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Chapter: 3 Chemistry
    Section: 3.10 Chemical thermodynamics
        SubSection: 3.10.4 Cryoscopic and ebullioscopic constants and enthalpies of fusion and of evaporation of some common solvents

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3.10.4    Crysocopic and ebullioscopic constants and enthalpies of fusion and of evaporation of some common solvents

The lowering ΔTfus,A = |T*fus,ATfus,A| of the freezing temperature T*fus,A of a mass mA of the solvent substance A by a dissolved amount nB of solute substance B is given by the formula:

         ΔTfus,A = (nB/mA)T*fus,A(RT*fus,A1sh*A) = (nB/mA)kfus,A,

where R denotes the gas constant (R = 8.314 51 J·K−1· mol−1), Δ1sh*A denotes the massic (formerly ‘specific’) enthalpy of fusion of the pure (*) solvent A, and kfus,A is called the cryoscopic constant of A, and where it has been assumed that the solution is ideal-dilute and that the solid phase is that of the pure solvent.
    Similarly, the corresponding elevation ΔTvap,A = |Tvap,AT*vap,A| of the boiling temperature at a given pressure is given by the formula:

         ΔTvap,A = (nB/mA)T*vap,A(RT*vap,A1gh*A) = (nB/mA)kvap,A,

where Δ1gh*A is now the massic enthalpy of evaporation of the pure solvent A and kvap,A is called the ebullioscopic constant of A, and where it has been assumed that the solution is ideal-dilute and that the solute B is involatile.

A

kfus/(K·kg·mol1)

kvap/(K·kg·mol1)

Δls h*A/(J·g1)

Δgl h*A/(J·g1)

CH3CO2H

             3.90

3.07

195

  405

CH3COCH3

             2.40

1.71

  98

  524

C6H5NH2

             5.87

3.22

  88

  460

C6H6

             5.12

2.53

126

  394

C10H16O (camphor)

      40

  

  45

  

CS2

           3.8

2.37

  58

  351

CCl4

      30

4.95

  16

  195

CHCl3 

             4.90

3.66

  80

  246

c-C6Hl2

         20.1

2.79

  32

  356

(C2H5)2O

             1.79

1.82

  97

  358

C10H8 (naphthalene)

             6.94

5.8  

152

  316

C6H5NO2

             6.90

5.26

  94

  331

C6H5OH

             7.27

3.04

122

  485

c-C5H5N

             4.75

  

  94

  460

H2O

             1.86

0.51

 333

 2257

 

 

 

 

 

Enthalpies of evaporation at several temperatures

Approximate values of the molar enthalpy of evaporation can be computed from tables of vapour pressure against temperature (see section 3.4.4) by the use of Clapeyron's equation. For conditions where the molar volume of the liquid is small compared with the molar volume of the vapour and where the vapour can be treated as an ideal gas, the molar enthalpy of evaporation Δ1g HA can be calculated from the expression:

         Δ1g HA = RT2p1(dT/dp)1.

Values of dT/dp for p = 101 325 kPa are given for many substances in Section 3.4.4.
    The following table gives values for the massic enthalpy of evaporation, Δ1g hA/(J·g−1), for several substances A at several temperatures. Some values at the normal boiling temperatures are given in the preceding table.

ΔlghA/(J·g1)

T/K

273.15

293.15

313.15

333.15

353.15

393.15

413.15

A

 

 

 

 

 

 

 

CH3COCH3

585

563

541

519

498

458

438

CS2

375

367

357

345

331

300

286

CCl4

214

208

206

191

171

163

CHCl3

280

273

265

257

248

229

219

(C2H5)2O

393

385

374

361

344

342

277

SO2

384

352

 

 

 

 

 

 

 

 

 

 

 

 

 


The following highly accurate massic enthalpies of evaporation of water were calculated, with temperatures T90 on the International Temperature Scale of 1990 (ITS-90), from those measured by Osborne, Stimson, and Ginnings (1939) J. Res. Nat. Bur. Stand., 23, 197, 261.

T90/K

ΔlghA/(J·g 1)

T90/K

ΔlghA/(J·g 1)

T90/K

ΔlghA/(J·g 1)

273.150

2500.58

303.143

2429.95

365.126

2282.72

278.149

2488.87

313.140

2406.14

373.124

2256.57

283.148

2477.14

323.137

2382.10

403.118

2174.17

288.147

2465.39

333.134

2357.82

423.114

2114.39

293.145

2453.59

343.132

2333.20

443.112

2049.60

298.144

2441.78

353.129

2308.19

473.110

1940.56

 

 

 

 

 

  

M.L. McGlashan

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