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The Thermodynamic Sinks of this World

What would an elemental soup cook up to?

Roald Hoffmann

Two Limits I Want to Avoid

If the temperature is very high, the entropy term in ΔG will steer things, so that reactions in which the entropy is increased are favored. All solids will be spontaneously converted into gases. Yes, even solid NaCl (boiling point [b.p.] = 1,413 degrees Celsius) and NaOH (b.p. 1,388 degrees Celsius). All molecular gases will decompose into atoms, a reaction with a nice positive ΔS, as translational degrees of freedom are created. And some ions will be created (for example, H ↔ H+ + e-), depending on the temperature. From a chemist’s point of view, the surface or interior of a star (now that is high T!) is boring—there are no molecules there. But from a nuclear physicist’s point of view, these are the greatest fun.

The low temperature limit, T approaching absolute zero, poses different problems. For then the history of the model world really, really matters. Let me explain. At very low temperatures, the atoms are not moving quickly enough to overcome in their collisions any barriers (remember the ubiquitous activation energies introduced above?). So if you take that hydrogen balloon with its mixture of H2 and O2 down to close to absolute zero, you will have to wait a very, very long time, a time approaching infinity, to get any water. Here H2 and O2 are metastable relative to H2O, perfectly happy on their own. You can see what I mean by history—at low T what you get (in a human, finite observation time) depends on whether the reactant molecules were first heated to overcome activation barriers. Or if they were not, just allowed to cool.

The high vacuum interstellar medium poses another challenge. If a rare collision between two molecules or atoms that are prone to react were to take place—say H2 + O (atomic)—the reaction to H2O being highly exothermic (by 491 kJ/mol, gas phase) the product molecule is born with a large energy. In the absence of collisions, that energy will have nowhere to go, and the reaction, exothermic, will not happen.

2013-07MargHoffmannFB.jpgClick to Enlarge ImageThe surfaces of dust grains and cooled planetary objects present a special environment: high vacuum on one side, but an inert, or potentially catalytic, solid surface on the other. Low temperatures (unless there is volcanic activity)—but a long, long time. And sporadic influx of energy in the form of light. This is a wonderfully interesting set of conditions, important for the evolution of complex chemistry or life. But it isn’t quite my model world.

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