Phase equilibrium IITwo component systems

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Phase equilibrium IITwo component systems
Is there any relation between the composition of
the liquid mixture and the vapour pressure that
produced?
Vapour mixture of component A and component B
Liquid mixture of component A and component B
2
Phase equilibrium IITwo component systems

• Objectives v.p. vapour pressure of the system
• Find out the factor affecting vapour pressure
• Able to use Raoults laws to calculate v.p.
• Able to draw vapour composition curve and
  boiling point composition curve of an ideal
  system.
• Predict how the positive deviation from Raoults
  law happen.

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Phase equilibrium IIfactors affecting vapour
pressure

• Two component system
• Vapour pressure of a mixture of A, B depend on
• Intermolecular force between A and B.
• Composition of the mixture, XA and Xb.

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Phase equilibrium IIcomposition and b.p. curve
Two components (A and B)system
Ideal solution/intermolecular force between A and
B is similar
98
69
b.p. of pure A 69 oC, b.p. of pure B 98 oC
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Phase equilibrium II use Raoults laws to
calculate v.p.
PA XA PAo
v.p. of A is directly proportional to the mole
fraction of A (in liquid) and v.p. of pure
component
Ptotal PA PB XAPAo XBPBo
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Phase equilibrium II Concept of ideal solution
and positive deviation from Raoults law
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22.2 Raoults Law for Ideal Solutions (SB p.263)
Ideal Solution
The tendency of the molecules of A and B in the
mixture to change from liquid phase to the vapour
phase is almost equal to that in pure A and pure
B.
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Phase equilibrium II Concept of ideal solution
and positive deviation from Raoults law
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Phase equilibrium II positive deviation from
Raoults law

• Objectives
• Point out the character of positive deviation.
• Able to give e.g. for the positive deviation.
• Able to deduce the situation of negative
  deviation.
• Understand the boiling point composition for the
  fractional distillation.

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Phase equilibrium II the character of positive
deviation

• Characters
• broken interaction between molecules.
• raise in vapour pressure and lower in boiling
  point.
• absorbing heat from surrounding.
• expansion in volume.

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Phase equilibrium II the characters of negative
deviation

• Characters
• forming interaction between molecules.
• lower in vapour pressure and raises in boiling
  point.
• releases heat from surrounding.
• reduces in volume.

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Phase equilibrium II the boiling point
composition for the fractional distillation.

• Characters
• A is more volatile and X gt X
• Vapour gets richer in more volatile component.
• A found in final distillate while B found in
  final residue.

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Phase equilibrium II the boiling point
composition for the fractional distillation.
(2)
(4)
(1)
(3)
A (more volatile one) found in final distillate
while B found in final residue
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Phase equilibrium II the boiling point
composition for the fractional distillation.

• Summary for fractional distillation
• Vapour curve (vapour composition curve) is above
  the liquid curve (liquid composition curve) in
  b.p. composition curve.
• By the process of successive condensation and
  boiling, the vapour of pure component A (which is
  more volatile) if finally obtained.
• The residual liquid becomes gradually richer in B
  (which is less volatile).

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• Highlights
• After 3 distillation condensation process,
  liquid composition of A increases from X to X
  .
• In residual liquid, XB increases from x to 1.0
  gradually.
• ????XB ???1.0 ???

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Phase equilibrium II Azetropic Mixtures

• Mixture of Negative deviation
• Maximum boiling point mixture obtained.
• Distillate can be pure A (start from Y) or pure
  B(start from x).
• Residual liquid is an azeotropic mixture.

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Phase equilibrium II Azetropic Mixtures

• Mixture of Positive deviation
• Minimum boiling point mixture obtained.
• Residual liquid can be pure A (start from x) or
  pure B (start from Y).
• Distillate is an azeotropic mixture.

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Phase equilibrium II Public exam Questions Q.3
(92 chem 1)
3.ai) Vapour pressure is higher than that predict
from Raoults law. Intermolecular forces
between molecules in mixture are less than that
between molecules in pure components. 3.aii) P is
vapour pressure of pure A. Q is vapour pressure
of pure B. 3.aiii) Interaction between molecules
in mixture is same as that between molecules in
pure component. Ptotal PA PB XAPAo
XBPBo
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Phase equilibrium II Public exam Questions Q.5
(93 chem 2)
5ai) Ptotal PA PB XAPAo
XBPBo Ptotal 0.6 (24) 0.4 (12)
19.2kPa 5aii) XA PA / Ptotal 0.6 (24)/19.2
0.75 XB PB / Ptotal 0.4(12) /19.2
0.25 PA XA Pao 0.75(24) 18 kPa PB
0.25(12) 3 kPa and Ptotal 21 kPa XA
PA / Ptotal 18 / 21 0.857 The composition
of the first droplet of second distillate is
85.7 mol by A and 14.3 mol by B.