two component systems limited to the mixtures two miscible liquids graphs include: 1.vapour pressure...
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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
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Ideal Solutions
• Shows linear relationship between vapour pressure at constant temp. and composition.
• Obeys Raoult’s Law
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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 + PB
o x B
• Read p.256 Example 22-1, 22-2 and TRY Check point 22-1.
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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.
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Examples of Ideal Solutions
• Propan-1-ol and propan-2-ol
• bromomethane and iodomethane
• hexane and heptane
Hydrogen bond
Dipole-dipole attraction
VDW forces
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Phase diagram for ideal solutionsVapour pressure vs mole fraction (w. constant temperature)
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Converting a vapour pressure/composition diagram (a) into a boiling point/composition diagram (b) for an ideal solution
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Boiling point vs mole fraction (w. constant pressure)
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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.
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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 intermolecular
attraction between < attraction between + attraction between
A and B A and A B and B
(in mixture) (in pure A) (in pure B)
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Example of non-ideal solution showing positive deviation
Dipole-dipole attraction VDW forces
Mixture of tetrachloromethane and ethanol
Intermolecular
attraction between
CCl4 molecules
Intermolecular
attraction between
CCl4 and C2H5OH
Intermolecular
attraction between
C2H5OH molecules
Hydrogen bond
< +
** Weakening of intermolecular attraction in mixture results in
1. volume expansion,
2. absorption of heat (i.e. temp. drop) when mixing.
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Phase Diagram for Positive Deviation
Weaker intermolecular attraction Easier for molecules to escape
Higher vapour pressure
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Weaker intermolecular attraction Lower boiling temperature
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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 intermolecular
attraction between > attraction between + attraction between
A and B A and A B and B
(in mixture) (in pure A) (in pure B)
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Example of non-ideal solution showing negative deviation
Hydrogen bond Dipole -dipole attraction
Mixture of trichloromethane and ethoxyethane
Intermolecular
attraction between
CHCl3 molecules
Intermolecular
attraction between
CHCl3 and C2H5OC2H5
Intermolecular
attraction between
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.
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Phase Diagram for Negative Deviation
Stronger intermolecular attraction More difficult for molecules to escape
Lower vapour pressure
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Stronger intermolecular attraction Higher boiling temperature