The entropy change accompanying a phase transition

We can suspect that the entropy of a substance increases when it melts cuid when it vaporizes because its molecules become distributed in a more disorderly way is it changes from solid to liquid and from liquid to vapor. Likewise, we expect the unfolding of a protein from a compact, active three-dimensional conformation to a more flexible conformation, a process discussed in In the laboratory 1.1, to be accompanied by an increase of entropy because the secondary structure of the polypeptide chain is lost. 

Notice how we must use the enthalpy of fusion at the melting temperature and that this expression appHes only at the melting temperature. We get the standard entropy of fusion, AjuS, if the solid and liquid are both at 1 bar we use the melting temperature at 1 bar and the corresponding standard enthalpy of fusion at that temperature. All enthalpies of fusion are positive (melting is endothermic it requires heat), so all entropies of fusion are positive too disorder increases on melting. The entropy of water, for example, increases when it melts because the orderly structure of ice collapses as the liquid forms (Fig. 2.6). 

The protein lysozyme, sm enzyme that brejiks down bactericd cell walls, unfolds at a tTcuisition temperature of 75.5 C, md the stcmdcffd enthalpy of transition as determined using differential scanning calorimetry is +509 kj mol . It follows that 

At the molecular level, the positive entropy change can be explained by the dispersal of matter and energy that accompanies the unraveling of the compact three-dimensional structure of lysozyme into a long, flexible chain that can adopt many different conformations as it writhes about in solution. 

The entropy of other types of transition may be discussed similarly. Thus, the entropy of vaporization, AyapS, at the boiling temperature, Tt, of a liquid is related to its enthalpy of vaporization at that temperature by 

---