13. How To Measure Disorder   Previous PageNext Page

B. The entropy increases when a solid or liquid is dissolved in water (below). This can be illustrated by liquid methanol and crystalline ammonium chloride.

The entropy of ammonium chloride in solution is the sum of the entropies of the hydrated ions; this is based on a convention that assigns an entropy of zero to the hydrated H+ ion.

Notice that the entropy changes that result from dissolving suggest that ammonium chloride solution is much more disordered than crystalline NH4Cl, whereas the difference in disorder between liquid methanol and methanol in aqueous solution is small.

This is explained by the break-up of a regular array of NH4+ and Cl- ions in the crystal lattice when NH4Cl dissolves, whereas liquid methanol has no such regular structure.



  Page 19 of 45 HomeGlossary