How Air Separates Water And Goes Boom

Fertilizer bombs typically use ammonium nitrate, NH₄NO₃. Bombers use it because the stuff is so ubiquitous and so plentiful.

Wiki draws a pretty picture of ammonium nitrate, which partially reveals its explosive potential:

Don't skip over that structure--it's a work of art: two very different nitrogen atoms separate elements of hydrogen and oxygen. On the left, four hydrogens surround a central nitrogen while on the right, three oxygen atoms surround a central nitrogen. There's an unbalanced symmetry. Those H's and O's would love to get together and quench, making water, leaving the naked nitrogens to couple, making N₂ which is essentially air. The chemical structure of ammonium nitrate is a depiction of air separating water.

We never hear about spontaneous explosions of ammonium nitrate. Why is that? An old word from the Greek called stoichiometry answers why. Try and balance the chemical equation of NH₄NO₃ making N₂ and H₂O:

             NH₄NO₃  =>  N₂  +  H₂O

Look easy? I gave up. The reason is that the hydrogen to oxygen ratio is inherently 4 to 3 on one side and 2 to 1 on the other. No amount of tweaking the coefficients will balance that reaction. The stoichiometry just doesn't work.

This brings up just how fertilizer bombs do work, because they do make air and water from ammonium nitrate. There is always a little admixture of carbon (typically fuel oil) but charcoal would work too. The carbon combines with the "excess" oxygen present in NH₄NO₃, making CO₂. This trick even has a name: cf. detonation vs. deflagration

Here's a question for any munitions experts out there: were ANFO's involved in Boston? I've heard that the differing sounds of detonation versus deflagration are diagnostic.

A second question is technical: is it possible (in theory) to isotopically label the nitrogen in manufactured ammonium nitrate and thus, after isotopic analysis of air samples at the bombing site prior to mixing, identify the source?
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[Added]  Well this breaking news should cause a spike in ammo and food prices. The report says that water sprayed on NH₄NO₃ made it explode. That may actually have been heat from the fire or could have resulted from of what's called heat of hydration when a dry salt suddenly mixes with water. Some salts suddenly mixed with water actually make the water colder.

Ammonium nitrate (AN) is not known to chemically react with water in an explosive way. However, if heated it reacts explosively:

             2NH₄NO₃  =>  2N₂  + O₂  + 4H₂O

Now that reaction really produces a sky-full: nitrogen, oxygen, & cloud.

"Don't call it transmutation. They'll have our heads off as alchemists"

The transmutation of elements was an ancient, discredited notion promulgated by alchemy (alchemy is to chemistry what astrology is to astronomy). Alchemists sought to turn base metals like lead into gold. They failed or were quacks and charlatans. And yet transmutation has always occurred naturally and has been practiced since 1917.

Natural transmutation was first discovered when Frederick Soddy, along with Ernest Rutherford, proved that radioactive thorium converted to radium in 1901. At the moment of realization, Soddy later recalled shouting out: "Rutherford, this is transmutation!" Rutherford snapped back, "For Christ's sake, Soddy, don't call it transmutation. They'll have our heads off as alchemists."

Transmutation became a fait accompli. But it was one thing to discover that atoms could naturally and spontaneously lose little pieces like an alpha particle or a beta particle or even a gamma ray. It was quite another thing to discover that atoms could add little pieces too.

In 1917, Rutherford projected alpha particles from radium decay through air and discovered a new type of radiation which proved to be hydrogen nuclei (Rutherford named these particles protons). Further experiments showed the protons were coming from the nitrogen component of air, and he deduced that the reaction was a transmutation of nitrogen into oxygen:

¹⁴N + α → ¹⁷O + proton

This was also the first demonstrative proof of artificial transmutation and the proton's existence.  All that was needed now was the neutron which led to division and multiplication.

Conversations with Henry

Henry: "Isoelectronic" is a perfectly fine concept. No need for you to feel it's inadequate. I'll give you an even easier example. We can play the same game with carbon, nitrogen, and oxygen:

[Henry sketches the Lewis structures for N₂ and CO]:

Henry:  Forget about the labels "C", "N", and "O" for a moment.  Don't let them color your thoughts. Think of them as the numbers 12, 14, and 16:

Me:  OK, but what does the little curved arrow 1p, 1n mean in your picture?

Henry:  That's your little Maxwell's Demon, moving a proton and a neutron from one side to the other. There's no net loss or gain, but rather just a transfer.

Me: Are you pushing electrons too?

Henry: No! The electrons haven't moved yet but they feel the polarization: suddenly there's an extra charge on the oxygen side and one less charge on the carbon side. The electrons rearrange, being drawn slightly closer to the oxygen, but not completely, and the carbon, having less positive nuclear charge, is polarized negatively by the electrons. The molecules electron's are polarized like this:

Me: Ah, that explains why carbon monoxide binds to metals like iron in hemoglobin via its carbon.

Animal Magnetism and the Lure of Bad Ideas

[This post was inspired by seeing a horse with magnetic bracelets around its fetlocks; the animal was undergoing "magnetic therapy treatment."]

Magnets fascinate us because they act at a distance -- both attracting and repelling.  Lodestone (the name literally means "leading stone") was so named for its navigational utility. There was a time when magnetism was consigned to inanimate things -- and then just mostly to iron. Ironically, magnetic navigation by birds and other animals appears to rely on magnetite.

As long as magnetism remains a mysterious force, it will find uses in medicine and alternative therapies. Discredited science and pseudo-science can die out and then return in vigor, once new discoveries occur. Such is the case for animal magnetism.

Around 1784, pre-revolutionary Paris was enthralled by Franz Mesmer, whose schtick was to pass magnets over people and cure them of alleged physical and mental ailments. Louis XVI (or his courtiers) was concerned enough to task the learned heads at the time to look into the matter. A Royal Commission included Antoine Lavoisier and Benjamin Franklin, among others. To make a long but interesting story short, the Royal Commission debunked Mesmer and he faded into near obscurity, leaving behind only the verb "to mesmerize." The whole story was brilliantly translated and retold by Stephen Jay Gould in his Bully For Brontosaurus.

Around 1845, Michael Faraday discovered that all matter responds to magnetic fields, albeit weakly and then only in the presence of very strong fields. Faraday named this remarkable property diamagnetism (though he did not explain its origins). Faraday's discovery briefly reenergized a belief in animal magnetism.

Faraday also discovered the paramagnetism of oxygen (he used soap bubbles filled with O₂ rather than liquid oxygen). Faraday developed a theory that atmospheric oxygen is seasonally influenced by the earth's magnetic field via temperature. He later abandoned this brainchild.*
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*Frank A.J.L. James, Michael Faraday: A Very Short Introduction, Oxford University Press (2010)

Saltpeter Is Not Rock Salt



Rock Salt (Halite)


If word histories made any sense, saltpeter should be rock salt. But instead rock salt is halite.

LL made a nice comment back here:

I thought that you would take this thread in the direction of Potassium Nitrate (saltpetre), accused falsely of causing impotence in men and at the same time allowing gunpowder to go boom.

Saltpeter

The Lewis structure of potassium nitrate invokes all that separated charge, which in a way depicts internal potential energy. Nitrates give explosions not just an oxidant (oxygen) but also nitrogen atoms badly wanting to make nitrogen gas. Whence the added oomph first discovered by the Chinese. The same principle is behind nitroglycerin (dynamite), trinitrotoluene (TNT), and nitromethane racing fuel: enhanced combustion.

I find nitrogen oxides intrinsically interesting and I even wrote a bit about them back here already; I mentioned impotence, but it's at the very end of the post. It's an odd coincidence that NO gets one hard while NO₃^- stands accused of the opposite.

Buh-Bye NOxious Waste

Nitrogen oxides are everywhere. There are "good" ones and "bad" ones and they're all slightly different. A collection of pretty pictures is here: link  Nitrogen oxides include laughing gas (good), nitrate and nitrite salts in foods (controversial), and nitrate plant fertilizers (good). Nitrogen oxides are even important in sex (good) because the molecule NO (nitric oxide) is key in sexual arousal.* Some nitrogen oxides do more harm than good: nitrogen oxides in clouds help form acid rain and the color of "brown cloud" comes from NO₂. But worst of all, nitrogen oxides come out the tail pipes of more efficient diesel vehicles. I wrote about that back here.

The reason there are so many different nitrogen oxides is because nitrogen and oxygen are Periodic Table neighbors, and being so close together, each covets its neighbor's electrons when excited. But only when provoked. Air does not react with itself under normal circumstances. But give things some radiation, a spark, or a diesel motor, and O₂ and N₂ do the nasty together--they recombine:
  
          N₂ + O₂  --->  2NO_x

There's an "x" subscript there because NO can react further to make NO₂ and other nitrogen oxides. "NOx" is so noxious that the State of California forbade the sale of new diesel vehicles beginning in 2004 (I got one of the last VW Golf models in 2003). Daimler-Benz wanted to get those sales back (leave it to the Germans to invent all the new cool diesel technology) and Daimler's BlueTec technology returns NOx emissions whence they came--back to nitrogen and water:

         4NO + 4NH₃ + O₂  --->  4N₂ + 6H₂O 

That equation is mass-balanced by the way, unlike-ahem, what one finds in the Internets (FTFY). The chemistry looks like complex gas-phase chemistry but actually a metal surface mediates the rearrangements (thankfully not another precious metal catalyst). In practice, NOx abatement injects ammonia into the exhaust stream upstream of the catalyst. Ammonia is the best "blue" additive in theory, but urea, which is more easily transported and handled, can also be used in Daimler's AdBlue technology. From what I've read and heard, the technology works as well but is very expensive. So what else is new. Owners are required to periodically charge up with fresh aqueous urea, which led one owner to ask whether he couldn't just "piss in a tank instead"  hahah.

A good primer on how the technology works can be found here or GIY (Google it yourself)

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*Has anyone noted the irony of "NO" (nitric oxide) being a key factor in male sexual arousal?

Magnesium Is Where The Photons Hit The Protons

According to van der Krogt, magnesium was so-named because:

The names magnesia alba and magnesia nigra are derived from Magnesia, Μαγνησια, a prefecture in Thessaly (Greece), with the capital Volos). Manganese and magnesium were abundant in oxide and carbonate ores in this region, and they therefore became referred to as Μαγνητις λιθος, or stones from Magnesia. The region also contained large amounts of iron oxides (magnetite, or lodestone, for example) so that the ores were magnetized. That explains why magnesium as well as magnet (and magnetism) are derived from Magnesia, while magnesium is not magnetic. link

Magnesium is essential to both plants and animals. Green plants are a great dietary source of the element. A magnesium ion sits right at the heart of each chlorophyll molecule in chloroplasts, part of the molecular machinery responsible for photosynthesis:

The drawing is a cartoon -- there are really several different forms of chlorophyll, each differing at its periphery.

Chlorophyll is an example of a metal porphyrin complex. A geometric array of linked rings (a porphyrin) surrounds a magnesium ion core. Another common metal porphyrin complex is the iron-centric hemoglobin food in blood; another related complex is cobalt-centric cobalamin or (vitamin B₁₂) in the liver. Such metallo-centric molecules are also ubiquitous in synthetic organometallic chemistry, especially among catalysts.

As an aside, the notion that "Cartesian" is an artificial construct imposed upon nature is clearly not supported here. These molecules show a sort of rectilinear coordinate system: four nitrogens define the four cardinal directions. Such molecular geometry is a consequence of the underlying electronic structure which we take for granted.

Chlorophyll uses visible light photons to convert water into protons and electrons (whence my title) with oxygen as a waste product. The oxygen we breathe (approximately 20% of air) comes solely from bacteria, algae, and plants. The electrons and protons harvested by chlorophyll are subsequently used to reduce carbon dioxide into sugars, completing one half of global respiration.

Flogging Phlogiston

Oxygen burned two great ideas in chemistry. Not literally, but understanding oxygen and oxidation undid two great ideas. One was called Valence Bond theory and I considered its undoing back here. VB theory is still useful and is taught in middle and high school chemistry curricula. The other great idea was called Phlogiston theory. Of course the Chinese had their own take on oxygen which you can read about here: link

The word "phlogiston" came from the ancient Greek word φλογιστόν ("burning up") and was promulgated by German scientists beginning in the 17th century. The notion had probably been around much longer because the idea was a very intuitive one. Phlogiston theory taught the existence of an element called phlogiston, a substance without color, odor, taste, or mass. Phlogiston was liberated when something burned or slowly rusted. Think of what you feel in front of an open flame. Not really so far-fetched, the notion was close to our modern notion of energy consumption, in so far as we suppose substances like fuels have "energy content." We speak of hydrocarbon's energy content in BTUs as if it were something we could distill and put in a bottle.

Phlogiston was a German notion and was undone by men like Antoine Lavoisier who showed that metals increased their mass when they burn or rust, inconsistent with something being lost or given up. Unfortunately, Lavoisier lost his head in the French Revolution for his royalist sympathies.  A dead cat bounce for Phlogiston occurred around the turn of the 19th century, just after water electrolysis was discovered.


Alessandro Volta's Pile 

When William Nicholson and Anthony Carlisle inserted the two wires from their voltaic pile together into a vessel of water, they also galvanized the entire scientific world, creating a sensation as great as any scientific discovery ever made. In Nicholson's words:

It was with no little surprise that we found the hydrogen extricated at the contact with one wire, while the oxigen [sic] fixed itself in combination with the other wire at a distance of almost two inches.

What actually happened depended on the type of metal wire used: when they used copper, hydrogen gas evolved at one wire while the other wire became "fixed with oxygen" meaning it turned to copper oxide (greenish blue). But with platinum or gold wires, hydrogen gas evolved cleanly at one wire while oxygen gas evolved cleanly at the other electrode. The great puzzle was not that the two different gases were produced, but rather that they were produced at different electrodes. It seemed to everyone that if the gases both came from the decomposition of water they should both appear at the same place.

Now the notion that hydrogen and oxygen were distinct elements was not universally accepted. It was not settled science. One of the doubters was a German named Johann Ritter. Ritter was no slouch.*  He repeated the Nicholson and Carlisle experiments and concluded that it was impossible for the gases to be produced from the decomposition of water since there was no way that a gas could travel through one wire, through the pile, and out through the other wire. The truth, Ritter argued, was that:

Water is an element

In Ritter's view, "oxygen" was just water plus positive electricity and "hydrogen" was just water plus negative electricity. He nearly set science back 2000 years. That water was an element and electricity was like phlogiston was ancient thinking. Great minds, including Michael Faraday, puzzled over water electrolysis for years. Bear in mind that in the early 1800's nobody had yet thought that water could ionize into H⁺ and OH^-. The proton (and the electron) had not yet been discovered. But the Germans ultimately lost the argument.

We now understand that water is consumed at both electrodes and electrons flow into one electrode and out the other:

At one electrode we have:  2H₂O   +   2e^-   ---->    H₂(g)   +    2OH^-
At the other electrode:        2H₂O    -   4e^-   ---->    O₂(g)   +    4H⁺  

I remember this stuff by recalling the origin of the word "oxygen" which means "acid-forming." The electrode which forms oxygen also forms acid. Of course the H⁺ and the OH^- swam the two inches back towards each other in Nicholson's experiment and remade neutral water, and completed the circuit.
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*Ritter was no slouch:

"William Herschel discovered infrared radiation because thermometers, which had recently been developed in Europe, showed a higher temperature just beyond the red end of the visible spectrum of sunlight. The German chemist Johann Wilhelm Ritter (1776-1810), after hearing about Herschel’s discovery from 1800, identified another “invisible” radiation which we now know as ultraviolet (UV) in 1801. He experimented with silver chloride since blue light was known to cause a greater reaction to it than did red light, and he found that the area just beyond the violet end of the visible spectrum showed the most intense reaction of all." reference

Why doesn't the world just spontaneously combust?

Rhetorically speaking, the question seems absurd. We don't so why even care.  It's like asking why don't we all die tomorrow--the potential is there--but it's just not a physical reality.

Half of the recipe for world conflagration is the fuel--the other half is the oxygen. Thermodynamics say that we should burn up. So why don't we? Why don't we ignite like a puddle of gasoline or at least slowly rust away like a piece of iron? Rust never sleeps--ask Neil Young.

I threatened a while back here to explain why oxygen in the air doesn't spontaneously ignite with all the organic fuel on earth in one big conflagration.  The short answer is a little matter of about 23 kilocalories of energy per mole needed for oxygen to react with most things. 23 kcals/mol is not a high barrier.

Spin provides why we abide (that's not a political jingle)

Molecular oxygen is chemically inert in the presence of hydrocarbons and carbohydrates because of the unusual nature of its electrons.  It's really not hard to understand this uniqueness of oxygen if you've had even rudimentary chemistry. One trick is to consider it in light of nitrogen.

Nitrogen is oxygen's periodic table next door neighbor. Nitrogen is surrounded by five valence electrons, while oxygen is surrounded by six.  If we put two atoms together, we get 10 and 12 valence electrons respectively. Lewis might have drawn N₂ like this:

:N:::N:  

The more usual Lewis structure for nitrogen is:

:N≡N:

Both Lewis structures predict a nitrogen-nitrogen triple bond between the atoms. [true chem nerds will appreciate that the bond order of three is understood as a single hot dog shaped sigma bond along the line connecting the atoms and two bun-shaped pi-bonds at right angles (orthogonal) to each other so as to not overlap. This is how 6 electrons can occupy space between atoms without violating Pauli's exclusion rule].

Now let's use our little Maxwell's Demon to go in and transmute each nitrogen atom into an oxygen atom, leaving everything else the same.  We have to add two electrons to the picture because we added one proton on going from nitrogen to oxygen (never mind the neutron count for now). So we initially get something that looks like this:

 :Ȯ≡Ȯ:

That Lewis structure predicts that there will be two unpaired electrons, one on each oxygen. In fact, oxygen is paramagnetic, having two unpaired electrons. But O₂ doesn't have a triple bond--it has a double bond. Hmmm. What to do. We could simply dissolve one bond between the two atoms, giving two lone pairs on each atom:

:Ö=Ö:

This picture is consistent with the observed double bond in dioxygen. But that picture fails to account for the unpaired electrons in dioxygen.  What is going here?  Nothing less than the failure of simple Valence Bond Theory and Lewis' overly simplistic view of chemical bonding!

There is no acceptable Lewis structure for the garden variety dioxygen molecule.

If you've gotten through this far I'm afraid that I've led you into a little trap.  The answer as to what is the electronic structure of O₂ and why don't we spontaneously combust are in part given by what's called Molecular Orbital Theory, a theory which triumphed over Lewis' theory way back in 1930s.

The two "extra" electrons in O₂ aren't parked on individual atoms nor do they form a bond. Instead, they each reside in separate identical "beds," unpaired with each other. An oxygen molecule possesses two so-called antibonding orbitals which look like this:

One electron goes into the pi*_y anti-bonding orbital and the other goes into an "orthogonal" pi*_x anti-bonding orbital.  Each orbital is singly occupied and electron is unpaired--just as observed.  Oxygen still has a six bonding electrons--just like nitrogen does, but the two anti-bonding electrons partially negate the triple bond. The electrons tally like debits and credits, giving a net bond order of two as observed.

UV light can convert normal O₂ into so-called so-called singlet oxygen which is drawn as:

:Ö=Ö:

When singlet O₂ is loosed on organic material, all hell breaks loose, for example, sunburn and as a growing body of evidence shows, mutations and cancers.

Acerbic Wet

The Danes and the Brits pioneered graphic depictions of simple chemical reactivity. Brønsted and Lowry independently shocked early 20th century chemists with their notions of spontaneous self-ionization of water:

Brønsted-Lowry theory explains how even the purest distilled water conducts electricity (which requires something charged). In their scheme, one water acts like a base by accepting a proton, while the other one acts like an acid, donating a proton. The slight but measurable extent of such H-swapping is real enough--a normal glass of water has a measurable concentration of H₃O⁺ of about 10^-7 units or a pH of 7 (pH is like a Richter scale). An equal & countervailing amount of hydroxide, OH^- neutralizes the acid.

Now consider adding anything to that glass of water which increases the amount of H₃O⁺ (but not OH^-).  Such a thing which donates an H⁺ to a neutral water molecule is called an acid in English.  The Germans call them Säure, which is related to our word sour. Svante Arrhenius (the august savant who also thought up AGW), came up with the idea first.

Lewis Structures: What Is Essential Is Invisible

After physicists discovered and defined the basic properties of the naked electron, the next big question was how to describe and understand them in the context of atoms and molecules, thus encroaching the natural domain of chemistry.

G.N. Lewis invented Lewis structures as a way to describe and understand how electrons surround atoms and also how they hold molecules together.  He did this in a non-mathematical, pictorial way in the early 20th century before the birth and subsequent ascent of quantum mechanics. Lewis depicted atoms and their electrons as cubes which could be joined at their edges, vertices, and faces:

The physicists regarded Lewis's theory as laughably crude, particularly the notion that electrons were fixed at certain positions. Lewis in turn was critical of the physicists' idea that electrons were completely fluid, as for example, in J. J. Thomson's plum-pudding model of the atom, because it seemed incapable of explaining the definitive shapes of molecules, for example, the tetrahedral geometry of carbon in countless organic compounds.

By the mid 1920's Lewis had dropped his 3D cubic portrayal of electronic structure; what survives today is rather like a flat 2D projection of those cubes onto a plane.  Lewis would have drawn hydrogen cyanide (the molecule that may have killed him) as:

"What is essential is invisible to the eye"

The lasting importance of Lewis's theory is that it provided chemists with a way (albeit simplified) of visualizing the electronic structures of atoms and molecules. That is perhaps why it endures.

Maxwell's Demon

The Scottish mathematician James Clerk Maxwell introduced a useful little helper later dubbed Maxwell's Demon. Maxwell's demon could open and shut little windows for individual molecules and Maxwell used it to enable some thought experiments which violated the Second Law of Thermodynamics.

I need a nuclear equivalent to Maxwell's molecular demon: something that can go in and transmute elements into those one higher or lower in atomic number while leaving things like the electronic configuration alone. What I really want to do is to mentally change carbon atoms into boron atoms or carbon atoms into nitrogen atoms, or nitrogen atoms into oxygen atoms, all just to make a couple pedantic points:

Carbon - (1 proton, 1 neutron, & 1 electron)  = Boron

or

Carbon + (1 proton, 1 neutron, & 1 electron) = Nitrogen

or

Nitrogen + (1 proton, 1 neutron, & 1 electron) = Oxygen

The new little demon can transmute dinitrogen into carbon monoxide, dinitrogen into dioxygen, and ethane into amino borane:

N₂ --->  CO

N₂ --->  O₂

CH₃CH₃ --->   BH₃NH₃

Oxygen Reductio Ad Absurdum

Oxygen comes after nitrogen.  The verb "to oxidize" connotes rusting, tarnishing, aging, and decay. What exactly does it mean to oxidize? One definition of oxidize clearly states "to turn something into an oxide." Here's the reductio ad absurdum for that definition (pun intended): Oxidized oxygen is oxide.  That sentence is not true. Rather, reduced oxygen is oxide. In other words, when oxygen is oxidized we don't get an oxide as in the dictionary definition. There's no great mystery here: the common meanings for "to oxidize" and "to reduce" (chemically) were derived from what oxygen does to metals and not what happens to oxygen itself.  Confusing?  Read it again or move along.

Oxygen was unknown to the ancients but was present in all their classical elements: earth, wind, fire and water. Today we know that:

• in earth: rocks, sand, and dirt are mostly oxides.
• in wind: oxygen is about 20% of what we breathe.
• in fire: if there is no oxygen, there is no fire.
• in water: H₂O.

Oxygen is more subtle in the life sciences: cellular respiration--breathing itself--is a life-giving natural process. Our cars and power plants need oxygen to burn fuels. At one level, much of what we do is just "global respiration." Oxygen chemically cycles through all these activities.

Unlike nitrogen, which preferentially* resides in the atmosphere, O₂ is present in air solely due to bacteria, algae, and plants: they consume water and breathe in CO₂, exhaling O₂ and making food, essentially the reverse of what we do. Oxygen is pumped into the sky at a high energy cost (free energy-I'm tempted to say free will just to make a point). Atmospheric oxygen is like an enormous storehouse of chemical potential which works in tandem with chemically stored forms of energy like fossil fuels and plant stuffs. Oxygen in the air is in a sense one half of an enormous battery (the anode). A recent abstract explains:

Through the photosynthetic action of cyanobacteria more than 2 billion years ago, dioxygen (O₂) converted the earth’s atmosphere from a reducing medium to one that is powerfully oxidizing. As a result, we are now awash in a sea of chemical instability, literally ready at all times to combust to yield carbon dioxide and water (H₂O). In other words, we are surrounded by enormous quantities of a gas that, from a thermodynamic point of view, is poised to react spontaneously with organic compounds and a wide variety of other reductants. While useful for generating heat, such reactions must be controlled if the oxidizing power of O₂ is to be harnessed for the production of more tractable forms of energy and more complex (partially and selectively oxidized) chemical compounds. (link)

The meeting abstract goes on to mention "spin":

Luckily, kinetic and spin barriers inhibit the direct reaction of O₂ with organic materials and its reduction to H₂O, extending the time we can exist in our current metastable atmospheric state.

Explaining what is meant by "spin" nicely brings together the electronic structures of nitrogen, oxygen, and G.N. Lewis. It requires some additional chemical theory, but it's worth it to explain exactly why we don't all just spontaneously ignite (see next post). Anybody want to second guess me in the comments? Here's a hint: watch this YouTube video starting here; then back it up and watch the whole thing.

P.S. That crazy-haired scientist is Prof. Martyn Poliakoff (I once had dinner at his house in Nottingham). Special thanks to Annie Gottlieb for first telling me about Martyn's series of chemistry videos.
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* "Preferentially" is meant in the thermodynamic sense. Making N₂ from nitride (or many reduced species is energetically favored whilst making O₂ from oxide (water) is energetically uphill. Plants do this feat by harvesting sunlight.