A question of zeppelins
May 27, 2006 6:16 AM   Subscribe

Why is Hydrogen inflammable? And not Helium?

What's in the chemical makeup of Hydrogen that makes in imflammable. How is it different than a gas like Helium or Nitrogen?

And why does gasoline go boom and not water?
posted by DieHipsterDie to Science & Nature (15 answers total) 1 user marked this as a favorite
Best answer: Some chemical reactions produce energy when they occur. Chemists call these "exothermic". The opposite, requiring energy to occur, is called "endothermic". A sufficiently exothermic reaction produces quite a lot of energy, which leads to things like fire and explosions.

Hydrogen combines with oxygen to make H20, water. This is an exothermic reaction.

Helium doesn't combine with much of anything. Helium is called "inert". The "why" is related to how many electrons helium has - you'll have to read about this in a basic chemistry text.

There is a whole class of reactions involving hydrocarbons (molecules with hydrogen and carbon) and oxygen. The class looks like this:

Hydrocarbon + oxygen = carbon dioxide + water

That reaction is the basis for all life on earth. It is exothermic. Gasoline looks like that, natural gas burning looks like that, when you eat food and turn it into energy it looks almost like that, and so on. The reverse reaction is endothermic - plants do it using the energy from sunlight, it would not happen by itself:

carbon dioxide + water + sunlight = sugar + oxygen

Another example of an exothermic reaction you see every day is rusting:

iron + oxygen = iron oxide

This reaction is actually quite exothermic, but it usually happens slowly. However, when a large amount of rust is occuring, the amount of energy given off is large. Cargo ships have caught fire because their cargo of iron got wet during the trip and started to rust.
posted by jellicle at 6:33 AM on May 27, 2006

Just to add to that, the reaction is exothermic because it takes less energy to form a stable H20 molecule than it does to maintain stable hydrogen and oxygen molecules on their own. Which is why it's exothermic (which is, really, a measure of kinetic energy, so can be dealt with through all sorts of fun formulas).
posted by klangklangston at 6:44 AM on May 27, 2006

It all comes down to a single electron and its accompanying proton.
posted by caddis at 7:03 AM on May 27, 2006

Doesn't everything?
posted by klangklangston at 7:14 AM on May 27, 2006

The first orbital "shell" in atoms has room for 2 electrons. Since neutral hydrogen has only 1 electron, its shell can combine with unfull shells from other atoms to form compounds where this shell is in some sense full.

Helium has 2 electons so its shell is already full and it cannot combine with other atoms.
posted by MonkeySaltedNuts at 8:15 AM on May 27, 2006

Though I don't remember for sure, I also believe that it's possible to superheat helium to the point where electrons will shear off, making it able to "combust."
posted by klangklangston at 8:39 AM on May 27, 2006

Don't stars eat helium after all the hydrogen has gone?
posted by A189Nut at 9:22 AM on May 27, 2006

Best answer: The exact answer to this involves the use of equations (that I no longer remember exactly) deriving from the Schrödinger equation that describe the various positions electrons can occupy around a nucleus. In terms of chemical reactivity (though not in terms of catalysis), the most important of these positions are refered to as the "valence shells" -- these are the outermost positions (i. e. farthest away from the nucleus) that electrons can occupy and thus when two or more atoms approach each other are what interact in a covalent chemical reaction (that is a chemical reaction that involves bond formation or, in simpler terms, electron sharing/orbital shell merging).

Now, these valence shells can only hold 8 electrons (except in the case of hydrogen and helium where the outermost only has 2 slots) for each energy level and a complete set of 8 (or, again, 2) generally occupies an energy minima and thus an atom likes to either gain or lose electrons to statisfy having a full valence shell. When you look at a periodic chart, each of these energy levels is represented by the rows. Starting at the left side of the chart, these elements -- sodium, potassium, etc. -- have 1 electron in their valence shell and tend to lose an electron in order to have their outermost shell (the previous energy level) full. This is why you don't usually find metallic sodium just hanging around outside and why it explodes when you through it into water -- it attacts the water molecules trying to shed that one electron. On the next to rightmost side of the chart, you find the halogens -- chlorine, bromine, etc. -- that need just one more electron to have a full outershell and are likewise very chemically reactive due to their hunger for that electron. On the rightmost side of the chart you find the noble gasses -- your helium, neon, argon, and so forth -- elements that now have a full valence shell and thus have a strong avoidance to anything that might withdraw or add an electron to their shells, making them chemically inert.

Thus, hyrdrogen, with its need for one more electron will readily partake in a reaction to either gain or lose that electron and thus two hydrogen atoms can react with a reactive oxygen atom that needs two electrons to satisfy its valence shell. Helium has a very stable outer shell and thus will not react. Nitrogen is not very reactive because it exists in the atmosphere as a diatomic atom -- two nitrogen atoms come together and share their electrons such that their shells are filled. Mollecular nitrogen happens to have a very low energy level, thus it is stable. It takes a lot of energy to break the bond and make it reactive again (which is why certain nitrogen containing compounds tend to be very reactive -- they want to revert back to the molecular nitrogen state).
posted by The Bishop of Turkey at 9:31 AM on May 27, 2006

Hydrogen also exists diatomically (H2) under standard conditions. However, the enthalpy of a H—H bond is 435kJ mol-1, compared the enthalpy of a N—N bond, which is 945kJ mol-1.
posted by matthewr at 9:43 AM on May 27, 2006

Don't stars eat helium after all the hydrogen has gone?
posted by A189Nut 20 minutes ago

That's a nukular meal, not a chemical one.
posted by caddis at 9:44 AM on May 27, 2006

When two atoms share an electron, is the electron doing a figure eight?
posted by airguitar at 12:08 PM on May 27, 2006

When two atoms share an electron, is the electron doing a figure eight?

No, electrons don't actually "orbit" the center of the atom as you would think. The sort of exist in 'clouds' where you don't know the actual position. Each point around the atom has a certan probability of having the electron, with some places having a higher probability then others. There are lots of diffrent 'cloud shapes' and those determine the orbits.
posted by Paris Hilton at 12:42 PM on May 27, 2006

The electron is somewhere in the pink, though we can't really say where (that's how quantum mechanics goes). You want a molecule more complex than molecular hydrogen and we start getting some weird shapes.

For more, you want to look up molecular orbital theory
posted by The Bishop of Turkey at 1:06 PM on May 27, 2006

Those two 'best marked' answers were both excellent, and are examples of the reason I read AskMe. Woo chemistry!
posted by wolftrouble at 3:59 PM on May 27, 2006

Looking back at it, I do have to call out a point in the first "best answer" -- just because a reaction is endothermic does not mean that it won't happen spontaneously. If the increase in entropy, or "disorder" (little more complicated than that but that is a close enough description), is enough it can offset the increase in energy that the products have relative to the reactants. A good example of this would be the instant cold packs that you can get at any drug store as that is a spontaneous endothermic reaction (which is why they are cold and instant). Likewise, just because a reaction is exothermic also does not mean that it will occur spontaneously.
posted by The Bishop of Turkey at 5:42 PM on May 27, 2006

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