Burdge/Overby, Chemistry: Atoms First, 2e Ch14 | Page 10

SEC TION 14.3 ? Entropy Changes in a System

Ta b le 1 4 .3

Entropy Changes for the Dissolution
(?S° ) of Some Ionic Solids at 25°C
soln

Dissolution equation

579

Student Annotation: Note that the subscript “rxn” in ?S° changes to “soln” to refer
rxn
specifically to the solution process.

????S° (J/K ? mol)
soln

NH4NO3(s)

NH+(aq) + NO–(aq)
4
3

108.1

AlCl3(s)

Al3+(aq) + 3Cl–(aq)

–253.2

FeCl3(s)

Fe3+(aq) + 3Cl–(aq)

–266.1

ionic solid dissociates and is dispersed throughout the solution. In general, this is the case for
ionic solutes in which the charges on ions are small. (In the case of NH4NO3, they are +1 and
–1.) However, when ions are dispersed in water, they become hydrat