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MARGINALIA

The Thermodynamic Sinks of this World

What would an elemental soup cook up to?

Roald Hoffmann

Thermodynamics, Light

That those gleaming brass 19th-century steam engines of the Midlands could not deliver energy at more than about 10 percent efficiency was not understandable until heat and work and energy and entropy were precisely defined. So it is with stability. There are three sorts of what is colloquially called “energy”—energy (E), enthalpy (H = E + PV), and what used to be called Gibbs free energy and now is termed Gibbs energy (G = E + PV – TS), where P, V and T are pressure, temperature and volume, respectively, and S is the entropy. (Skipped here are some fine points of which thermodynamic variables are kept constant.)

The Gibbs energy change (ΔG) in a reaction determines the position of the equilibrium in a reaction

A + B ↔ C + D,

that is, whether there is a lot of reactant A + B, or product, C + D. If ΔG is negative, product dominates; if ΔG is positive, reactant is present in greater amount. We also need the words exothermic and endothermic—these refer to heat changes and can be related to negative ΔH (exothermic, heat released in a reaction as written, left to right) or positive ΔH (endothermic, heat absorbed in a reaction). At low temperatures, as T > 0 kelvin, exothermic reactions go to the right, endothermic ones to the left.




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