Lec20 - Chapter Outline 1. Spontaneity of the Dissolution...

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LMALaput Chemistry 16 Lecture 20 Chapter Outline 1. Spontaneity of the Dissolution Process 2. Dissolution 3. Molality and Mole Fraction 4. Henry’s Law 5. Vant hoff factor and dissociation of electrolytes 6. Colligative properties 7. Lowering of Vapor Pressure and Raoult’s Law 8. Boiling Point elevation 9. Freezing Point Depression 10. Osmotic pressure 11. Determination of Molecular weight of solute using colligative properties THE DISSOLUTION PROCESS •Solutions are homogeneous mixtures of two or more substances. –Dissolving medium is called the solvent . –Dissolved species are called the solute . •There are three states of matter (solid, liquid, and gas) which when mixed two at a time gives nine different kinds of mixtures. –Seven of the possibilities can be homogeneous. –Two of the possibilities must be heterogeneous TWO HETEROGENOUS POSSIBILITIES Solute Solvent Example Solid Gas dust in air Liquid Gas clouds, fog SPOTANEITY OF THE DISSOLUTION •As an example of dissolution, let’s assume that the solvent is a liquid. •Two major factors affect dissolution of solutes 1. Change of energy content or enthalpy of solution, ΔH solution – If ΔH solution is exothermic (< 0) dissolution is favored. –I f ΔH solution is endothermic (> 0) dissolution is not favored. 2. Change in disorder, or randomness, of the solution ΔS mixing - If ΔS mixing increases (> 0) dissolution is favored. - If ΔS mixing decreases (< 0) dissolution is not favored. Thus the best conditions for dissolution are: –For the solution process to be exothermic. Δ H solution < 0 –For the solution to become more disordered. Δ S mixing > 0 a. Weak solute-solute attractions favor solubility b. Weak solvent-solvent attractions favor solubility c. Strong solvent-solvent attractions favor solubility •Disorder in mixing a solution is very common. ΔS mixing is almost always > 0. •What factors affect ΔH solution ? –There is a competition between several different attractions. Solute-solute attractions such as ion-ion attraction, dipole-dipole, etc. –Breaking the solute-solute attraction requires absorption of E. Solvent-solvent attractions such as hydrogen bonding in water. –This also requires an absorption of E. Solvent-solute attractions, solvation , releases energy. –If solvation energy is greater than the sum of the solute- solute and solvent-solvent attractions, the dissolution is exothermic , ΔH solution < 0 . –If solvation energy is less than the sum of the solute- solute and solvent-solvent attractions, the dissolution is endothermic, ΔH solution > 0 . DISSOLUTION OF SOLID IN LIQUID •The energy released (exothermic) when a mole of formula units of a solid is formed from its constituent ions (molecules or atoms for non-ionic solids) in the gas phase is called the crystal lattice energy .
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Lec20 - Chapter Outline 1. Spontaneity of the Dissolution...

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