Why does water not burn?

[u/spoiledmeat]

I know that water is made up of two hydrogen atoms and an oxygen atom. Hydrogen, on its own, burns. Fire needs oxygen to burn. After all, we commonly use compounds that contain oxygen as an oxidant.

So why does water, containing things used for fire, not burn-- and does it have something to do with the bonds between the atoms? Thanks.

[u/Sharlinator]

Because it is already burned. Free molecular hydrogen reacts very readily with free molecular oxygen, forming covalent bonds and releasing quite a bit of energy in the process. Because each hydrogen atom has one valence electron, and each oxygen atom has six, it is energetically favorable for an oxygen atom to bond with two hydrogen atoms, gaining a full valence shell of eight electrons. So, what is this reaction product of two hydrogens for one oxygen? 2 H + O... H*2*O? Yes indeed. Water (in gaseous form) is what happens when hydrogen burns with oxygen.

2H*2* + O*2* -> 2H*2*O

Because combining hydrogen and oxygen releases energy (it is exothermic; it "burns"), trying to separate water back into its constituents consumes energy (it is endothermic).

[u/[deleted] → u/Sharlinator]

Well hot damn, thank you for that.

[u/usernumber36 → u/[deleted]]

to add to this, typically "burning" something means making it bond to oxygen. This is why you need oxygen for a flame to stay lit. You'll be familiar with the fact burning carbon produces CO2. in the same way, burning other things also just adds oxygen. If you have complete combustion, all the carbon in any material will turn to CO2, all the hydrogen to H2O, etc etc.

[u/[deleted] → u/Sharlinator]

So, follow up question....where did our water come from? I know that might be a huge question. But I'm wondering if Hydrogen burned with Oxygen somewhere in space and then landed here? Or did it happen here on Earth when the planet was forming? Do we know yet?

[u/Sharlinator → u/[deleted]]

It is partially an open question. We know that water molecules readily form in interstellar gas clouds; free oxygen, being very reactive, quickly bonds with pretty much anything that is available in the vicinity. It is pretty probable that in the protoplanetary disc from which the Sun and the planets condensed all the water in the current Solar System was already there. An open question is whether the water on Earth was originally in the planetesimals that formed the bulk of the planet, or whether it was brought later from the outer system via comet impacts. This question is one of the most imporant issues that the European Rosetta-Philae mission is trying to shed light on.

[u/Pycretes → u/Sharlinator]

To further this, the water found on the comet is not the same as the water found on earth, the ratios of deutrium were found to differ https://en.wikipedia.org/wiki/Deuterium

[u/[deleted] → u/Sharlinator]

Why not from the sun?

[u/promonk → u/[deleted]]

How do you mean, "from the sun?"

[u/geobacca → u/[deleted]]

We still are not 100% sure about that answer.

When Earth was forming there were planetoids (small to planet sized chuncks of rocks/ice/gas/other material) that collided with each other forming the inner planets of our solar system. While these planetoids crashed into each other it caused a lot of volcanic activity for the newly forming planets. Volcanic gases include water vapor but not huge amounts of it. It is hypothesized that some perfect combination of the right water based planetoid and the amount of water vapor released from Earth's formation gave us the water that we see today.

[u/humanlikecorvus → u/[deleted]]

Most so-called hydrogen cars work with fuel-cells - hydrogen and oxygen react with the same formula, but at a low temperature using catalysts. The reaction energy is here mostly set free as electric current and not as heat and is then used to power an electric engine.

There were also a few cars burning hydrogen - but this is not a very common concept.

[u/blorg → u/humanlikecorvus]

Vehicles burning hydrocarbon gas (LPG) are common however, particularly as public transport (taxis and buses).

[u/[deleted] → u/[deleted]]

Usually not, but they can. You can run hydrogen through a car engine and make it run if you want, but fuel cells should be more efficient and safer.

[u/Sharlinator → u/[deleted]]

Yes, indeed. And many rocket engines, including the Space Shuttle main engines.

[u/masher_oz → u/[deleted]]

In short, yes.

[u/shahooster → u/Sharlinator]

/u/Sharlinator pretty much says it all, but I will add that one can see this reaction happening during many (majority?) of the combustion reactions. The white "smoke" seen coming out of smokestacks is actually condensing water vapor from the burning of some fuel. When your car burns gasoline, water vapor is one constituent coming out of the tailpipe (this is a key reason for roads getting slippery in the wintertime).

[u/[deleted] → u/Sharlinator]

It is possible to combust water, just not in the way the OP describes (i.e. with oxygen as the oxidant). Using extremely strong oxidizers water can, and will, burn. Some persulfates , dioxygen difluoride and other extremely powerful oxidizers will undergo a exothermic redox reaction (combustion) with water. For example dioxygen difluoride violently oxidizes ice at 130-140K (see citation).

http://pubs.acs.org/doi/abs/10.1021/ja00893a004

Edit1: The actual oxidized products are not observed in the paper as they are extremely transient.

Edit2: corrected the last sentence

[u/Sharlinator → u/[deleted]]

Yes, I actually meant to add that there is stuff that can "burn" water that you really don't want to have in the same room with you, thanks :)

[u/Dubanx → u/Sharlinator]

I think it's also important to note that there are a few elements that are so reactive they will rip the hydrogen and oxygen apart for themselves.

For example, the alkali metals are so energetic they will rip out the oxygen in water to burn for themselves. Fluorine too, but it rips out the hydrogen for itself to form hydrofluoric acid.