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Chapter 13: Properties of Solutions

Chapter 13: Properties of Solutions. Sam White Pd. 2. Introduction. A solution is any homogenous mixture, which means the components are uniformly intermingled on a molecular level The Solvent is the most abundant component. It does the dissolving.

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Chapter 13: Properties of Solutions

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  1. Chapter 13: Properties of Solutions Sam White Pd. 2

  2. Introduction • A solution is any homogenous mixture, which means the components are uniformly intermingled on a molecular level • The Solvent is the most abundant component. It does the dissolving. • The Solute are any of the other components. They are the ones being dissolved

  3. Formation of Solutions • With the exception of gas solutions, solutions form when the attractive forces between solute and solvent are comparable or greater than the intermolecular forces in either component

  4. Formation of Solutions • Example: Salt Water- Attractive forces between Na+ or Cl- and the polar water molecules overcome the lattice energy of solid NaCl • Once separated, the Na+ and Cl- are surrounded by water. This interaction is known in all solutions as solvation • When the solvent is water, this interaction is known as hydration

  5. Energy Change in Solution Formation • In order to form a solution, the solvent must form space to house the solute and the solute must be dissolved, both of which take energy

  6. Enthalpy of Solution • Overall Enthalpy Change: • DHsolution=DH1+DH2+DH3 • Example with Salt Water: • DH1 accounts for the separation of NaCl to Na+ and Cl- • DH2 accounts for the separation of solvent molecules to accommodate the solute • DH3 accounts for the attractive interactions between solute and solvent

  7. Overall Enthalpy Change

  8. Saturated Solutions • As concentration of a solid solute increases, so does it’s chance of of colliding with the surface of the solid and becoming reattached to the solid • This is called crystallization • Solute + Solvent Solution

  9. Saturated Solutions • When the rates of crystallization and dissolving become equal, no increase of solute in solution will occur • When a solution will not dissolve any more solute, it is a saturated solution • When a solution that can still dissolve solute into it is an unsaturated solution

  10. Supersaturation • Under suitable conditions, it is sometimes possible to form a solution with more solute than that needed for a saturated solution • These solutions are supersaturated

  11. Supersaturation • Supersaturation usually occurs because many solutes are more soluble at one temperature than another • Example: Sodium acetate, NaC2H3O2, will dissolve in water more readily at higher temperatures. When a saturated solution is made at higher temperatures then slowly cooled, all of the solute may remain dissolved even though the solubility decreases

  12. Factors Affecting Solubility • The stronger the intermolecular attractive forces between solute and solvent, the greater the solubility • As a result of favorable dipole-dipole attractions, polar liquids tend to dissolve more readily in polar solvents • Water is not only polar, but has hydrogen bonds, making solutes that have hydrogen bonds able to dissolve in water as well

  13. Factors Affecting Solubility • Pairs of liquids that mix in all proportions are miscible • Liquids that do not dissolve significantly in one another are immiscible

  14. Hydrocarbons vs. Alcohols • Many hydrocarbons are immiscible in water because they are nonpolar molecules • Alcohols have an OH group, which are both polar and have hydrogen bonds, making them more readily soluble in water • As the carbon chain become larger, the effect of the OH group becomes smaller, meaning that larger alcohol chains begin to become less soluble

  15. Pressure Effects • Pressure only affects the solubility of gas in a solvent • As pressure increases, solubility of the gas increases

  16. Henry’s Law • Cg = kPg • Cg is the solubility of the gas in solution (usually expressed in molarity) • Pg is the partial pressure of the gas over solution • k is the Henry’s Law Constant, which is unique for all solute-solvent pairs as well as the temperature

  17. Temperature Effects • As temperature increases, the solubility of solid solutes (such as salts) normally increases • In contrast, as temperature increases, the solubility of gaseous solutes normally decreases

  18. Solubility Charts Gas Solubility Curve Solids Solubility Curve

  19. Ways of Expressing Concentration • Mass percentage, ppm • Mole Fraction • Molarity • Molality

  20. Mass Percentage and ppm • Mass % of component = (mc / mt) x 100 • mc = mass of component in solution • mt = total mass of solution • ppm of component = (mc / mt) x 106 • mc and mt denote the same things for ppm as they denote for mass % of component

  21. Mole Fraction • Mole Fraction of Component = (molc / molt) • molc = moles of component • molt = total moles of all components

  22. Molarity • Molarity = (mols / Ls) • mols = moles solute • Ls = liters solution

  23. Molality • Molality = (mols / kgs) • mols = moles solute • kgs = kilograms solvent

  24. Colligative Properties • Colligative properties depend on the quanity of solute, not the type of solute • The colligative properties are: • Vapor-Pressure Reduction • Boiling-Point Elevation • Freezing-Point Depression • Osmotic Pressure

  25. Vapor-Pressure Reduction • As the amount of solute increases, the vapor pressure of solution decreases • This relationship can be expessed through Raoult’s Law: • PA = XAPoA • PA = Partial pressure exerted by solvent • XA = Mole fraction of solvent • PoA = Vapor pressure of pure solvent

  26. Boiling-Point Elevation • As amount of solute increase, boiling point increases • This relationship can be expressed as: DTb = dKbm • DTb = total boiling point elevation • d = dissociation factor of the solute • Kb = molal boiling point elevation constant of the solvent • m = molality of solution

  27. Freezing-Point Depression • As amount of solute increases, freezing point decreases • This relationship can be expressed as: DTf = dKfm • DTf = total freezing point depression • d = dissociation factor of the solute • Kf = molal freezing point depression constant of the solvent • m = molality of solution

  28. Osmotic Pressure • As amount of solute increases, osmotic pressure increases • This relationship can be expressed as: p = (n / V)RT = MRT • p = osmotic pressure • n = number of moles solute • V = volume of solution • R = ideal gas constant • T = temperature of solution • M = molarity of solution

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