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MARGINALIA

The Thermodynamic Sinks of this World

What would an elemental soup cook up to?

Roald Hoffmann

A Real Problem Lurking

I built my world as one of essentially limitless amounts of each element. If instead I began with equal and large finite masses, I would immediately run into the constraints of a limiting reagent. Consider for instance that simple water-forming reaction. If you have a finite amount (say 16 grams each) of H2 and O2, you will get at equilibrium 18 grams of H2O, with 14 grams of H2 left over and a miniscule amount of O2. Thirty-two grams we began with, 32 grams we have at the end—from Lavoisier’s time we knew that nothing is gained, nothing is lost. And let’s not worry about what mass/energy equivalence leads to in an exothermic reaction. There just wasn’t enough O2 there to react with the H2 supplied.

Here’s the nagging thought then about the utility of my Gedankenexperimentany real world must begin with pretty constrained if not fixed local concentrations of the elements. And so the outcome of all reactions, not just for the equal-mass example above, will be constrained by those initial conditions. We will come back to this.





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