PHYSICAL CHEMISTRY IN BRIEF

CHAP. 6: THERMODYNAMICS OF HOMOGENEOUS MIXTURES [CONTENTS] 145

∆GE = ∆GM−RT

∑k

i=1

xilnxi, (6.34)

∆FE = ∆FM−RT

∑k

i=1

xilnxi. (6.35)

6.2.3 Heat of solution (integral)

The heat of solution is defined by the relation

∆solHi=

Q

ni

, [T, p], (6.36)

whereQis the heat exchanged with the surroundings, which accompanies the solution ofni
moles of a substanceiinnsolvmoles of a solvent at specified temperature and pressure.

Note: The heat of solution defined in this way is sometimes termed the integral heat of

solution in order to prevent confusion with the differential heat of solution of a component

[see6.3.5].

∆solHi depends on temperatureT, pressurep, and on the composition of the resulting
mixture.
Dependence of the integral heat of solution on composition can be expressed in two ways:
a) using a relative amount of solvent

nrel=

nsolv

ni

(6.37)

in which case the heat of solution ∆solHiis defined as the amount of heat accompanying

solution of 1 mole of a substance innrelmoles of a solvent;

b) using molalitymiof the dissolved substance in the resulting mixture.

Example

During the dissolution of 0.2 mol NH 3 (g) in 54.045 g H 2 O(`)at 25◦C and pressure 101.325 kPa,

it was found out that a heat of 6870 J passed to the surroundings. Calculate the heat of solution

of ammonia andnrel.