Physical Properties of Solutions

1. Physical Properties of Solutions Chapter 12

2. Definitions A solution is a homogenous mixture of 2 or more substances The solute is(are) the substance(s) present in the smaller amount(s) The solvent is the substance present in the larger amount

3. A saturated solution contains the maximum amount of a solute that will dissolve in a given solvent at a specific temperature. An unsaturated solution contains less solute than the solvent has the capacity to dissolve at a specific temperature. A supersaturated solution contains more solute than is present in a saturated solution at a specific temperature. Sodium acetate crystals rapidly form when a seed crystal is added to a supersaturated solution of sodium acetate. Rapid Crystallization

4. Enthalpy of solution(or enthalpy of dissolution) • The heat of solution (DHsol) is defined as the sum of the energy absorbed (endothermic), or released (exothermic) as the solute dissolves in a solvent at constant pressure. • The value of the overall enthalpy change is the sum of the individual enthalpy changes of each step.

5. Examples • Dissolving ammonium nitrate in water is endothermic (solvation does not weigh up against energy spent in breaking down the crystal lattice) • Adding potassium hydroxide is exothermic, the solute-solvent attractions are stronger than the other steps. • Solutions with negative heats of solution have lower vapor pressures.

6. “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 • CCl4 in C6H6 • polar molecules are soluble in polar solvents • C2H5OH in H2O • ionic compounds are more soluble in polar solvents • NaCl in H2O or NH3 (l)

7. The Cleansing Action of Soap

8. Fat Soluble and Water Soluble Vitamins • Vitamin A is soluble in nonpolar compounds (like fats). • Vitamin C is soluble in water.

9. Generally, solubility increases with increasing temperature Temperature and Solubility Solid solubility and temperature Some substances, solubility decreases with increasing temperature (gives off heat as they dissolve)

10. Temperature and Solubility Gas solubility and temperature solubility usually decreases with increasing temperature

11. low P high P low c high c 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 is the concentration (M) of the dissolved gas c = kP P is the pressure of the gas over the solution k is a constant (mol/L•atm) that depends only on temperature

12. moles of A XA = sum of moles of all components x 100% mass of solute x 100% = mass of solution mass of solute mass of solute + mass of solvent Concentration Units The concentration of a solution is the amount of solute present in a given quantity of solvent or solution. Percent by Mass % by mass = Mole Fraction(X) 12.3

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

14. moles of solute moles of solute m= m= moles of solute M = mass of solvent (kg) mass of solvent (kg) liters of solution 5.86 moles C2H5OH = 0.657 kg solvent What is the molality of a 5.86 M ethanol (C2H5OH) solution whose density is 0.927 g/mL? • Assume 1 L of solution: • 5.86 moles x MM ethanol = 270 g ethanol (solute) • 1000 mL x 0.927 g/mL = 927 g of solution mass of solvent = mass of solution – mass of solute = 927 g – 270 g = 657 g = 0.657 kg = 8.92 m

15. Convert % mass to Molarity • What is the Molarity of a 95% acetic acid solution? (density = 1.049 g/mL) 1000 mLx 1.049g/ml = the mass of solution = 1049 g 95% of the solution is acetic acid 1049 g solution x 0.95 = 997 g solute 997 g X 1 mol/60.05 g Acetic acid = 16.6 mol solute Since we assumed 1 L, that’s 16.6 mol / 1 L or 16.6 M

16. Colligative Properties of Solutions Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles. • Vapor Pressure Lowering • Boiling Point Elevation • Freezing Point Depression • Osmotic Pressure

17. = vapor pressure of puresolvent 0 P1 = X1 P 1 0 P 1 Raoult’s law Vapor-Pressure Lowering; The vapor pressure of a Solution is lower than pure solute. X1= mole fraction of the solvent DP1 = X2P01 X2 = mole fraction of the solute

18. Lowering the Pressure Above the Solution (by opening bottle) Decreases Gas Solubility

19. 0 DTb = Tb – T b 0 T b boiling point of the pure solvent DTb = Kbm Boiling-Point Elevation of Nonelectrolyte Solutions T b boiling point of the solution 0 m is the molality of the solution Kb is the boiling-point elevation constant (0C/m)

20. Osmotic Pressure • Osmotic pressure is the “funky” colligative property, but it is very important biologically • Osmotic pressure is the pressure required to prevent osmosis. P= M RT where P is osmotic pressure

21. van’t Hoff factor i • Colligative properties depend on the concentration of particles • Strong electrolytes, like NaCl, should produce (nearly) two moles of solute particles for mole of NaCl that dissolves • The van’t Hoff factor i scales the solute molatity to the correct number of particles

22. van’t Hoff factor i

23. van’t Hoff factor i Δ T f = i K f m D Tb = iKfm P1 = X1P01 P = iMRT