Binary liquids

Physical chemistry

1. WELCOME

2. BINARY LIQUID SYSTEMS Topics covered Vapour pressure, Raoult’s law, Ideal and non ideal solutions, Solubility of liquids in liquids-Types Lever rule Prepared by Dr. Sree Remya T S Asst Professor S. N. College ,Kollam

3. VAPOUR PRESSURE At any given temperature for a particular solid or liquid, there is a pressure at which the vapor formed above the substance is in dynamic equilibrium with its liquid or solid form. This is the vapor pressure of the substance at that temperature. At equilibrium, the rate at which the solid or liquid evaporates is equal to the rate that the gas is condensing back to its original form. All solids and liquids have a vapor pressure, and this pressure is constant regardless of how much of the substance is present.

4. RAOULT’S LAW In the 1880s, French chemist François-Marie Raoult discovered that when a substance is dissolved in a solution, the vapor pressure of the solution will generally decrease. This observation depends on two variables: the mole fraction of the amount of dissolved solute present and the original vapor pressure (pure solvent). P solution = χ solvent P solvent Raoults law is applicable for dilute solutions only Ideal solutions obey Raoults law whereas real solutions does not

5. IDEAL SOLUTIONS Solutions which obey Raoults law at all concentrations and at all temperatures. If A and B form ideal solution, then in terms of interactions A-A~ B-B~A-B Also the solute-solute interaction and solvent-solvent interaction is almost similar to the solute-solvent interaction. ∆ H mixing =0 and ∆ V mixing =0 Eg : n- hexane and n -heptane Benzene-toluene

6. VAPOR PRESSURE-COMPOSITION GRAPH FOR IDEAL SOLUTIONS

7. Real or Non ideal solutions Deviate from Raoults law Three types Type 1- Show small positive deviations from Raoults law, eg : benzene-toluene Type 2- Show large positive deviations from Raoults law, eg : Water-propanol, water-ethanol Type 3- Show large negative deviations from Raoults law, eg:water-HNO3, water-H2SO4

8. Positive Deviation from Raoult’s Law Positive Deviation from Raoult’s Law occurs when the vapour pressure of component is greater than what is expected in Raoult’s Law. For Example , consider two components A and B to form non-ideal solutions. Let the vapour pressure, pure vapour pressure and mole fraction of component A be P A , P A and x A respectively and that of component B be P B , P B and x B respectively. These liquids will show positive deviation when Raoult’s Law when: P A > P A x A and P B > P B x B , as the total vapour pressure (P A x A + P B x B ) is greater than what it should be according to Raoult’s Law. The solute-solvent forces of attraction is weaker than solute-solute and solvent-solvent interaction that is, A – B < A – A or B – B The enthalpy of mixing is positive that is, Δ mix H > 0 because the heat absorbed to form new molecular interaction is less than the heat released on breaking of original molecular interaction The volume of mixing is positive that is, Δ mix V > 0 as the volume expands on dissolution of components A and B

9. Negative Deviation from Raoult’s Law Negative Deviation occurs when the total vapour pressure is less than what it should be according to Raoult’s Law. Considering the same A and B components to form a non-ideal solution, it will show negative deviation from Raoult’s Law only when: P A < P A x A and P B < P B x B as the total vapour pressure (P A x A + P B x B ) is less than what it should be with respect to Raoult’s Law The solute-solvent interaction is stronger than solute-solute and solvent-solvent interaction that is, A – B > A – A or B – B The enthalpy of mixing is negative that is, Δ mix H < 0 because more heat is released when new molecular interactions are formed The volume of mixing is negative that is,   Δ mix V < 0 as the volume decreases on dissolution of components A and B

10. Limitations of Raoult’s Law There are a few limitations to Raoult’s law. Raoult’s law is apt for describing ideal solutions . However, ideal solutions are hard to find and they are rare. Different chemical components have to be chemically identical equally. Since many of the liquids that are in the mixture do not have the same uniformity in terms of attractive forces, these type of solutions tends to deviate away from the law. There is either a negative or a positive deviation. The negative deviation occurs when the vapour pressure is lower than expected from Raoult’s law. An example of negative deviation is a mixture of chloroform and acetone or a solution of water and hydrochloric acid. Alternatively, positive deviation takes place when the cohesion between similar molecules is greater or that it exceeds adhesion between unlike or dissimilar molecules. Both components of the mixture can easily escape from the solution. An example of positive deviation includes the mixtures of benzene and methanol or ethanol and chloroform.

11. SOLUBILITY OF LIQUIDS IN LIQUIDS

12. ie ., 1-1 > 1-2

13. Cases of partial miscibility

14. The Lever Rule The composition and amount of material in each phase of a two phase liquid can be determined using the lever rule . This rule can be explained using the following diagram.      

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