phase equilibria ( total recall )

Phase Equilibria ( Total Recall )

All prepared by [email protected]

( A ) Raoult’s law & azeotropic mixture

( A ) Raoult’s law

( i ) Definition [ STPM 1999 ] ( ii ) Formulae / Calculations

The vapour pressure , PA of a mixture of miscible liquids is given by the product of the vapour pressure of that pure component Po

A and its mole fraction . PA = Po

A × XA

PA = PoA × XA / PB = PoB × XB

PT = PA + PB

XA = 𝒏(𝑨)

𝒏 𝑨 + 𝒏 (𝑩) / XB =

𝒏(𝑩)

𝒏 𝑨 + 𝒏 (𝑩)

XA + XB = 1

( B ) Azeotropic mixture

( i ) Definition [ STPM 2007 ]

A mixture with a constant boiling point and has a fixed composition which cannot be separated by distillation .

( ii ) Limitations on the separation of two components forming an azeotropic mixture

~ Complete separation cannot be done .

( iii ) How to obtain pure ethanol from an azeotropic mixture with 96% of ethanol ??

~ Use anhydrous calcium chloride ( CaCl2 – dehydrating agent )



( B ) Ideal mixture

( i ) Properties

Attractive forces : same strong

Vapour pressure : same as estimated by Raoult’s law

Volume of solution : exactly same as the total volume of liquids

Enthalpy change : zero ( thermally neutral ) [ heat released during bond formation = heat absorbed during bond breaking ]

Temperature of solution : remain unchanged

( ii ) Examples

Mixture of liquids Type of intermolecular forces

1. Benzene & methylbenzene ( toluene ) [ STPM 1999 ]

van der Waals forces

2. Heptane & octane [ STPM 2007 S ] / hexane & heptane


3. Methanol & water

Hydrogen bonds

4. Propan-1-ol & propan-2-ol / butan-1-ol & butan-2-ol

Hydrogen bonds



( iii ) Diagram [ Benzene & methylbenzene ]

(a) Vapour pressure-composition diagram ( STPM 1999 ) (b) Boiling point-composition diagram ( Pahang 2011 )



( C ) Negative deviation

( i ) Properties

Attractive forces : ( solution ) stronger than( pure liquids )

Vapour pressure : less than estimated by Raoult’s law

Volume of solution : less than the total volume of liquids

Enthalpy change : exothermic ( negative ) [ heat released during bond formation > heat absorbed during bond breaking ]

Temperature of solution : increase

( ii ) Examples


1. Nitric acid and water ( STPM 2006 )

Pure liquid Solution

Nitric acid : Permanent dipole( van der Waals forces )

Water : hydrogen bonds

Electrostatic force / Ionic bond

2. Chloroform ( trichloromethane ) and acetone ( propanone )

Chloroform : Permanent dipole( van der Waals forces )

Acetone : Permanent dipole( van der Waals forces )

Hydrogen bonds

3. Acetone and tribromomethane ( bromoform )

Bromoform : Permanent dipole( van der Waals forces )

Acetone : Permanent dipole( van der Waals forces )

Hydrogen bonds

4. Methanoic acid ( formic acid ) and water

Methanoic acid : hydrogen bonds

Water : hydrogen bonds

Ion-dipole attractions



( iii ) Diagram [ Nitric (V) acid and water ]

(a) Vapour pressure-composition diagram ( STPM 2007 S ) (b) Boiling point-composition diagram ( STPM 2006 E )



( D ) Positive deviation

( i ) Properties

Attractive forces : ( solution ) weaker than ( pure liquids )

Vapour pressure : more than estimated by Raoult’s law

Volume of solution : more than the total volume of liquids

Enthalpy change : endothermic ( positive ) [ heat released during bond formation < heat absorbed during bond breaking ]

Temperature of solution : decrease

( ii ) Examples


1. Ethanol and water ( STPM 2009 ) Pure liquid Solution

Ethanol : hydrogen bonds ( weaker )

Water : hydrogen bonds ( strong )

Hydrogen bonds ( weakest )

2. Propan-1-ol and water

Propan-1-ol : hydrogen bonds ( weaker )

Water : hydrogen bonds ( strong )

Hydrogen bonds

3. Butan-1-ol and methylbenzene

Butan-1-ol : hydrogen bonds Methylbenzene : van der Waals

forces


4. Chloroform ( trichloromethane ) and ethanol

Chloroform : van der Waals forces

Ethanol : hydrogen bonds

Hydrogen bonds ( weak )



( iii ) Diagram [ ethanol and water ]

(a) Vapour pressure-composition diagram (b) Boiling point-composition diagram ( STPM 2004 E / 2009 S )



( E ) Fractional distillation

( i ) Principle ~ Answering skill * Refer full notes !!!

Mixture with C1 composition will be heated to boil at T1 temperature .

This will produces a vapour with C2 composition with more ( ) .

The vapour will be cooled in the column and forms liquid .

The liquid with C2 composition will be reheated to form a vapour at T2 temperature with C3 composition which is riched in ( ) .

Boiling , cooling and condensation process will be repeated .

( ) will be obtained as distillate and ( ) will be the residue .

( ii ) For ideal mixture

Distillate : ( pure liquid ) ~ a lower boiling point / higher vapour pressure Residue : ( pure liquid ) ~ a higher boiling point / lower vapour pressure

( iii ) For negative deviation

Distillate : ( pure liquid ) ~ a lower boiling point / higher vapour pressure

Residue : ( azeotropic mixture ) ~ a higher boiling point / lower vapour pressure * Must write the composition !!!

( iv ) For positive deviation

Distillate : ( azeotropic mixture ) ~ a higher boiling point / lower vapour pressure * Must write the composition !!!

Residue : ( pure liquid ) ~ a lower boiling point / higher vapour pressure



( F ) Fractional distillation under reduced pressure

( i ) Advantages

The fractional distillation can be carried out at lower temperature .

So that , the organic substance which are not stable at high temperature can be extracted .

*Extra !!!

At reduced pressure , the boiling point of organic substance will be lowered .

Thus , the organic substance can be extracted without any decomposition .

( ii ) Disadvantages

At reduced pressure , apparatus that are strong and can withstand low pressure have to be used .

This will increases the cost of extraction .

phase equilibria ( total recall )

Documents