sense of how groups of things behave.  The sort of question you're asking is "I know this could never happen in the real world but what if...?"  To those of us who do work in the real world this kind of question is a waste of time and effort.  The fact that you have been given an answer but can not understand it suggests to me that there is little point in trying to help you because your grasp of physics and chemistry are not at a high enough level for it to be explained to you in any meaningful way.  The water molecule you have chosen is very stable and will probably last for billions of years in your little vacuum cell before the subatomic particles that form the hydrogen and oxygen decay to something else.

[vex]

I don't see why it's necessary to scold the OP here for asking a question. If it's a waste of time for you to answer his question, it's even more of a waste of time for you to get on his case about it.

All I mean by "lyse" is for the chemical bonds to break, or for the water molecule to separate into its elements.

[DrCMS]

I don't see why it's necessary to scold the OP here for asking a question. If it's a waste of time for you to answer his question, it's even more of a waste of time for you to get on his case about it.

Because the OP has been given 2 good answers in this thread but chose to scold the answerers because the OP did not like those answers.  Also this is the 2nd time they have asked the same thing and the second time they have been given the answer.

All I mean by "lyse" is for the chemical bonds to break, or for the water molecule to separate into its elements.

Yes and the OP could have found that out by using Google if it was not clear.  

Personally I would never use "lyse" in this type answer as I consider it to be a term most applicable to cell biology.

[JoGo]

First off, I would really like to apologize for my rudeness. I understand that people who answer my threads takes time out of there day to answer my questions. I really appreciate your answers and efforts. Once again, very much sorry, I feel terrible about how rude I was...<

Anyway, your answers helped me realize how strange the question was in the first place. I did take a trip this morning to my university and asked my professor and he gave a me a very clear answer using physics.

Once again, thanks guys for all your great answers! 

[Borek]

asked my professor and he gave a me a very clear answer using physics

Out of curiosity - can you share the answer you got?

[juanrga]

I have posted a thread on the decomposition of molecules, now I am curious about a similar topic. My reply to this subject was that molecules only decompose when they have received enough energy. But someone else replied that even the most stabilized particles will decay over time, such as protons.
Now, if we look at this with a theoretical view point: we take ONE water molecule and put it in a perfect vacuum space, will this molecule decompose over a certain amount of time (assuming for some reason that the hydrogen and oxygen molecules will never decay themselves).

In the first place, according to the Standard Model of matter protons are stable. There are some hypothetical models where protons decay, but currently there is not experimental basis for that. I.e. never a proton have been seen to decay.

Now your main question. A water molecule in a perfect vacuum space is not a stable quantum system and would disappear. Not because it would decay or decompose into fragments, but because nuclei would delocalizate over the whole molecular space and the molecule would cease to exist.

[Caustikola]

@ Juanrga
Theory guides, experiment decides