11. Phase Equil

Chapter 11Chapter 11

Type of solution Example

Liquid in liquid Water & ethanolBenzene & toluene

Solid in liquid Sodium chloride aqIodine in CCl4

Gas in liquid Aqueous CO2

Gas in gas Mixture of O2 & N2

Solid in solid Alloys

Mixtures of Liquids2 types of liquid mixture2 types of liquid mixture

Mixture of miscible liquids

•Dissolve completely in each other in all proportions

•A homogeneous solution.

•eg.: Ethanol & water

Immiscible liquids

•2 layers are formed when the 2 liquids are mixed together.

•eg. benzene & water

A mixture of miscible liquids

A mixture of immiscible liquids

11.111.1 Mixtures of Mixtures of miscible liquidsmiscible liquidsA homogeneous mixtureA homogeneous mixture

Liquid mixtureLiquid mixture

Ideal solution Non-Ideal solution

Negative deviation Positive deviation

11.211.2 Ideal Ideal solutionsolution

Ideal SolutionsIn an ideal solutionideal solution that contains A & B, the intermolecular forces of attractionintermolecular forces of attraction between A & A, between B & B and between A & B are equal/ similar.

• When an ideal solution is formed, there is:

no change in volume on mixing A with B

no enthalpy change on mixing (heat is neither liberated nor absorbed)

Substance A Substance B

(A ... A) is similar to (B ... B)

Ideal Solution:A + B

(A ... A) and (B ... B) are similar to (A ... B)

Examples of ideal solution:

1. Mixture of hexane and heptane– Both are alkanes and non-polar.– Forces between hexane molecules

Ξ Forces between heptane moleculesΞ Forces between hexane and heptane

moleculesΞ Dispersion forces

2. Mixture of water and methanol– Forces between water molecules

Ξ Forces between methanol moleculesΞ Forces between water and methanol moleculesΞ Hydrogen bonding

11.3 Vapour Pressure

Use gas equation:Use gas equation:PV = nRTPV = nRT

Vapour pressure

Vapour

Liquid

When the liquid is in equilibrium with vapour,

Liquid Vapourthe pressure of the vapour is SATURATED VAPOUR PRESSURE.

• When the temperature is increased, the vapour pressure ___________

• Liquids that have strong intermolecular forces have _______ boiling point, and _______ vapour pressure.

• Liquids that have weak intermolecular forces have _______ boiling point, and _______ vapour pressure.

• When the temperature is increased, the vapour pressure increases.

• Liquids that have strong intermolecular forces have higher boiling point, and lower vapour pressure.

• Liquids that have weak intermolecular forces have lower boiling point, and higher vapour pressure Volatile.

11.6 Vapour Pressure of an Ideal

Liquid mixture & Raoult’s Law

Vapour Pressure of an Ideal Solution

the tendency of A to escape from the pure liquid A

=

the tendency of B to escape from the pure liquid B

=

• In an ideal solution that contains A & B, the tendency of A to escape from the ideal solution

the tendency of B to escape from the ideal solution

• This is because the intermolecular intermolecular forces of attractionforces of attraction in the ideal solution are the same as in the pure liquids.

• However, in an ideal solution, the [A] [A] is less than in the pure liquidis less than in the pure liquid.

• As a result, partial vapour pressure partial vapour pressure of Aof A on the surface of an ideal liquid an ideal liquid mixture mixture is less than the vapour less than the vapour pressure of pure Apressure of pure A.

B too!

Pure Solvent A in aclosed container

Solution of Solvent A andsolute B

P(A) is lower than P°(A)

We’re able to proof this fact…

But, how ???

Raoult’s Law• The vapour that exists in equilibrium

with a mixture of 2 miscible liquids is a mixture of 2 vapoursmixture of 2 vapours.

• The total pressuretotal pressure of the vapour above the liquid mixture is the sum of sum of the partial pressures of the 2 vapoursthe partial pressures of the 2 vapours.

• The vapour above the liquid mixture & the partial pressures of the component vapours vary with vary with temperature & the temperature & the composition of the solutioncomposition of the solution.

• Raoult’s Law states that the partial vapour pressure of each component in a solution at a given temperature is equal to the mole fraction of the component in the liquid mixture multiplied by the vapour pressure of pure liquid at the same temperature.

PA = PoA XA

where PA = partial pressure of A in the ideal solution Po

A = vapour pressure of pure A XA = mole fraction of A in liquid mixture

• Total vapour pressure of the solution, PT = PA + PB

and XA + XB = 1 ...... in liquid mixture refer example pg.171refer example pg.171

• A solution/mixture that obeys obeys Raoult’s LawRaoult’s Law is known as ideal ideal liquid mixtureliquid mixture.

11.7 The composition of

Vapour above the Ideal Liquid mixture

Dalton’s Law (vapour mixture)

Total

AvapourA P

P X

,mixture vapour the in A of fraction Mole

Refer Example 6 on Page 172:

The composition of vapour above the ideal

solution• Raoult’s law is used to calculate calculate

the mole fractionthe mole fraction of the components in the ideal liquid ideal liquid mixturemixture.

• Dalton’s lawDalton’s law is used to calculate calculate the mole fractionthe mole fraction of the components in the vapourvapour.

Vapour pressure-Composition graph

• Refer Table 11.2• Then Figure 11.8

100

80

60

40

20

0 0.2 0.4 0.6 0.8 1.0Mole fraction of benzene

Vap

our p

ress

ure

(kP

a)

PTotalPBenzene

PMethylbenzene

100

80

60

40

20

0 0.2 0.4 0.6 0.8 1.0Mole fraction of benzene

Vap

our p

ress

ure

(kP

a)

PTotal

P Benzen

e

PMethylbenzene

• Benzene has a higher vapour pressure (100 kPa) than methylbenzene.

• Hence, benzene is the more volatile component & has lower boiling point.

Vapour Pressure-Composition Diagram

A

B

Liquid composition curve (x = composition in liquid mixture;

Vapour Pressure-Composition Diagram

A

B

Vapour composition curve Coordinate-x = Composition in vapour mixture

For example…

11.811.8 Vapour Vapour pressure and pressure and TemperatureTemperature

Vapour pressure increases with Vapour pressure increases with temperaturetemperature

Effect of Temperature on the Vapour Pressure Phase

Diagram• In the next slide, I’m going to

show you a plot showing the variation of total vapour pressure variation of total vapour pressure against temperatureagainst temperature for liquids A & B in an ideal solution.

• When the temperature temperature increasesincreases, PPAA & P & PBB in the liquid mixture increaseincrease.

• The figure following shows the variation of variation of Total vapour pressureTotal vapour pressure for a liquid for a liquid mixturemixture A & B A & B at 4 increasing temperatures.

• T1<T2<T3<T4

• For a liquid mixture with composition X4, PT increases from P1 atm at T1 to 1 atm at T4 (T4 > T1)

11.10

• When the vapour pressure of a liquid reaches reaches atmospheric pressureatmospheric pressure (1 atm), the liquid boilsliquid boils.

• In the figure, x1 boils at T1, x2 boils at T2, x3 boils at T3 & x4 boils at T4.

• This shows that pure A has a higher vapour pure A has a higher vapour pressurepressure than pure B.

• Pure A is more volatilePure A is more volatile than pure B.• Pure B has a higher boiling pointPure B has a higher boiling point than pure A.• As XA decreases & XB increases, boiling point boiling point

of the mixture also increasesof the mixture also increases.

• By using a phase diagram, the variation in variation in boiling point against compositionboiling point against composition can be plotted.

11.11

• By using a phase diagram, the variation in variation in boiling point against compositionboiling point against composition can be plotted.

11.11

Vapour

Liquid

T°A

T°B

Boiling point composition curve

•T°A < T°B •A is more volatile•Vapour composition is richer in A.

Boiling Point-Composition Diagrams for Ideal Solutions

• The VP-composition curvesVP-composition curves are important when consider the conditions for consider the conditions for separating liquidsseparating liquids in a liquid mixture through distillation.

• So, VP-composition curve is converted to converted to a boiling point-composition curvea boiling point-composition curve at constant pressure.

•A high vapour pressurehigh vapour pressure corresponds to a low boiling pointlow boiling point & vice versa.

• Both of the curves have the same shapessame shapes except that they are reversedreversed.

• Notice that liquid curveliquid curve in VP-composition curve is not a straight lineis not a straight line because

the mixture deviates slightly from ideal deviates slightly from ideal behaviourbehaviour.

DistillationDistillationDistillation is a laboratory technique used for Distillation is a laboratory technique used for separating and purifying liquids by exploiting separating and purifying liquids by exploiting the boiling points of the components in the the boiling points of the components in the

mixturemixture

11.911.9 Simple Simple distillation of an distillation of an

Ideal mixtureIdeal mixture

•SuitableSuitable to separate liquids that differ liquids that differ widely in their boiling pointswidely in their boiling points.

•Not effectiveNot effective to separate ideal solution.

• For example, the simple distillation of benzene-methylbenzene mixture.

• Pure benzene can be separated from a mixture of methylbenzene & benzene through successive distillation.

• If a liquid mixture contains 50% benzene is heated, the mixture will boil at T1 (C).

• Vapour of composition a% benzene is obtained at D (Richer in benzene the more volatile component).

• Xbenzene in the vapour at D is higher than 0.5.

• Vapour D condensed to give liquid mixture (E), the composition of the liquid obtained is the same as its vapour.

• Thus, the first portion formed does not consist of pure benzene, but merely a mixture richer in benzene.

• If liquid E is collected & redistilled, it will boil at T2 to produce a distillate (G) with composition b (yet richer in benzene).

• If the process is repeated over & over again, the final distillate is pure benzene.

11.10 Fractional 11.10 Fractional distillationdistillation

Fractional Distillation of an Ideal Solution

• The most efficient methodmost efficient method to separate ideal solution.

• Principle: The process of vaporisation and condensation takes place continuously in a fractionating column

• The longer the better.

• Give more time for:Vapor liquid to be established.

• Especially important when mixture A & B have very close TB.

Insulated To prevent heat

loss

• Provide large contact surface (v + l) so that:Vapor liquid

is established for better separation

• Smaller beads preferred.

To measure TB of distillate (A or B)

A + B

Slowly enough to allow the system to continuously re-equilibrate (Liquid & vapour composition changes, and new equil (l + v) is established.

Ascending column, 1. Temperature (TB)

decreased2. Composition of

vapour becomes richer in the more volatile component

DistillateThe more volatile component is distilled over first, followed by the less volatile component.

Going UP the fractionating column

• If a liquid with composition C1 is heated, the liquid will boil at T1.

• Vapour w1 (richer in A, vapour composition C2) is produced.

Vapour

Liquid

1

• The vapour, w1 (now richer in A) rises up the column.

• Vapour w1 is condensed on the glass bead.

• The liquid formed will have Liquid composition C2.

Vapour

Liquid

2

• This liquid (C2) is heated by the ascending vapour; it will boil at T2 to produce a vapour w2 (even richer in A).

3

• When the vapour (w2) condensed, the liquid with composition C3 is produced.

4

• By repeating the process, pure A (more volatile component) is distilled over, followed by pure B.

5

Simple distillation Fractional distillation

Advantages

• simpler setup • faster distillation

times • consumes less energy

• much better separation

• can more readily purify complex mixtures

Dis-advantag

es

• requires the liquids to have large boiling point differences (>70oC)

• gives poorer separation

• only works well with relatively pure liquids

• more complicated setup than simple distillation

• takes longer for liquids to distill

• consumes more energy than simple distillation

Best used for:

separating relatively pure liquids with large boiling differences or liquids with solid impurities

separating complex mixtures of liquids with smaller boiling point separations.

• Page 181: Example 13

11.1111.11 DEVIATION DEVIATION FROM RAOULT’S FROM RAOULT’S

LAWLAW

Miscible-LiquidsLiquid mixture

Ideal solution Non-Ideal solution

Negative deviation Positive deviation

Negative deviation from Raoult’s law

• A mixture that gives vapour pressure less than ideal is said to show negative deviation from Raoult’s Law.

• (A … B) is stronger than (A … A) and (B … B)

• The mixing process is exothermic• The volume of the mixture decreases

• For example: 1. Water + HCl

• HCl … HCl = Van der Waals forces• H2O … H2O = Hydrogen bonds• HCl … H2O = Ion-dipole interaction

forces (stronger than Van der Waals and hydrogen bonds)

E.g: Other Mixtures that exhibits negative

deviation

•CHCl3 + CH3COCH3

•HNO3 + H2O

•H2SO4 + H2O

Formation of Hydrogen bonding in mixtures

Example 2:• When CHCl3 is mixed with propanone,

heat is liberatedheat is liberated because H bonds are H bonds are formedformed between the molecules.

Negative Deviation & Vapour Pressure

Curves

• Liquid composition curve has a minimum.

Maximum Boiling Point Mixture• The VP-composition diagram for nitric acid &

water mixture shows a min. in the VP curvea min. in the VP curve.• This min. corresponds with a max. in the a max. in the

boiling point-composition diagram.boiling point-composition diagram.

• At M, the HNO3-H2O system has the minimum VP & the max. boiling point.

• At this point, both the liquid & vapour in equi have same composition.

• This mixture is known as azeotropic mixture/ constant boiling mixture/ azeotrope.

Azeotropic mixtureMixture of 2 liquids that boils at that

fixed composition.

• Also known as “Azeotrope”• Composition is constant• i.e.: Composition read from the x-

coordinate of maximum boiling point• Both components will boil at same

temperature.

Azeotropic mixture

Examples of maximum boiling points azeotropes:

• Table 11.3• Example:

– Fractional distillation of water-nitric (V) acid mixture at different composition.

Fractional distillation of dilute nitric(V) acid

• HNO3-H2O mixture has max. boiling point max. boiling point at 121at 121°°C when the mixture contains 68.2% C when the mixture contains 68.2% of HNOof HNO33.

• When a solution contains less than 68.2% HNO3 (C2) is subjected to fractional distillation, boiling occurs at T2.

• The first vapour is richer in water.

• The vapour condenses to a liquid of the same composition (which is richer in water).

121

• The continuous vaporisxn & condensxn ascending the fractionating column results in vapour/ liquid that is even richer in water.

• Water is distilled over first.

121

•As distillation continues, the remaining mixture becomes richer in HNO3 until the composition of the mixture = Azeotrope (68.2% HNO3)

•The azeotrope will boil constantly at 121 °C.

•Hence, the azeotrope is collected as the second distillate at 121 °C.

121

Fractional distillation of concentrated nitric(V) acid

• When a solution contains more than 68.2% HNO3 (C1)is subject to fractional distillation, boiling begins at T1, to give vapour (richer in acid).

• The vapour condenses to a liquid of the same composition (which is richer in acid).

• The continuous vaporisxn & condensxn ascending the fractionating column results in vapour/ liquid that is even richer in acid.

• Nitric (V) acid is distilled over first.

• The remaining mixture becomes richer in water until the composition = Azeotrope (68.2% HNO3)

• The azeotrope will boil constantly at 121 °C.

• Hence, the azeotrope is collected as the second distillate at 121 °C.

Fractional distillation of hydrochloric acid

• The liquid mixture contains 20.2% HCl (P) is called azeotropic mixture.

• Acid solution with [ ] less than 20.2% HClAcid solution with [ ] less than 20.2% HCl will yield pure waterpure water followed by azeotropic mixture.

• Conversely, acid solution with [ ] greater acid solution with [ ] greater than 20.2% HClthan 20.2% HCl will yield pure HClpure HCl followed by azeotropic mixture.

• Take note,1. On heating an azeotropic mixture until it

boils, the vapour liberatedvapour liberated has the same same composition as the original liquid mixturecomposition as the original liquid mixture.

2. Azeotropic mixture cannot be separated cannot be separated through through fractional distillationfractional distillation.

Positive Deviation from Raoult’s Law

• A mixture of miscible liquids that gives a vapour pressure much more than that of an ideal mixture.

• What causes positive deviation from Raoult’s Law?

• The higher than ideal vapour pressure shows that molecules in the non-ideal solution have greater tendency to escapegreater tendency to escape than from their respective pure liquid.– because (A ... B) are weakerweaker than (A ...

A) & (B ... B).• When the solution is prepared,

Heat is absorbedabsorbed (endothermic reaction)

Volume increasesincreases slightly

Vapour Pressure Curve (has a max. vp)Slight positive deviation

Severe positive deviation

0 Mole Fraction of B 1.0

Boiling point-Composition diagram has a minimum boiling point (Azeotrope)

Boiling point (°C)

Minimum boiling point T°B

T°A

Examples:• Page 189: Table 11.5

• A mixture which shows slight slight positive deviationpositive deviation (cyclohexane-tetrachloromethane) can be can be separated by fractional distillationseparated by fractional distillation.

• In contrast, mixture which shows gross positive deviationgross positive deviation (benzene-ethanol, ethanol-water) cannot be cannot be separated completely by fractional separated completely by fractional distillationdistillation.

Fractional Distillation of Benzene-Ethanol Mixture

• When the 2 liquids are mixed, There is a slight increase in volume Heat is absorbed

• The absorption of heat energyabsorption of heat energy shows that some of the intermolecular forces between some of the intermolecular forces between moleculesmolecules have been broken.

• Addition of benzene to ethanol decreases decreases the degree of H bondingthe degree of H bonding between ethanol molecules.

–As a result, tendency of the tendency of the components to escapecomponents to escape increases.

Examples

• If a solution contains less than 32.4% ethanol less than 32.4% ethanol (C(C11)) is subjected to fractional distillation,

it will boil at T1 the vapour liberated will have composition X1

(higher percentage of ethanolhigher percentage of ethanol than original mixture).

[ethanol] in the residual liquid decreases.• In the fractionating column, the vapour

undergoes condensation & vaporisationcondensation & vaporisation & finally an azeotropic mixture is obtained.

• As distillation continues, boiling point of the boiling point of the residual liquid increasesresidual liquid increases until temperature reaches 80oC (boiling point of benzene).

• At this temperature, entire solution consists of pure benzene.

• If a solution contains more than 32.4% more than 32.4% ethanolethanol (C2) is subjected to fractional distillation,

it boils at T2, the vapour escapes has the composition X2

(richer in benzene).• The more volatile componentmore volatile component (azeotropic

mixture) with lower boiling point is distilled with lower boiling point is distilled over firstover first.

• As distillation proceeds, the [ethanol] in the residual liquid increases.

• Finally, temperature of residual liquid reaches 78.5oC (boiling point of pure

ethanol) & pure ethanol distills overpure ethanol distills over.

Summary• When subjected to fractional distillation,Solution which contains less than 32.4% less than 32.4%

ethanolethanol can be separated into azeotropic mixture, followed by pure benzene.

Solution which contains more than 32.4% more than 32.4% ethanolethanol can be separated into azeotropic mixture, followed by pure ethanol.

Methods to separate 2 components in Azeotropic

Mixturea)a) Using a third componentUsing a third componentb)b) Using a chemical methodUsing a chemical method Calcium oxide (CaO) can be used to

absorb water from the liquid mixturec)c) Using an adsorbentUsing an adsorbent Charcoal or silica gel can be used to

remove 1 component from AMd)d) Using solvent extractionUsing solvent extraction

A component from AM can be extracted using a suitable solvent.

FRACTIONAL FRACTIONAL DISTILLATION UNDER DISTILLATION UNDER REDUCED PRESSUREREDUCED PRESSURE

- PRESSURE VS BOILING POINT- PRESSURE VS BOILING POINT- ADVANTAGE- ADVANTAGE

- DISADVANTAGE- DISADVANTAGE

Fractional Distillation under Reduced Pressure

• Uses: distil out substances that have high boiling points.

• A substance boilssubstance boils when its VP reaches VP reaches the external pressurethe external pressure.

• A reduction in reduction in pressurepressure will enable the distillation to be carried out at lower temperature.

• For organic liquids which decompose decompose at temperatures below their boiling at temperatures below their boiling point at atmospheric pressurepoint at atmospheric pressure, it is undesirable to distil such liquids at their normal boiling points.

• It is necessary to distil the liquid at reduced pressurereduced pressure in order to lower lower the the boiling pointboiling point.

• To do this, a vacuum pump is connected to the side arm of the collection flask.

• This method is called vacuum distillation.

Advantages of distillation under reduced pressure

• Organic liquids which decompose at decompose at temperatures below their boiling temperatures below their boiling point at atmospheric pressurepoint at atmospheric pressure can be distilled at temperature lower than their normal boiling points.

• Less heat is required.

DISadvantages• The apparatus must be strong

enough to withstand breakage under reduced pressure.

• No cracks on the distilling flask• The liquid boils irregularly• Impurities with low boiling point

may also be distilled.

FINISH