Multicomponent Equilibrium Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 77

TABLE 3.11
Thermodynamic Functions for Transfer of Nonpolar
Molecules from theVapor Phase (A) to Water (W) at 298 K
Solute G i H i S i Δ (kJ/mol) Δ (kJ/mol) Δ (kJ/mol)

Methane 26.15 − 12.76 − 38.91

Ethane 25.22 − 16.65 − 41.89

Propane 26.02 − 23.85 − 49.87

n -Butane 26.52 − 25.10 − 51.62

Source: From Nemethy, G. and Scheraga, H.A. 1962. Journal of Chemical
Physics 36, 3401-3417.

slightly polar compounds with water is called the hydrophobic effect ; this property
is characterized by the activity coefficient of the solute in water or the octanol-water
partition constant of the solute.
The entropic contribution toward the free energy of solution increases as the solute
sizeincreases.Ithasbecomeevidentfromthedatathatalthoughforsmallermolecules
theentropiccontributionpredominates,forlargermoleculestheenthalpiccontribution
is equally important in making the excess free energy of solution positive.

E XAMPLE 3.12 E XCESS T HERMODYNAMIC F UNCTIONS OF S OLUTION OF L ARGE
H YDROPHOBIC M OLECULES IN W ATER

(a) The following data were obtained by Biggar and Riggs (1974) for the aqueous
solubility of a chlorinated insecticide, namely, heptachlor. It has a molecular weight of
373 and a melting point of 368 K.

Temperature t ( ◦ C) Solubility ( μ g/L)

15 100

25 180

35 315

45 490

The enthalpy of melting of heptachlor is 16.1 kJ/mol. Calculate the excess functions for
solution of heptachlor in water at 298 K.
Since heptachlor is a solid at 298 K, we need to properly account for the enthalpy of
melting of the solid. The equation for solid solubility given above should be used. The
data required are x i and 1 /T .

x i

1 / T /( 1 / K )

10 − 9 0.00347 4.82 ×

8.68 × 10 − 9 0.00335

10 − 8 0.00324 1.52 ×

2.36 × 10 − 8

0.00314

continued