Thermodynamics and Chemistry

CHAPTER 11 REACTIONS AND OTHER CHEMICAL PROCESSES

11.4 ENTHALPIES OFSOLUTION ANDDILUTION 324

B(s) (A + B)(l)

sol

(a)

A(l) (A + B)(l)

dil

(b)

Figure 11.8 Two related processes in closed systems. A: solvent; B: solute. The

dashed rectangles represent the system boundaries.

(a) Solution process.

(b) Dilution process.

We can also find the effect of temperature on the molar differential reaction enthalpy
ÅrH. From Eq.11.3.5, we have.@ÅrH=@T /p;DÅrCp. Integration from temperature
T^0 to temperatureT^00 yields the relation

ÅrH.T^00 ; /DÅrH.T^0 ; /C

ZT 00

T^0

ÅrCp.T; /dT (11.3.11)

This relation is analogous to Eq.11.3.9, using molar differential reaction quantities in place
of integral reaction quantities.

11.4 Enthalpies of Solution and Dilution

The processes of solution (dissolution) and dilution are related. The IUPAC Green Book^8
recommends the abbreviations sol and dil for these processes.
During asolution process, a solute is transferred from a pure solute phase (solid, liquid,
or gas) to a solvent or solution phase. During adilution process, solvent is transferred
from a pure solvent phase to a solution phase. We may specify the advancement of these
two kinds of processes bysolanddil, respectively. Note that both processes take place
inclosedsystems that (at least initially) have two phases. The total amounts of solvent
and solute in the systems do not change, but the amounts in pure phases diminish as the
processes advance andsolordilincreases (Fig.11.8).
The equations in this section are about enthalpies of solution and dilution, but you
can replaceHby any other extensive state function to obtain relations for its solution and
dilution properties.

11.4.1 Molar enthalpy of solution

First let us consider a solution process in which solute is transferred from a pure solute
phase to a solution. Themolar differential enthalpy of solution,ÅsolH, is the rate of
change ofHwith the advancementsolat constantTandp, wheresolis the amount of

(^8) Ref. [ 36 ], Sec. 2.11.1.