Thermodynamics and Chemistry

CHAPTER 12 EQUILIBRIUM CONDITIONS IN MULTICOMPONENT SYSTEMS

PROBLEMS 417

conventionsÅfH.HC, aq/D 0 andSm.HC, aq/D 0 (Secs.11.3.2and11.8.4). (Your calcu-

lated values will be close to, but not exactly the same as, those listed in AppendixH, which are

based on the same data combined with data of other workers.)

For the reaction^12 H 2 .g/C^12 Cl 2 .g/!HCl.g/, the standard molar enthalpy of reaction

at298:15K measured in a flow calorimeter^21 isÅrHD92:312kJ mol^1.

The standard molar entropy of gaseous HCl at298:15K calculated from spectroscopic

data isSmD186:902J K^1 mol^1.

From five calorimetric runs,^22 the average experimental value of the standard molar en-

thalpy of solution of gaseous HCl at298:15K isÅsol,BHD74:84kJ mol^1.

From vapor pressure measurements of concentrated aqueous HCl solutions,^23 the value

of the ratiofB=am;Bfor gaseous HCl in equilibrium with aqueous HCl at298:15K is

5:032 10 ^7 bar.

12.19The solubility of crystalline AgCl in ultrapure water has been determined from the electrical
conductivity of the saturated solution.^24 The average of five measurements at298:15K issBD
1:337 10 ^5 mol dm^3. The density of water at this temperature isAD0:9970kg dm^3.
(a)From these data and the Debye–Huckel limiting law, calculate the solubility product ̈ Ks
of AgCl at298:15K.
(b)Evaluate the standard molar Gibbs energy of formation of aqueous AgCion at298:15K,
using the results of part (a) and the valuesÅfG.Cl, aq/D 131:22kJ mol^1 and
ÅfG.AgCl, s/D109:77kJ mol^1 from AppendixH.

12.20The following reaction was carried out in an adiabatic solution calorimeter by Wagman and
Kilday:^25

AgNO 3 (s)CKCl(aq,mBD0:101mol kg^1 )!AgCl(s)CKNO 3 (aq)

The reaction can be assumed to go to completion, and the amount of KCl was in slight excess,

so the amount of AgCl formed was equal to the initial amount of AgNO 3. After correction

for the enthalpies of diluting the solutes in the initial and final solutions to infinite dilution,

the standard molar reaction enthalpy at298:15K was found to beÅrHD43:042kJ mol^1.

The same workers used solution calorimetry to obtain the molar enthalpy of solution at infinite

dilution of crystalline AgNO 3 at298:15K:Åsol,BH^1 D22:727kJ mol^1.

(a)Show that the difference of these two values is the standard molar reaction enthalpy for

the precipitation reaction

AgC.aq/CCl.aq/!AgCl.s/

and evaluate this quantity.

(b)Evaluate the standard molar enthalpy of formation of aqueous AgCion at298:15K,

using the results of part (a) and the valuesÅfH.Cl, aq/ D 167:08kJ mol^1 and

ÅfH.AgCl, s/D 127:01kJ mol^1 from AppendixH. (These values come from cal-

culations similar to those in Probs. 12. 18 and 14. 4 .) The calculated value will be close to,

but not exactly the same as, the value listed in AppendixH, which is based on the same

data combined with data of other workers.

(^21) Ref. [ 146 ]. (^22) Ref. [ 72 ]. (^23) Ref. [ 138 ]. (^24) Ref. [ 67 ]. (^25) Ref. [ 164 ].