Properties of Solutions. Chapter 13

Transcription

1 Properties of Solutions Chapter 13

2 Sodium acetate crystals rapidly form when a seed crystal is added to a supersaturated solution of sodium acetate. Saturated solution: contains the maximum amount of a solute that will dissolve in a given solvent at a specific temperature. Unsaturated solution: contains less solute than the solvent has the capacity to dissolve at a specific temperature. Supersaturated solution: contains more solute than is present in a saturated solution at a specific temperature.

3 H soln = H 1 + H 2 + H 3 Properties of Solutions The interactions in the solution process: solvent-solvent interaction solvent-solute solute interaction solute-solute interaction H 1 >0 H 2 >0 H 3 <0 H soln?

4 Solutions Processes The Energy Changes and Solution Formation H soln <0 H soln >0

5 Solutions Processes The Energy Changes and Solution Formation NaOH added to water has H soln = kj/mol. NH 4 NO 3 added to water has H soln = kj/mol. If H soln is too endothermic a solution will not form. NaCl in gasoline: the ion-dipole forces are weak because gasoline is non-polar. Therefore, the iondipole forces do not compensate for the separation of ions. Water in octane Water in octane: water has strong H-bonds. There are no attractive forces between water and octane to compensate for the H-bonds.

6 Solutions Process Solution formation is entropy-driven NaCl dissolving in water: 1) Interrupting H-bonds in water 2) NaCl dissociates into Na+ and Cl-, 3) ion-dipole forces form: Na + δ - OH 2 and Cl - δ + H 2 O. (ions are hydrated)

7 Consider: Solutions Process Ni (s) + 2HCl(aq) NiCl 2 (aq) + H 2 (g). Ni(s) HCl(aq) (Ni NiCl 2 ). NiCl 2.6H 2 O

8 Like dissolves like Two substances with similar intermolecular forces are likely to be soluble in each other. Non-polar molecules are soluble in non-polar solvents CCl 4 in C 6 H 6 Polar molecules are soluble in polar solvents C 2 H 5 OH in H 2 O Ionic compounds are more soluble in polar solvents NaCl in H 2 O or NH 3 (l)

9 Concentration Units The concentration of a solution is the amount of solute present in a given quantity of solvent or solution. Percent by Mass mass of solute % by mass = x 100% mass of solute + mass of solvent Mole Fraction (X) mass of solute = x 100% mass of solution X A = moles of A sum of moles of all components

10 Molarity (M) Concentration Units M = moles of solute liters of solution Molality (m) m = moles of solute mass of solvent (kg)

11 Temperature and Solubility Solid solubility and temperature

12 Fractional crystallization is the separation of a mixture of substances into pure components on the basis of their differing solubilities. Suppose you have 90 g KNO 3 contaminated with 10 g NaCl. Fractional crystallization: 1. Dissolve sample in 100 ml of water at 60 0 C 2. Cool solution to 0 0 C 3. All NaCl will stay in solution (s = 34.2g/100g) g of PURE KNO 3 will precipitate (s = 12 g/100g). 90 g 12 g = 78 g

13 Temperature and Solubility Gas solubility and temperature solubility usually decreases with increasing temperature

14 Pressure and Solubility of Gases The solubility of a gas in a liquid is proportional to the pressure of the gas over the solution (Henry s law). c = kp C: the concentration (M) of the dissolved gas P: the pressure of the gas over the solution k: a constant (mol/l atm) that depends only on temperature low P high P low c high c

15 Colligative Properties of Nonelectrolyte Solutions Colligative properties: properties that depend only on the number of solute particles in solution and not on the nature of the solute particles. Vapor-Pressure Lowering P = X P 0 P = vapor pressure of pure solvent 0 A A A Raoult s law P A X A = mole fraction of the solvent If the solution contains only one solute: X B 0 0 = 1 X P P = P = X P A A A B A X B mole fraction of the solute

16 Colligative Properties of Nonelectrolyte Solutions Vapor-Pressure Lowering: P 0 A PA

17 Ideal solution with two or more volatile components Ideal Solution 0 P A = X A P A 0 P B = X B P B P T = P A + P B 0 P T = X A P A0 +X B P B

18 Fractional Distillation Apparatus

19 Boiling-Point Elevation T b = T b T b 0 0 T b is the boiling point of the pure solvent T b is the boiling point of the solution T b > T b 0 T b > 0 T b = K b m M: the molality of the solution K b : the molal boiling-point elevation constant ( 0 C/m)

20 Freezing-Point Depression T f = T f 0 T f > T f 0 T f 0 T f : the freezing point of the pure solvent T f : the freezing point of the solution T f > 0 T f = K f m m : the molality of the solution K f : the molal freezing-point depression constant ( 0 C/m)

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22 What is the freezing point of a solution containing 478 g of ethylene glycol (antifreeze) in 3202 g of water? The molar mass of ethylene glycol is g. m = T f = K f m moles of solute = mass of solvent (kg) K f (water) = C/m 478 g x 1 mol g kg solvent = 2.41 m T f = K f m = C/m x 2.41 m = C 0 T f = T f T f 0 T f = T f T f = C C = C

23 Osmotic Pressure (π) Osmosis : the selective passage of solvent molecules through a porous membrane from a dilute solution to a more concentrated one. A semipermeable membrane: allows the passage of solvent molecules but blocks the passage of solute molecules. Osmotic pressure (π):( the pressure required to stop osmosis dilute more concentrated

24 Osmotic Pressure (π) High P Low P π = MRT M : the molarity of the solution R : the gas constant T : the temperature (in K)

25 Osmosis through the semipermeable membrane of a red blood cell isotonic solution hypotonic solution hypertonic solution

26 Colligative Properties of Nonelectrolyte Solutions Colligative properties : properties that depend only on the number of solute particles in solution and not on the nature of the solute particles. Vapor-Pressure Lowering P 1 = X 1 P 1 0 Boiling-Point Elevation Freezing-Point Depression Osmotic Pressure (π) T b = K b m T f = K f m π = MRT

27 Colligative Properties of Nonelectrolyte Solutions 0.1 m NaCl solution 0.1 m Na + ions & 0.1 m Cl - ions Colligative properties : the number of solute particles in solution 0.1 m NaCl solution 0.2 m ions in solution van t Hoff factor (i) = actual number of particles in soln after dissociation number of formula units initially dissolved in soln nonelectrolytes NaCl CaCl 2 i 1 2 3

28 Colligative Properties of Nonelectrolyte Solutions Boiling-Point Elevation T b = i K b m Freezing-Point Depression Osmotic Pressure (π) T f = i K f m π = imrt

29 Colloids A colloid: a dispersion of particles of one substance throughout a dispersing medium of another substance. Tyndall effect: ability of a Colloid to scatter light. The beam of light can be seen through the colloid.

30 Colloids Colloid versus solution Colloidal particles are much larger than solute molecules Colloidal suspension is not as homogeneous as a solution Size of particles (nm) Type of Dispersions <1 Solutions or Mixture of gas Colloids >200 emulsion

31 Colloids Hydrophilic and Hydrophobic Colloids:

32 The Cleansing Action of Soap Grease Hydrophilic Head Hydrophobic Tail

33 Desalination Chapter 13: 4, 8, 12(b), 16, 30, 38, 50(c), 52, 80

34 Homework Chapter 13: 4, 8, 12(b), 16, 30, 38, 50(c), 52, 80