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MARGINALIA

The Thermodynamic Sinks of this World

What would an elemental soup cook up to?

Roald Hoffmann

Metastable

Time for an example or two. Here are two possible equilibria in my model world, at room temperature:

Na (s) + ½ Cl2 (g) ↔ NaCl (s) ΔH = –411 kJ/mol
H2 (g) + ½ O2 (g) ↔ H2O (l) ΔH = –286 kJ/mol

The units of the enthalpy here and throughout this essay are kilojoules per mole (kJ/mol), and s, l and g stand for solid, liquid and gas, respectively. The source of the heats is the wonderfully useful (yet not infallible) NIST Chemistry WebBook, NIST Standard Reference Database Number 69, http://webbook.nist.gov/chemistry/. They are all rounded off to 1 kJ.

I have not chosen these reactions for nothing; they are examples of general salt (not just sodium chloride) and oxide formation, two of the avenues to great stability that we will encounter. Both reactions are highly exothermic. The second is a lecture demonstrator’s standby, the hydrogen balloon. As you have seen, nothing much happens … until a flame or a spark is brought up. Then the reaction goes, with a satisfying bang. For the sodium-chlorine reaction, to get it going one needs to melt the sodium, or put a drop of water on it.

Click to Enlarge ImageFor both reactions there is an activation energy, Ea, larger for the water formation. A schematic diagram of what is happening, energy-wise, in the course of the reaction is shown in the figure above. Here is the reason for specifying high temperatures in my Gedankenexperiment—I want to be able to overcome all barriers and just wind up with the most stable materials.




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