Page 61 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:36 9in x 6in b985-ch06 Elementary Physical Chemistry

46 Elementary Physical Chemistry

The subscripts vap, liq, subl, and fus refer respectively to vapor, liquid,
sublimation and fusion. Equation (6.5b) is also known as the Claussius–
Clapeyron equation.

Note: ∆H subl > ∆H vap and, in general, V liq >V sol. (H 2 O is an excep-
tion.)

Problem: Explain why, in general,

(dP/dT ) liq−gas < (dP/dT ) sol−liq and (dP/dT ) sol−liq > 0.

6.2. Chemical Equilibrium. Mixtures
There are three ways to characterize mole fractions:

(1) Molar concentration, [x]= n x /V solution (x is the solute) (6.8)
(2) Molal concentration,m x = n x/M solvent (usually mole/kg of (6.9)
solvent)
(3) Mole fraction,x A /n (n = total number of moles) (6.10)

6.2.1. Ideal Solutions. Raoult’s Law
Let us adopt the convention that the solvent will be denoted as A and the
solute as B.
The solvent of all substances, when sufficiently dilute, obeys Raoult’s
Law,thatis
P A = x A P A ∗ (6.11)

∗
where P is the vapor pressure of pure A.
A
The reason for this behavior can be rationalized that in very dilute
solutions, a molecule A is surrounded essentially by other A molecules,
but at a concentration of x A . Hence, P A = x A P .
∗
A
An Ideal Solution is a solution in which every component obeys Raoult’s
Law over the entire range of compositions.