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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,A − Tfus,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,A/Δ1sh*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.
ΔTvap,A = (nB/mA)T*vap,A(RT*vap,A/Δ1gh*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.
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:
HA = RT2p −
1(dT/dp) − 1.
Values of dT/dp for p = 101 325 kPa are given for
many substances in Section
ΔlghA/(J·g − 1)
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.
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