Sunday, November 4, 2007

More on Spontaneity/Equilibrium, Making Diamonds

During the last lecture of Week 7, we completed "remixing" the Second Law in terms of more convenient system variables. Before we might have said that, for spontaneous processes, the entropy of the universe tends towards a maximum until it reaches zero, at which point we have reached equilibrium. Now we can look entirely at system functions and say that the gibbs energy G of the system tends towards a minimum when T, P are constant (with the same argument applying for the helmholtz energy A when T,V are constant).

Calculating rxnG and rxnA for chemical reactions is straightforward if we remember that Hess' Law works for all extensive thermodynamic properties. Typically, however, only fH, fG and S are tabled in appendices, but the other thermodynamic functions can readily be calculated from these.

As an example showing how to use one of the eight fundamental relations, we looked at the Superman problem: How much pressure is necessary to convert graphite to diamond? Using the equation (∂∆G/∂P)T=∆V and densities of graphite and diamond, we calculated 14.8 kbar, a pressure trivially attained by the Last Son of Krypton.

Rather than mining diamonds (which has political and human costs), we can now make synthetic diamonds. What a great topic for, say, undergraduate seminar.

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