Wednesday, April 28, 2010

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.
* "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.


  1. How can you do a post on oxygen and not point out that if it didn't interact with hydrogen just right, ice cream would not be possible?

  2. Because that's your job here Jason. :)

  3. I detect a higher hand at work in this chemical behavior of oxygen. Ice cream, like beer, is evidence that God wants us to be happy.

  4. I think the active versus passive voice matters in that reductio.

  5. I think the active versus passive voice matters in that reductio.

    To oxidize is transitive and so can be used in the passive or active voice: Oxygen oxidizes iron or iron is oxidized by oxygen.

  6. Along the same lines is "Oxidation state" as applied to oxygen vs. say metals.

  7. Ritmo: I run into this problem all the time explaining what "oxidation" means to attorneys.
    The verb has a generic meaning of "to remove valence electrons from." Conceptually, that is quite clear, but our language still lacks a word other than "to oxidize". The situation is slightly better with the converse of oxidation i.e., to add valence electrons to, "to reduce" which derives from reducere "to lead back to."