atkins’ physical chemistry eighth edition chapter 5 – lecture 2 simple mixtures copyright ©...
Post on 20-Dec-2015
548 views
TRANSCRIPT
Atkins’ Physical ChemistryEighth Edition
Chapter 5 – Lecture 2Simple Mixtures
Copyright © 2006 by Peter Atkins and Julio de Paula
Peter Atkins • Julio de Paula
The Chemical Potential of Liquids
• Ideal solutions
• Need to know how Gibbs energy varies with composition
• Recall that at equilibrium: μA (liq) = μA (vapor)
• Use * to designate pure substances
Fig 5.10 Eqilibrium between liquid and condensed phases
*A
oA
*A P ln RTμμ
AoAA P ln RTμμ
For pure substance A:
When B is added:
Combining :
*A
A*AA
P
P ln RTμμ
Raoult’s law:
*AAA PxP
A and Bboth
volatile
Fig 5.11 Ideal binary mixture
*AAA PxP
A*AA x ln RTμμ
Definition of idealsolution
*BBB PxP
Fig 5.12 Near-ideal mixture of benzene and toluene
Note:
and
are straight lines,
indicating a nearly
ideal solution:
P,Tmb
mb
XP
P,Tb
b
XP
A*
A X ln RTμ(liq)μA
*A
A*A P
Pln RTμ(liq)μ
A
which becomes:
Fig 5.13 Molecular basis of Raoult’s law for a volatilesolvent and volatile solute
solvent molecules
Fig 5.14 Strong deviations from Raoult’s law
Nonpolar
Polar
Notice that Raoult’s law
is obeyed increasingly
closely as the
component in excess
(solvent) approaches
purity
Henry’s Law: PB = XBKB
• 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
Ideal-dilute solutions
Raoult’s law: PA = XAPA*
where KB is an empirical
constant in units of P
Fig 5.14 Very dilute solution behavior
Henry’s law
BBBBB KmKxP
PA = XAPA*
PB = XBKB
Fig 5.16 Henry’s law description of solute molecules ina very dilute solution
• Solvent moleculesenvironment differs onlyslightly from that of pure solvent
• However, solutemolecules are inan entirely differentenvironment from that of the pure solute
solventsolvent
Fig 5.17 Experimental vapor pressures of a mixtureof acetone and chloroform
The Properties of Solutions
Liquid mixtures of ideal solutions
)x ln xx ln nR(xΔS BBAAmix
)x ln xx ln nRT(xΔG BBAAmix
ΔHmix = 0
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
Three types of interactions in the mixing process:
• solute-solute interaction• solvent-solvent interaction• solvent-solute interaction
Hmix = H1 + H2 + H3
ΔH1
ΔH2
ΔH3
Solutions
The enthalpy change of the overall process depends on H for each of these steps
Enthalpy changes accompanying solution processes:
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
Solutions
Factors Affecting Gibbs Energy of Mixing
Acetone is miscible in water
H2O
C6H14Hexane is immiscible in water
can hydrogen bond with water
ΔGmix < 0
ΔGmix > 0