Nils Walter: Chem 260

Non-ideal solutionsStrong deviations from ideality are shown by dissimilar substances Raoult’s law

obeyed for a close-to-pure

solvent

William Henry(1775-1836)

observedHenry’s law (fora dilute solute):

pB = xBKB (e.g., gas solubility)

Nils Walter: Chem 260

Ideal and real solutions: ActivitiesFrom both Raoult’s (solvent) and

Henry’s laws (solute) follows:O

][ln)(ln)()(

solvCRTlxRTll

solv

solvsolvsolv

+=+=

µµµ

O

]ln[JRTJJ += µµ O⇒⇒⇒⇒

standard chemical potential @ 1 M

The chemical potential is a measure of the ability of J to bring about physical or

chemical change

BUT:

JJJ aRT ln+= µµ O

⇓⇓⇓⇓to

preserve equation for real

solutions:

Effective concentration = activity aJ = γγγγJ[J]

Nils Walter: Chem 260

Consequences of chemical potential changes in mixtures: Colligative properties

Boiling point elevation:∆∆∆∆TB = KBbB

Freezing point depression:∆∆∆∆Tf = KfbB

cryoscopic constantmolalitySolute is insoluble

in solid solvent:

Chemical Potential

lowered by solute

ebullioscopic constant

Solute is not volatile:

Chemical Potential

lowered by solute

Nils Walter: Chem 260

Phase diagrams of binary mixturesPhase rule: F = C - P + 2

for binary mixtures = 2p,T

p = constant:Temperature-composition diagram

for binary mixture of volatile liquids

in equilibrium

Nils Walter: Chem 260

Finally, as promised: Whisky distillery

Fractional distillation:High-boiling azeotrope,

e.g., nitric acid/water

Non-ideal mixtures

Low-boiling azeotrope, e.g., ethanol/water

Nils Walter: Chem 260

Liquid-liquid phase diagrams of partially miscible liquids

E.g., hexane/nitrobenzene:Upper critical

solution temperature

(thermal motion)

Lever rule

"'

'"

ll

ancompositioofphaseofAmountancompositioofphaseofAmount =

E.g., triethylamine/water:

Lower Tcrit(complex formed)