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Properties of Solutions

Properties of Solutions. Brown, LeMay Ch 13 AP Chemistry. CaCl 2 (aq). 13.1: Types of Solutions. When do solutions form?. Solutions form (the solute and solvent will mix) when: Energy: solute-solvent interactions are stronger than solute-solute or solvent-solvent interactions.

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Properties of Solutions

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  1. Properties of Solutions Brown, LeMay Ch 13 AP Chemistry CaCl2 (aq)

  2. 13.1: Types of Solutions

  3. When do solutions form? • Solutions form (the solute and solvent will mix) when: • Energy:solute-solvent interactions are stronger than solute-solute or solvent-solvent interactions. • Disorder:Solutions result in a more disordered state than the separate solute and solvent states, since molecules will be “mixed” that were once “well organized”. • NaCl (s) + H2O (l) → Na+ (aq) + Cl- (aq) • Ion-dipole interactions > H-bonds (H2O···H2O) < Ionic bonds (Na+ Cl-) • The increase in disorder also drives the dissolving process.

  4. 13.2: Ways to Express Concentration • Mass Percent • Mole Fraction:commonly used for gases • Molarity:commonly used for solutions • Molality:commonly used for colligative properties • Varies with T • Does not vary with T

  5. 13.3: Solubility Vocabulary • Solvation:dissolving; the interactions between solute and solvent • Hydration: solvation with water • Crystallization: “un-dissolving”; process by which solute particles leave the solvent. • Solute + solvent ↔ solution (equilibrium)

  6. 13.3: Solubility Vocabulary • Saturated: a solution that is in equilibrium with undissolved solute (appears as solution and crystals) • Solubility: the amount of solute needed to form a saturated solution • Unsaturated:a solution containing less than the saturated amount (appears as solution only) • Supersaturated: a solution containing more than the saturated amount, yet appears unsaturated.

  7. 13.4: Factors Affecting Solubility • “Like dissolves like.” • Miscible: liquids that mix (polar or ionic solute with polar solvent, or nonpolar with nonpolar) • Immiscible:liquids that do not mix (polar or ionic solute with nonpolar solvent) • Covalent network solids do not dissolve in polar or nonpolar solvents.

  8. 13.4: Factors Affecting Solubility • Pressure: does not significantly affect solubility of liquids and solids • Gases: increased P means increased solubility Henry’s law: Cg = k Pg Cg = solubility of gas in solution (M) k = Henry’s law constant Pg = partial pressure of gas over solution William Henry(1775-1836)

  9. 13.4: Factors Affecting Solubility • Temperature • Most solids: increased T means increased solubility • * Exception: Ce2(SO4)3 • Gases: increased T means decreased solubility

  10. 13.5: Colligative Properties • Properties that are dependent on the number of solute particles present in solution • Vapor pressure lowering: the greater the concentration of a nonvolatile solute, the lower the vapor pressure of the solvent • Solute takes up surface area • Introduction of solute-solvent IMF Raoult’s law: PA = XA P°A PA = vapor pressure of solvent vapor above solution (solute A is nonvolatile) XA = mole fraction of solute A P°A= normal vapor pressure of solvent François-Marie Raoult(1830-1901)

  11. Ideal solution:described by Raoult’s law • Has low concentration of solute • Solute and solvent have similar types of IMF & molecular sizes

  12. Extension of the Liquid Phase • Boiling point elevation: DTb = i Kb m Kb (H2O) = 0.51 ºC•kg/mol • Freezing point depression: DTf = i Kf m Kf (H2O) = 1.86 ºC•kg/mol • i = van’t Hoff factor: Unitless constant associated with the degree of dissociation of a solute in a solvent Jacobus van’t Hoff(1852-1911)

  13. Ideal i values • i = 1 Substances which do not ionize in solution Ex: sucrose (sugar) • i = 2 Substances which ionize into 2 ions Ex: NaCl • i = 3 Substances which ionize into 3 ions Ex: MgCl2 Ex: Determine the solute “equivalent molality” (factoring in i) for the following solutions: • 1-m sucrose • 1-m NaCl • 1-m CaCl2

  14. P Solvent “wants” to flow Osmotic Pressure (P) • Pressure required to prevent osmosis of solute particles P = iMRT = (n/V)RT R = 0.0821 L-atm/mol-K • Applied on solution side to stop net movement of solvent from the pure solvent side. • Osmosis: net movement of solvent toward the solution with the highest solute concentration Prevents flow of solute particles

  15. 13.6: Colloids • Mixtures containing particles intermediate between: • A solution (homogeneous, < 10 Å) and • A suspension (heterogeneous, > 2000 Å) • Tyndall effect:scattering of lightseen in a colloid John Tyndall(1820-1893)

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