I’ll get right to the point: how, practically and physically, do you make a compound? I know that water is H[sub]2[/sub]0 and sulphuric acid is H[sub]2[/sub]SO[sub]4[/sub] and everything, and I know that although air is roughly one part oxygen and four parts nitrogen it isn’t ON[sub]4[/sub], because it isn’t a compound (and you couldn’t even make a compound that was ON[sub]4[/sub]). Right?
So how exactly do you make a compound? Let’s say I want to make laughing gas (N[sub]2[/sub]O). Both nitrogen and oxygen are in plentiful supply in the air. What would I do to turn it into nitrous oxide?
Well, usually putting two elements together with adequate heat and pressure will do the trick. For instance, you can easily produce carbon dioxide because carbon will combine with oxygen at atmospheric pressure, with just a little heat (relatively speaking). Nitrogen will combine with oxygen, but it needs a lot of encouragement – the kind of encouragement found inside an internal combustion engine, for instance (which is why we have NOx problems and catalytic converters).
As a rule, other elements don’t combine as readily as oxygen, so to prepare those compounds, you need to evacuate the reaction vessel of air. You need to prepare fine particles of solid reagents, or dissolve them in water if that’s involved. You may need to pressurize the vessel, or heat the reagents, or ignite them with an electric arc. Finally, the heat required may be greater than the reagents or the product can tolerate, in which case you must use a catalyst, which will reduce the thermodynamic hurdle for the reaction (catalysts also reduce the expense and danger of other reaction, making them economically feasible).
As I recall, NO[sub]2[/sub] is easy to make, and is made all the time by burning things. Unfortunately, so is NO[sub]3[/sub]. I believe that the proper initial conditions can prevent formation of the latter, or you can separate them physically, but I’m an analytical chemist, and I don’t remember that stuff.
Let me see if I’ve got this straight. There’s no single “make a compound” technique, you have to do something special for each compound you want to make, but usually it involves putting the two elements together under heat and pressure. The heat and the pressure varies with the elements and compound.
Back to nitrous oxide. By burning something under the proper conditions, nitrous oxide is formed from the nitrogen and oxygen in the air. What am I burning? Since air has to be present, doesn’t this simply result in a mixture of air and nitrous oxide?
Remember that “burning” usually just means “combining with oxygen.” That’s why we say that our bodies “burn” food, despite the fact that there’s no actual fire in our stomachs. (Unless we eat Mexican.)
If you put a bunch of nitrogen and oxygen together in a container, and throw in a match, you’ll get a big explosion. The match provides just enough energy to get a few nitrogen and oxygen atoms to join, and this reaction happens to be exothermic. (It produces heat. Or, to be more accurate, it releases heat that is already stored in the atoms.) Thus, you get a chain reaction, and all the nitrogen and oxygen starts bonding until you run out of one or the other, and you get a very satisfying FWOOMP!
As for why certain elements care to bond to one another, you have to learn about electron configurations, covalent and ionic bonds, and so forth. A good high school chemistry textbook has all this information, most of which I’ve forgotten.
Offhand, I would say that you’re burning nitrogen… burning is generally a combination of free oxygen from the air with other elements or componds to form oxygenated combounds. For instance, the burning of hydrogen gas, as in the space shuttle rockets or the classic high school chemistry demonstration, results in water vapour, steam.
As far as the notion of ‘mixture of air’, this will happen unless the combustion is done under sufficiently controlled circumstances to be certain of almost total consumption of the components. If you were using a rig with a stream of nitrogen and a stream of oxygen entering the combustion chamber, you could probably get a fairly pure sample of various nitrogen/oxygen compounds as a result.
[QUOTE=friedo]
If you put a bunch of nitrogen and oxygen together in a container, and throw in a match, you’ll get a big explosion. The match provides just enough energy to get a few nitrogen and oxygen atoms to join, and this reaction happens to be exothermic. (It produces heat. Or, to be more accurate, it releases heat that is already stored in the atoms.)
[QUOTE]
This brings up a question that occured to me while I was composing my own response. Any room is basically a container filled with a bunch of nitrogen and oxygen. Why doesn’t it begin to burn as soon as you strike a match??
Come to think of it, don’t the space shuttles use nitrogen as a fire supressant or something? Is there something special you have to do to get nitrogen to burn beside just mix with oxygen and add heat??
And even if you didn’t, you could liquify the product gas to get rid of any excess N2 or O2 (whichever was in excess.) I don’t remember their boiling points, but O2 has a higher one.
So pure nitrous oxide can not be made by burning in the presence of ordinary air, because then you’d have some nitrogen and ideal gases and stuff remaining? You need to have pure nitrogen and pure oxygen?
So, where do you get pure nitrogen and pure oxygen from? How do you extract oxygen from air?
I just realized that my line of questioning sounds a bit suspicious, so I’d just like to assure everyone that I’m not planning to manufacture nitrous oxide, and it’s legal where I am anyway.
That’s the answer to one of my questions. So I’d just lower the temperature on the gas until either nitrous oxide or the excess gas was a liquid, remove the liquid, and re-heat it to a gas?
Well, since ordinary air is about 80% nitrogen molecules, you will have five nitrogen atoms to every oxygen atom in a given sample of air. Since there isn’t a stable compound that uses those proportions, you’ll have some nitrogen left over.
If you enrich air with oxygen to a suitable proportion, then you’ll be able to get a much larger combination of nitrogen/oxygen compounds, once you figure out how to get the reaction to start at all. You’ll still have other components from the air left after the reaction, carbon dioxide, argon, neon etcetera… (unless the carbon dioxide starts combining with the nitrogen somehow.)
There’s nothing special about starting with ‘pure’ nitrogen and oxygen except as a way of making sure that you’re controlling the relative proportion of each. They have to mix to react with each other anyway.
Or, take a compound which already has Oxygen in (eg. Hydrogen Peroxide) and split that compound up (Hydrogen Peroxide will split into water and oxygen without any help, but adding Manganese Dioxide to it will help it along).
What exactly does that have to do with it? Most combustion reactions deal with atoms that are already bonded to something… very rarely do you come across atoms that are unconnected with anything, on a chemical level. (Noble gases and gas particles in space are about the only thing that come to mind, and I’m not even sure about the second.
When you burn hyrdogen in air, the hydrogen is bonded to itself. (So is the oxygen.) But the presence of a spark along with those two components is enough to get the molecules involved to split up and combine into H2O, liberating heat.
So, why doesn’t the same process work with nitrogen? Presumably it might have something to do with the fact that nitrogen is more strongly bound to itself? (Doesn’t nitrogen have three covalent bonds?) So, what do you need to burn nitrogen with oxygen? Higher temperatures than a match will provide? A greater concentration of oxygen than is present in our atmosphere? Higher or lower pressure?? Something I haven’t thought of?
Ah, yes, nothing like trying to boil a big pot of explosives :eek:
Heating a nitric acid solution under pressure will also yeild N[sub]2[/sub]O… but the pressures are outiside the range of your average pressure cooker. Of course, the reaction of iron with nitric acid can ALSO produce nitrous… so, that could be interesting all around (especially when it finally disolved the wall of the pressure cooker).
As for 2N[sub]2[/sub] + O[sub]2[/sub] --> N[sub]2[/sub]O… I think you’d need to subject them to coronal discharge (E.G. lightning) and have them react as plasma.
Because to combine, the atom or molecule has to break its existing bond. Unless the bond is not very strong, a match ain’t gonna provide enough energy. Take nitroglycerine for example. It is a highly explosive molecule, but just because it has the ability to explode doesn’t mean it does so without an impetus. When comparing nitro to something like RDX, what sets off one won’t even phase the other. You need to supply enough energy to break the bond.
N2 isn’t quite noble, but it’s a very stable molecule (the two Ns are triple-bonded, and the two lone pairs of electrons are quite happy to stay where they are, as, with many of the other electrons in the molecule spending most of their time in the molecular bonding orbital, there’s little negative charge to shield them from the nucleus, so they feel the pull quite strongly). You need to put a lot of energy into the system to get N2 to react with much of anything, because it’s really tough to get much net energy out of breaking this triple-bond. O2, however, reacts quite readily with a lot of things, but not N2, or your average bolt of lightning would fry the troposphere. No matter what the stoichiometric ratios or ratios of available products, how would trowing a match in a bottle of N2+O2 do anything but let the match burn down and yield stuff like CO and CO2?
