ABSTRACT
The work done on an isolated system is equal to the change in its total energy. Molecules in vacuum or in a gas can sometimes be approximated as isolated systems but such a description is guaranteed to be wrong for molecules in solution or in the solid state. Nevertheless, in the preceding chapter we used ideas from statistical mechanics to modify the above statement of energy conservation and to predict that the (average) work f dx done when a molecule is stretched, mechanically, by a force f equals the change of its free energy dG = G(x + dx) − G(x). This enabled us to further establish the connection between the equilibrium distribution of the extension x and the restoring force generated by the stretched molecule.
