1. Peter Atkins • Julio de Paula Atkins’ Physical Chemistry
Eighth Edition
Chapter 5 – Lecture 2
Simple Mixtures
Copyright © 2006 by Peter Atkins and Julio de Paula

2. The Chemical Potential of Liquids
Need to know how Gibbs energy varies with composition
Recall that at equilibrium: μA (liq) = μA (vapor)
Ideal solutions
Use * to designate pure substances

3. Fig 5.10 Eqilibrium between liquid and condensed phases
For pure substance A:
When B is added:
A and B
both
volatile
Combining :
Raoult’s law:

4. Fig 5.11 Ideal binary mixture
Definition of ideal
solution

5. Fig 5.12 Near-ideal mixture of benzene and toluene
Note:
and
are straight lines,
indicating a nearly
ideal solution:
which becomes:

6. Fig 5.13 Molecular basis of Raoult’s law for a volatile
solvent and volatile solute
solvent molecules

7. Fig 5.14 Strong deviations from Raoult’s law
Notice that Raoult’s law
is obeyed increasingly
closely as the
component in excess
(solvent) approaches
purity
Nonpolar
Polar

8. Ideal-dilute solutions
In ideal solutions, both solvent and solute
obey Raoult’s law.
In real solutions at low concentrations:
Solute = PB ∝ Solute
Called an ideal-dilute solution
Raoult’s law: PA = XAPA*
Henry’s Law: PB = XBKB
where KB is an empirical
constant in units of P

9. Fig 5.14 Very dilute solution behavior
Henry’s law
PB = XBKB
PA = XAPA*

10. Fig 5.16 Henry’s law description of solute molecules in
a very dilute solution
Solvent molecules
environment differs only
slightly from that of pure
solvent
However, solute
molecules are in
an entirely different
environment from that of
the pure solute
solvent

11. Fig 5.17 Experimental vapor pressures of a mixture
of acetone and chloroform

12. The Properties of Solutions
Liquid mixtures of ideal solutions
ΔHmix = 0

13. The Properties of Solutions
Liquid mixtures of real solutions
Recall that ΔG = ΔH - TΔS
Therefore: ΔGmix < 0 or ΔGmix > 0
Depends on relative magnitudes of ΔHmix and ΔSmix and T

14. Three types of interactions in the mixing process:
solute-solute interaction
solvent-solvent interaction
solvent-solute interaction
ΔH1
ΔH2
DHmix = DH1 + DH2 + DH3
ΔH3

15. Enthalpy changes accompanying solution processes:
The enthalpy change of the overall process depends on H for each of these steps
Enthalpy changes accompanying solution processes:

16. Enthalpy is only part of the picture
Increasing the disorder or randomness of a system tends to lower the energy of the system
Solutions favored by increase in entropy that accompanies mixing

17. Factors Affecting Gibbs Energy of Mixing
can hydrogen bond with water
Acetone is miscible in water
Figure UN
Title:
Polar liquids tend to dissolve readily in polar solvents.
Caption:
Water is both polar and able to form hydrogen bonds (Section 11.2). Thus, polar molecules, especially those that can form hydrogen bonds with water molecules, tend to be soluble in water. Acetone is a polar molecule with the structural formula shown in the figure and mixes in all proportions with water. Acetone has a strongly polar C=O bond and pairs of nonbonding electrons on the O atom that can form hydrogen bonds with water.
Notes:
Keywords:
ΔGmix < 0
Hexane is immiscible in water
C6H14
ΔGmix > 0
H2O