Pres sure-volume work

For chemical purposes, the internal energy (8.71) must include chemical work terms, one for each of the c independent chemical components participating in active equilibria. Together with the usual extensities XL for heat (S) and pres sure-volume work (V), the arguments of the internal energy function must be extended to include c additional chemical extensities (such as the mole numbers n2,. .., nc, with conjugate chemical potentials ... 

In the sodium-water reaction, the internal energy changes by —368.6 kJ because heat evolves and changes by —2.5 kJ because pres sure-volume work is done. The total change of internal energy is... 

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