Two Component Systems
• Limited to the mixtures two miscible
liquids
• Graphs include:
1.
Vapour pressure vs composition of mixture
2.
Boiling point/temp. vs composition of mixture
• Intermolecular interaction between the
two components
Ideal Solutions
• Shows linear relationship between vapour
pressure at constant temp. and composition.
• Obeys Raoult’s Law
Raoult’s Law
• The partial vapour pressure of a component
of a mixture is equal to the vapour pressure
of the pure component multiplied by its
mole fraction in the mixture.
• For a two-component mixture of A and B,
partial pressure of A PA = PAo x A
partial pressure of B PB = PBo x B
Ptotal = PA + PB = PAo x A + PBo x B
• Read p.256 Example 22-1, 22-2 and TRY
Check point 22-1.
Molecular Interaction in Ideal Solutions
• Intermolecular
Intermolecular
Intermolecular
attraction between attraction between attraction between
A and B
 A and A
 B and B
(in mixture)
(in pure A)
(in pure B)
 escape tendency of molecule A or B in the mixture
equals to their respective escape tendency in pure A
or pure B.
**
There would be no volume change and no
enthalpy change when A and B are mixed to
form an ideal solution.
Examples of Ideal Solutions
• Propan-1-ol and propan-2-ol
Hydrogen bond
• bromomethane and iodomethane
Dipole-dipole attraction
• hexane and heptane
VDW forces
Phase diagram for ideal solutions
Vapour pressure vs mole fraction (w. constant temperature)
Converting a vapour pressure/composition diagram (a) into a
boiling point/composition diagram (b) for an ideal solution
Boiling point vs mole fraction (w. constant pressure)
Deviations from Raoult’s Law
• Many liquid mixtures are not ideal
solution.
• Liquid mixtures do not obey Raoult’s
Law (or they deviate from Raoult’s
Law), are known as non-ideal solutions.
• Deviations from Raoult’s Law can be
positive or negative.
Positive Deviation from Raoult’s Law
* Vapour pressure of a liquid mixture is greater than
that predicted by Raoult’s Law,
i.e. PA > PAo x A and PB > PBo x B
*
intermolecular
intermolecular
attraction between < attraction between
A and B
A and A
(in mixture)
(in pure A)
intermolecular
+
attraction between
B and B
(in pure B)
Example of non-ideal solution showing
positive deviation
Mixture of tetrachloromethane and ethanol
Intermolecular
Intermolecular
Intermolecular
attraction between
attraction between
attraction between
CCl4 and C2H5OH < CCl4 molecules
Dipole-dipole attraction
VDW forces
+ C H OH molecules
2 5
Hydrogen bond
** Weakening of intermolecular attraction in mixture results in
1. volume expansion,
2. absorption of heat (i.e. temp. drop) when mixing.
Phase Diagram for Positive Deviation
Weaker intermolecular attraction
Easier for molecules to escape
Higher vapour pressure
Weaker intermolecular attraction
Lower boiling temperature
Negative Deviation from Raoult’s Law
* Vapour pressure of a liquid mixture is smaller than
that predicted by Raoult’s Law,
i.e. PA < PAo x A and PB < PBo x B
*
intermolecular
intermolecular
attraction between > attraction between
A and B
A and A
(in mixture)
(in pure A)
intermolecular
+
attraction between
B and B
(in pure B)
Example of non-ideal solution showing
negative deviation
Mixture of trichloromethane and ethoxyethane
Intermolecular
Intermolecular
Intermolecular
attraction between
attraction between
attraction between
CHCl3 and C2H5OC2H5
>
Hydrogen bond
CHCl3 molecules
Dipole -dipole attraction
+
C2H5OC2H5 molecules
Dipole-dipole attraction
** Strengthening of intermolecular attraction in mixture results in
1. volume contraction,
2. evolution of heat (i.e. temp. rise) when mixing.
Phase Diagram for Negative Deviation
Stronger intermolecular attraction
More difficult for molecules to escape
Lower vapour pressure
Stronger intermolecular attraction
Higher boiling temperature