Prof. Mueller Chemistry 451 - Fall 2003 Lecture 19 - 1

Phase Transformations

Chapter 8 of Atkins: Sections 8.4- 8.6

Temperature- Composition DiagramsDistillation of MixturesAzeotropesImmiscible Liquids

Liquid- Liquid Phase DiagramsPhase SeparationsCritical Solution TemperaturesDistillation of Partially Miscible Liquids

Liquid- Solid Phase DiagramsEutecticsReacting SystemsIncongruent Melting

Prof. Mueller Chemistry 451 - Fall 2003 Lecture 19 - 2

Temperature Composition Diagrams

Temperature composition diagrams show the boundaries of compositions of phases at equilibrium at different temperatures at a given pressure (typically 1 atm) - liquid phase in lower part of diagram

Region between lines: F’ = 1, compositions ofphases are fixed at given temperatureRegion outside lines, F’ = 2, temperature andphase composition are variable

a1: pure phase liquid is heated and boils atT2, with composition a2= a1, vapor hasminuscule component a2’ (location of thetie line gives us the b.p.T2)a3: first bit of condensation drawn off froma distillation, richer in the more volatilecomponent, with vapor composition a3’a4: Vapor is drawn off, liquid condensesto this composition, very rich in volatilecomponent, and almost pure A is obtained

Boiling and condensation cycle is known as fractional distillation

Prof. Mueller Chemistry 451 - Fall 2003 Lecture 19 - 3

High-Boiling Azeotropes

Sometimes favorable interactions between molecules reduce vapor pressure of the mixture below the ideal value: i.e., A-B interactions stabilize the liquid phase – here G E= negative, more favorable for mixing

Examples: propanone / trichloromethane and nitric acid/water mixtures•Composition a heated to boiling (a2’ >a2)•Vapor (rich in A) is removed, and liquid left is richer in B, composition a3, vapor with a3’ •Vapor removed, composition shifts to a4,vapor composition at a4’•Composition of remaining liquid shifts to B asmore A is drawn off, and b.p. of liquid,vapor becomes richer in B•Finally, at point b the vapor of A has thesame composition as the liquid•Evaporation occurs without change incomposition, and the mixture is anazeotrope (boiling without changing):distillation cannot separate the components

Prof. Mueller Chemistry 451 - Fall 2003 Lecture 19 - 4

Low Boiling Azeotropes

The diagram below also shows azeotropic behavior, except that themixture is destabilized relative to the ideal solution (A-B interactions areunfavorable, G E is positive, less favorable to mixing)

Examples: dioxane/H2O & ethanol/H2O

• Start at a1, boil at point a2, with vapor composition a2’

• Vapor condenses to a3, vapor at a3’composition, condensing higher up the fractional condensation tube to give composition a4

• Azeotropic vapor comes out of thecolumn at b, but not beyond

Prof. Mueller Chemistry 451 - Fall 2003 Lecture 19 - 5

Immiscible Liquids

Distillation of two immiscible liquids A and B (example, octane and water) and at equilibrium, there is a tiny amount of A dissolved in B, and a tiny amount of B dissolved in A - liquids are saturated with the other component (in figure a)

The total vapor pressure is close top = p A* + p B*

If p = atmospheric pressure, then boilingcommences, and the dissolved substancesare purged from solution

Mixture is agitated, each component is keptsaturated in the other component, purgingcontinues until very dilute solutions arereplenished

Mixing is essential, as separatedcomponents (figure b) would not boilat the same temperature

Presence of saturated solution means that components boil at a lower temperature than they would alone - basis of steam distillation

Prof. Mueller Chemistry 451 - Fall 2003 Lecture 19 - 6

Liquid-Liquid Phase Diagrams

Partially miscible liquids are liquids that do not mix in all proportions at all temperaturesWhen P = 2, F’ = 1 (prime denotes constant pressure), the selection of temperature makes the compositions of the immiscible phases fixedWhen P = 1 (two liquids are fully mixed) both temperature and composition can be changed

1 Add small amt. of B to A at fixed T, itdissolves completely, single phase, P=1

2 Add more B to the point where B nolonger dissolves, P = 2, major phase is Asaturated with B, minor phase is Bsaturated with A (relative abundancesare given by the lever rule)

3 Add enough B to dissolve all of the A,and system has single phase, P = 1

4 Addition of more B dilutes the solution

Compositions of two phases vary withchanging temperature

21 3 4

xB

Prof. Mueller Chemistry 451 - Fall 2003 Lecture 19 - 7

Hexane and Nitrobenzene

Mixture of 50. g hexane (0.59 mol C6H14) and 50. g nitrobenzene (0.41 mol C6H5NO2) was prepared at 290 K.What are the compositions of the phases? To what temperature must the sample be heated to obtain a single phase?At 290 K, point xN = 0.41 occurs in a two phase region of the diagram, with the tie line cutting the boundary at xN = 0.35 and xN = 0.83 (these are the compositions of the two phases)

Ratio of amount of each phase:

There is 7 times as much hexane richphase as there is nitrobenzene-rich phase.

If the sample is heated to 292 K, we go intoa single phase region

nα

nβ

=lβlα=

0.83− 0.410.41− 0.35

=0.420.06

= 7

Prof. Mueller Chemistry 451 - Fall 2003 Lecture 19 - 8

Critical Solution Temperatures

The upper critical solution temperature , Tuc , is the highest temperature at which phase separation occursThis temperature exists since the thermal motion finally overcomes the potential energy advantage of certain molecules being close together

The Pd/H solid solutionsystem has a solution ofH2 in Pd and palladiumhydride up to 300o C,single phase at highertemperatures

Gibbs energy of mixingvaries with T - doubleminima indicate partiallymiscible phases, and asthe temperature rises,single minimum occurs atthe upper criticaltemperature

Prof. Mueller Chemistry 451 - Fall 2003 Lecture 19 - 9

Critical Solution Temperatures, 2

The lower critical solution temperature , Tlc, is the lowest temperature at which phase separation occurs

For triethylamine andwater, the system ispartially miscibleabove Tlc, and singlephase below

Some systems haveboth Tuc and Tlc, witha famous examplebeing nicotine inwater, where Tuc =210o C and Tlc = 61o C

Prof. Mueller Chemistry 451 - Fall 2003 Lecture 19 - 10

Liquid-Solid Phase Diagrams

Solid and liquid phases can be present below the boiling point (e.g., immiscible pair of metals right up to their melting points (As and Bi)

Two component liquid of composition a1(1) a1 → a2 System enters “Liquid+B”

pure solid B comes out ofsolution, remaining liquidricher in A

(2) a2 → a3 More solid B forms, equalamounts from lever rule,liquid even richer in A

(3) a3 → a4 Less liquid than at a3,composition given by e,liquid now freezes into atwo component system ofA and B

Composition at e is known as theeutectic composition (easily melted)

Prof. Mueller Chemistry 451 - Fall 2003 Lecture 19 - 11

Eutectics

Liquid with eutectic composition freezes at a single temperature, without depositing A or B in advance of the freezing pointSolid with eutectic composition melts, without any composition change, at the lowest temperature of any mixture

Solutions to the left of e deposit A asthey cool, to the right deposit BOnly the eutectic solidifies at a singletemperature (F’ = 0 when C = 2 and P =3), no other components unloaded

Examples:• Solder, 67% tin and 33% lead,

m.p. 183oC• 23% NaCl, 77% H2O m.p. -21.1oC; salt

added to ice on a road (isothermal)mixture melts at T > -21.1oC

Prof. Mueller Chemistry 451 - Fall 2003 Lecture 19 - 12

Eutectic formation and thermal analysis

Eutectic formation happens in many binary alloy systems, important in alloy microstructure - eutectic solids have two phases, but crystallizes in a homogeneous mixture of microcrystals (microscopy, X- rays, NMR)Thermal analysis useful for detecting eutectics

This type of analysis is conducted bycooling down an isopleth like the “a”isopleth from liquid to complete solid.

The place where the temperature remains constant over time is the eutectic halt

This technique helps to make a phasediagram of the material

Prof. Mueller Chemistry 451 - Fall 2003 Lecture 19 - 13

Reacting Systems

Many binary systems react to produce different compounds - one important example is the formation of GaAs (gallium arsenide) which is very important for the manufacture of III/V semiconductors:

Ga + As GaAs

System prepared with A and excess of Bconsists of C and unreacted B

The binary B,C system forms a eutectic

The important part of the phase diagramare the compositions of equal amounts ofA and B (x = 0.5) and pure B

Solid deposited along the cooling isopleth“a” is compound C

Below a4 there are two solid phases, withsome C and some B

Prof. Mueller Chemistry 451 - Fall 2003 Lecture 19 - 14

Incongruent MeltingSometimes component C is not stable as a liquid (e.g., alloy Na2K)

(1) a1 → a2 Some Na deposited, liquidricher in K

(2) a2 → a3 Just belowa3, solid sample,with solid Na and solid Na2K

(1) b1 → b2 No change until Na begins todeposit at b2

(2) b2 → b3 Solid Na deposits, butreaction happens to makeNa2K (K atoms diffuse into solid Na)

Here, liquid Na/K in eqb. with Na2K solid(3) b3 → b4 Amount of solid increases

until b4, liquid hits eutecticpoint, now two phase solid is formed

If solid is reheated, reverse happens, but no liquid Na2K is formed at any point: too unstable to exist as a liquid -incongruent melting is when a compound melts into its own components, without itself forming a liquid phase