Nitrogen oxides

[spudpeel]

Ive thought about this before, but never really bothered about it. If the valency of Nitrogen is 3 and oxygen 2, how can the two form NO and NO2? Do they have orbitals that bond together, as between layers of graphite, or exist as dimers, or is there something weird going on? I'd really appreciate someone helping out!

[woelen]

Ive thought about this before, but never really bothered about it. If the valency of Nitrogen is 3 and oxygen 2, how can the two form NO and NO2? Do they have orbitals that bond together, as between layers of graphite, or exist as dimers, or is there something weird going on? I'd really appreciate someone helping out!

Yes, there is something "weird" going on.

 

Usually, all atoms in a molecule want to have filled shells. If you look at water, then the oxygen shares its two electrons with the hydrogens, but the electrons of the hydrogens also are shared. This makes all atoms in water happy.

 

In some compounds, some atoms are not happy at all. Look at a compound like methyl, CH3. This molecule contains a very unhappy C-atom, with just 7 electrons. It would be much better if a fourth H-atom comes into play, but two CH3-molecules also can become happy if they join together, forming H3C-CH3.

 

Such unhappy atoms are missing some electrons. In more scientific terms, such atoms have unpaired electrons. Molecules, containing such atoms are called radicals. So, CH3 is a radical and it is VERY reactive.

 

Now the surprising point is coming. NO and NO2 also are radicals. The nitrogen atom in both molecules contain an unpaired atom. NO and NO2 are very reactive molecules, but in terms of radicals they are surprisingly inert, such that it is even possible to store them. This is impossible with CH3. That would react at once with itself, forming ethane (H3C-CH3) and with the container, in which it is stored.

 

Another example of an 'inert' radical is ClO2. This bright yellow gas can be created in macroscopic quantities and can be stored. It still, however, is very reactive and easily explodes, but in terms of radicals it is remarkably inert.

[jdurg]

NO2, however, VERY easily dimerizes into N2O4. In fact, if you have a sealed tube of NO2, it will dimerize readily and even moreso depending on concentration. NO is VERY reactive and will quite easily oxidize itself to NO2 by reacting with atmospheric oxygen. This NO2 can then dimerize and everything is happy. So NO isn't as ultimately stable as one would make it to be. It's not nearly as unstable as CH3, but not stable like H2O.

[akcapr]

ya, same thing with CO. Does CO oxidize also to CO2 in air, or does it take some activation energy?

[Bluenoise]

Yeah they basically exist for the very short time it takes them to find something to react with to make a stabler arrangement

[The Thing]

Oh yes, but if CO can't find any excess oxygen to react with in an incomplete combustion, woosh things are gonna get nasty, expecially if there's a person right beside it breathing it in for an hour.

[woelen]

ya, same thing with CO. Does CO oxidize also to CO2 in air, or does it take some activation energy?

CO slowly oxidizes to CO2 in the atmosphere, but this is quite different from NO. NO is oxidized at once. If you have a syringe of colorless NO and you press the gas out of the syringe, then you'll see a brown plume of NO2 and all of it is oxidized within seconds. CO takes days or even weeks to be oxidized and that reaction also requires sunlight.

 

CO is not a radical. It only has paired electrons.

[woelen]

NO2, however, VERY easily dimerizes into N2O4. In fact, if you have a sealed tube of NO2, it will dimerize readily and even moreso depending on concentration. NO is VERY reactive and will quite easily oxidize itself to NO2 by reacting with atmospheric oxygen. This NO2 can then dimerize and everything is happy. So NO isn't as ultimately stable as one would make it to be. It's not nearly as unstable as CH3, but not stable like H2O.

NO2 readily dimerizes, but the dimer also readily decomposes back to NO2. This is a large difference with e.g. CH3. Dimerized CH3 (ethane) never will form CH3 again, with NO2 there is an equilibrium:

 

2NO2 <---> N2O4

 

Low temperature and high pressure drive this reaction to the right. This is nicely demonstrated if you have a syringe, filled with NO2. If you press a capped syringe with the gas, such that pressure increases, then you'll see the color become lighter (after an initial transient, where it is darker). If you then suddenly release the thing, then you'll see darkening instead of lightening of the color.

 

Very nice experiment, I have done that once. I made my NO2 by first making NO-gas from acidified ferrous sulfate and sodium nitrite, pressing the gas into another syringe through a thin tube (water spoils the demo). I next made O2 from H2O2 and KI and pressed the O2 gas also into the syringe with NO, approximately the same volume. Then you are ready to play.

Be careful though, NO and NO2 are insidiously toxic. Do this outside!

[jdurg]

That demonstration also works with temperature quite nicely. Place it in an ice-water bath and you'll see one color. Then dunk it in a hot water bath and the color rapidly changes. Really neat to see. (Also, I don't know if you could consider 2CH3 => Ethane a dimerization. I consider it more of a reaction because as you said, it's not something that's easily reversible while the 2NO2 <=> N2O4 is quite reversible. Those damn semantics again. hehe. )