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 + PB

o 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

• bromomethane and iodomethane

• hexane and heptane

Hydrogen bond

Dipole-dipole attraction

VDW forces

Phase diagram for ideal solutionsVapour 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 intermolecular

attraction between < attraction between + attraction between

A and B A and A B and B

(in mixture) (in pure A) (in pure B)

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.

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 intermolecular

attraction between > attraction between + attraction between

A and B A and A B and B

(in mixture) (in pure A) (in pure B)

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.

Phase Diagram for Negative Deviation

Stronger intermolecular attraction More difficult for molecules to escape

Lower vapour pressure

Stronger intermolecular attraction Higher boiling temperature