Properties of Solutions

1. Properties of Solutions Brown, LeMay Ch 13 AP Chemistry Monta Vista High School 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 http://phet.colorado.edu/en/simulation/soluble-salts • Crystallization: “un-dissolving”; process by which solute particles leave the solvent. • Solute + solvent ↔ solution (equilibrium) http://conference.merlot.org/2007/Thursday/Sattsangi_Crystalization.swf

6. 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 http://www.dynamicscience.com.au/tester/solutions/chemistry/solutions/solubilitycurves.html Work with your lab group on the above link. • 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. Solubility Solubility: go to the temperature and up to the desired line, then across to the Y-axis. This is how many g of solute are needed to make a saturated solution of that solute in 100g of H2O at that particular temperature. At 40oC, the solubility of KNO3 in 100g of water is 64 g. In 200g of water, double that amount. In 50g of water, cut it in half.

8. Supersaturated If 120 g of NaNO3 are added to 100g of water at 30oC: 1) The solution would be SUPERSATURATED, because there is more solute dissolved than the solubility allows 2) The extra 25g would precipitate out 3) If you heated the solution up by 24oC (to 54oC), the excess solute would dissolve.

9. Unsaturated If 80 g of KNO3 are added to 100g of water at 60oC: 1) The solution would be UNSATURATED, because there is less solute dissolved than the solubility allows 2) 26g more can be added to make a saturated solution 3) If you cooled the solution down by 12oC (to 48oC), the solution would become saturated

10. 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)http://www.lcc.ukf.net/KS3Chem/mixtures.htm • Covalent network solids do not dissolve in polar or nonpolar solvents.

11. 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 http://www.kentchemistry.com/moviesfiles/Units/GasLaws/HenrysLawMovie.htm William Henry(1775-1836)

12. 13.4: Factors Affecting Solubility • Temperature • Most solids: increased T means increased solubility • * Exception: Ce2(SO4)3 • Gases: increased T means decreased solubility

13. 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 solvent P°A= normal vapor pressure of solvent François-Marie Raoult(1830-1901)

14. (c) 2006, Mark Rosengarten Reference Table H: Vapor Pressure of Four Liquids

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

16. 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 http://dwb4.unl.edu/chemistry/dochem/DoChem066.html Jacobus van’t Hoff(1852-1911)

17. 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

18. P Solute “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

19. 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)