Two Component Systems

1. 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

2. Ideal Solutions • Shows linear relationship between vapour pressure at constant temp. and composition. • Obeys Raoult’s Law

3. Raoult’s Law • The partial vapour pressure of a component of a mixture is equal tothe 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 = PAox A partial pressure of B PB = PBox B Ptotal = PA + PB = PAox A + PBox B • Read p.256 Example 22-1, 22-2 and TRY Check point 22-1.

4. 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.

5. Examples of Ideal Solutions • Propan-1-ol and propan-2-ol • bromomethane and iodomethane • hexane and heptane Hydrogen bond Dipole-dipole attraction VDW forces

6. Phase diagram for ideal solutions Vapour pressure vs mole fraction (w. constant temperature)

7. Converting a vapour pressure/composition diagram (a)into a boiling point/composition diagram (b) for an ideal solution

8. Boiling point vs mole fraction (w. constant pressure)

9. 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.

10. 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)

11. Example of non-ideal solution showing positive deviation Mixture of tetrachloromethane and ethanol Intermolecular attraction between CCl4 and C2H5OH Intermolecular attraction between CCl4 molecules Intermolecular attraction between C2H5OH molecules < + Dipole-dipole attraction VDW forces Hydrogen bond ** Weakening of intermolecular attraction in mixture results in 1. volume expansion, 2. absorption of heat (i.e. temp. drop) when mixing.

12. Phase Diagram for Positive Deviation Weaker intermolecular attraction Easier for molecules to escape Higher vapour pressure

13. Weaker intermolecular attraction Lower boiling temperature

14. 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)

15. Example of non-ideal solution showing negative deviation Mixture of trichloromethane and ethoxyethane Intermolecular attraction between CHCl3 and C2H5OC2H5 Intermolecular attraction between CHCl3 molecules Intermolecular attraction between C2H5OC2H5 molecules > + Hydrogen bond Dipole -dipole attraction Dipole-dipole attraction ** Strengthening of intermolecular attraction in mixture results in 1. volume contraction, 2. evolution of heat (i.e. temp. rise) when mixing.

16. Phase Diagram for Negative Deviation Stronger intermolecular attraction More difficult for molecules to escape Lower vapour pressure

17. Stronger intermolecular attraction Higher boiling temperature