Phase Equilibrium Note

8/3/2019 Phase Equilibrium Note

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PHASE EQUILIBRIUM

Miscible liquids- 2 liquids dissolves completely to produce homogenous solution


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IDEAL SOLUTION = obeyRaoults Law

NON-IDEAL SOLUTION do not obey Raoults LawNegative deviation Positive deviation

The intermolecular forcesof attraction between A A,B B and A B (aftermixing) are equal

The intermolecular forces ofattraction between A B isstronger than B B and A A

The intermolecular forcesattraction between A B isweaker than B B and A

No change in volume aftermix A & B

Decreases in volume after mixing Increases in volume aftermixing

No enthalpy change onmixing( heat is neitherliberated nor absorbed)

Temperature increases aftermixing-Heat is liberated (the heat energy

released to form new bonds isgreater than the heat energyabsorbed to break the bonds inpure liquids)

Temperature decreases aftermixing-Heat is absorbed (the heat

energy released to form newbonds is less than the heatenergy absorbed to break thebonds in pure liquids)

Obey Raoults law :The partial vapourpressure of a component (A)in a solution at a giventemperature is equal to thevapour pressure of pure liquid(A) at the same temperature,multiplied by the mole

fraction of the component(A) in the solution.Or PA = PA XA .(solution)PT = PA+PB //// XA + XB = 1XA = PA / PT (gas)

Actual vapour pressure is lowerthan the predicted by RaoultsLaw

Actual vapour pressure ishigher than the predicted byRaoults Law

Decreases the tendency ofmolecules to escape from theliquid mixture

Increases the tendency ofmolecules to escape from theliquid mixture

Minimum vapour pressure Maximum vapour pressure

Produce azeotropic solution solution where the composition of thvapour is the same as the composition of the solution at constantboiling pointAzeotropic mixtures withmaximum boiling point

Azeotropic mixtures withminimum boiling point

Example : most liquids withsame functional group-ethane butane-ethene butane-benzene-methylbenzene-Ethanol-buthanol-water - methanol

HCl H2OPropanone-CHCl3HNO3 H2O

Ethanol-water

Ethanol-benzene

Benzene-methanol

Vapour pressure-compositiondiagram

NOTE** A is high in v.p , lowb.p, thus more

volatile



n -compos n -compos on Boiling point-compositiondia ram

100% A100% B100%100% B100% B0% A

b.p

B

Vp PA

Vp PB


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DISTILLATION :

1. Simple distillation for separation of solutions with a big differences in b.p

2. Fractional distillation for separation of solutions with small differences in b.p


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Miscible ideal solution(example : mixture of benzene-methylbenzene)

1. Benzene is more volatile thanmethylbenzene.

2. If 0.2 moles of benzene (0.8methylbenzene) is heated,itwill boil at T1 and producedmixture of C1 which is richerin benzene .

3. As vapour C1 pass the coolercolumn, it condenses andform a liquid of compositionC1.

4. As the beads graduallyheated,its hot vapour allow torevapourised the C1composition liquid

5. Repeating of condensationand vapourising increases thepercentage of benzene invapour.

6. First distillate is benzene(lower bp),methylbenzene will bethe residue// it will bedistilled at 110C(bp).

Miscible non-ideal solution Negative deviation (example : water -nitric acid)

Miscible non-ideal solution Positive deviation


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3. Steam distillation - to separate liquids with high bp which might decompose on heating

-mainly for two immiscible liquids with one of the component iswater.

- Steam distillation is used to extract fragrant oils from plants (makingperfume) and purify

organic compound

-To carry out this process, the liquid must be immiscible with water,have a relatively high RMM and

high vapour pressure(100C)

Mixture of two Immiscible liquids.

Immiscible liquid liquids do not dissolve in one another Two of the immiscible liquids (A & B) are mixed, it form two separate layers Lower density liquid top layer Higher density liquid bottom layer The intermolecular forces of attraction between their molecules are different Example: benzene and water, mercury and water, chlorobenzene and water,

nitrobenzene and water Water is a polar molecule Benzene, chlorobenzene and nitrobenzene are non-polar molecule with van der Waals

forces of attraction and mercury atoms are bonded to each other with metallic bond. PT = PA + PB where PT = Total pressure of the liquid mixture,

PA = Vapour pressure of pure liquid A,P

B= Vapour pressure of pure liquid B

Method of distillation and purification: Steam distillation and followed by separationprocess using separation funnel

mA / mB = PAMA / PBMB mA andmB = mass//percentageby mass of A and B respectively


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MA and MB = rmm A and Brespectively.

Exercises.

1999 Paper 2

2004

Paper 1


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2007 paper 2


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Distribution of a Solute between two solvents

1. The partition law a solute will distribute itself between two immiscible solvents suchthat the ratio of the concentration of the solute in both solvents is equal to a constant, at afixed temperature.

Solvent A Concentration of X in solvent A [g/dm3 or mol/dm3] = K

Concentration of X in solvent B [g/dm3 ormol/ dm3]

Solvent BK is a partition coefficient ..constant at fixed temperature and has

no unit.

2. The partition law can only be used under this condition:a) Solution must be dilutedb) The solute must exist in the same molecular state in both solvents.

-the solute can not undergo dissociation or association (usually formation of dimer in case of carboxylic acid) in one of the solvents.-e.g distribution between butanoic acid between water and methylbenzene

Benzoic acid will partially dissociate in water : CH3CH2CH2COOH + H2OCH3CH2CH2COO-- + H3O+

Benzoic acid will associate in methylbenzene : 2 CH3CH2CH2COOH (CH3CH2CH2COOH)2

(formation of dimer)

Or Benzene

Water

c) The temperature is constant

Solvent Extraction :

Apply partition law.

Organic compound- in aqueous layer

Ether extraction ---used ether asextracting solvent.

Advantage of ether extraction :

Ether almost immiscible with water

Most Nk

n

X

X

HI

H+

+ I-


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CHROMATOGRAPHY

- technique used to separate the components of a mixture by distributing the mixture betweentwo immiscible phases..stationary phase and mobile phase.

- 2 types of chromatography

1. adsorption chromatography -

Phase Equilibrium Note

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