Phase Equilibria ( STPM )

ST A PM lex T C a he n m is try

Phase Equilibria ( STPM 2011 ~ 2014 )

Phase

Equilibria

Content

1. STPM 2011 Objective question 2. STPM 2012 Objective question 3. STPM 2013 Objective question 4. STPM 2014 Objective question

* There is no any structured or essay questions between STPM 2011 & STPM 2014 .

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Phase Equilibria ( STPM 2011 ~ 2014 )

ST A PM lex T C a he n m is try

STPM 2011 OBJ

Answer : D

Explanation :

~ The curve is linear : mixture of A & B forms ideal solution 

Enthalpy change : zero ( thermally neutral )

~ A : lower vapour pressure → higher boiling point → residual liquid ~ B : higher vapour pressure →lower boiling point → distillate ~ Methanol is more volatile than water 

Thus , A is water while B is methanol .

The higher the volatility of liquids , the lower the vapour pressure of the liquid

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Phase Equilibria ( STPM 2011 ~ 2014 )

ST A PM lex T C a he n m is try

STPM 2012

Answer : B

Explanation :

~ The curve has maximum boiling point ( for azeotrope ) → negative deviation ~ During fractional distillation with composition G :  

Pure J : lower boiling point → distillate Azeotropic mixture , F : higher boiling point → residual liquid

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Phase Equilibria ( STPM 2011 ~ 2014 )

ST A PM lex T C a he n m is try

STPM 2013 Sem 1

Answer : D

Explanation :

~ The curve has minimum boiling point ( for azeotrope ) → positive deviation

~ Attractive force in solution ( X-Y ) is weaker than pure liquids ( X-X / Y-Y )

~At composition Q , azeotropic mixture will be formed and cannot be separated into pure liquids X and Y . ~ Mixture of ethanol and methanol : ideal solution ~ During fractional distillation ( composition P ) :  

Q ( azeotrope ) : lower boiling point → distillate Pure X : higher boiling point → residual liquid

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Phase Equilibria ( STPM 2011 ~ 2014 )

ST A PM lex T C a he n m is try

STPM 2014 Sem 1

Answer : B

Explanation :

~ Boiling point of azetrope : lower than pure liquids → minimum boiling point ~ Minimum boiling point → positive deviation

~ Ethanol and methylbenzene ( STPM 2014 Sem 1 )

Pure ethanol

Type of intermolecular forces

Hydrogen bonds

Pure methylbenzene ( toluene ) van der Waals forces

Solution of toluene and ethanol van der Waals forces

~ During fractional distillation ( 20% ethanol ) :  

Azeotrope : lower boiling point → distillate ( 68% ethanol , 32% toluene ) Pure toluene : higher boiling point → residual liquid

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