The Chemistry of Solutes and Solutions

1. Chapter 15 The Chemistry of Solutes and Solutions William Henry 1774-1836. Henry’s Law. Francois Raoult 1830-1901. Raoult’s Law.

2. H δ+ δ- Cl- O H Na+ O H H The Solution Process • Solutes and solvent are components of the solution. • In the process of making solutions with condensed phases, intermolecular forces become rearranged. • Consider NaCl (solute) dissolving in water (solvent): • the water H-bonds have to be interrupted, • NaCl dissociates into Na+ and Cl-, • ion-dipole forces form: Na+…-OH2 and Cl- …+H2O.

3. The Solution Process

4. . . X O . . H The Solution Process • “Rule”: LIKE DISSOLVES LIKE • polar solvents dissolve polar solutes. • Non-polar solvents dissolve non-polar solutes. Water is polar (because it’s bent). It will therefore tend to dissolve other polar molecules or ions. For example, most salts, alcohols and sugars dissolve in water. Alcohols and sugars all contain the O-H part of a molecule which makes them polar:

5. H H H H C C C C H H H H The Solution Process Most organic substances (compounds of carbon) are non-polar. That is why they in general do not dissolve in water (which is polar). Generally, carbon chains are non-polar (no dipole moment). Since the electronegativities of hydrogen and carbon are virtually the same, hydrocarbons are non-polar. Gasoline is non-polar, because it is a hydrocarbon.

6. Solute-Solvent Interactions • Miscible liquids: mix in any proportions. • Immiscible liquids: do not mix. • Soluble liquids: mix in certain proportions. • Intermolecular forces are important: water and ethanol are miscible because the broken hydrogen bonds in both pure liquids are re-established in the mixture. • The number of carbon atoms in a chain affect solubility: the more C atoms the less soluble in water.

7. Factors Affecting Solubility Solute-Solvent Interactions ethanol ---- water ethanol ---- ethanol

8. Factors Affecting Solubility Solute-Solvent Interactions • The number of -OH groups within a molecule increases solubility in water. Glucose – (a sugar)

9. mass of component in soln total mass of soln ppm of component Ways of Expressing Concentration • All methods involve quantifying amount of solute per amount of solvent or solution. • Amounts are masses, moles or liters. • Qualitatively solutions are dilute or concentrated. • Definitions: of component Mass fraction = mass of component in soln 6 = ´ 10 total mass of soln Mass % = mass fraction ´ 100

10. Mole fraction of solute Ways of Expressing Concentration Mole Fraction, Molarity, and Molality solute moles of = of total moles all components X M moles of solute = Molality kg of solvent m

11. The Composition of the Oceans Concentrations in ppm and Molarity Chloride 19350 0.55 Sodium 10760 0.47 Sulfate 2710 0.028 Magnesium 1290 0.054 Calcium 412 0.010 Potassium 400 0.010 Carbon dioxide* 106 2.3 x 10-4 Bromide 67 8.3 x 10-4 Boric acid 27 4.3 x 10-4 Strontium 7.9 9.1 x 10-5 Fluoride 1.3 7.0 x 10-5 * present as bicarbonate and carbonate

12. Factors Affecting Solubility Henry’s Law: Pressure Effects: solubility of a gas in a liquid is a function of the pressure of the gas as given by: Cg is the solubility of gas, Pg the partial pressure, k = Henry’s law constant. • Carbonated beverages are bottled under PCO2 > 1 atm. As the bottle is opened, PCO2 decreases and the solubility of CO2 decreases. Therefore, bubbles of CO2 escape from solution.

13. dissolving gases dissolving solids opening pop bottles thermal pollution in lakes sugar dissolves better in warm water Temperature effects

14. Lowering the Vapor Pressure • Non-volatile solutes reduce the ability of the surface solvent molecules to escape the liquid. • Therefore, vapor pressure is lowered. • The amount of vapor pressure lowering depends on the amount of solute (this is a colligative property)

15. Colligative Properties • Colligative properties depend on number of (nonvolatile) solute molecules. • Some colligative properties: • Vapor pressure lowering • Boiling point elevation • Freezing point depression • Osmotic pressure

16. Colligative Properties Raoult’s Law (used for vapor pressure lowering) • PA is the vapor pressure of A in a solution • PA is the vapor pressure of pure A • A is the mole fraction of A in solution,

17. Colligative Properties Boiling-Point Elevation

18. Colligative Properties Boiling-Point Elevation • At 1 atm (normal boiling point of pure liquid) there is a lower vapor pressure of the solution. Therefore, a higher temperature is required to reach a vapor pressure of 1 atm for the solution (Tb). • Molal boiling-point-elevation constant, Kb, expresses how much Tb changes with molality, m: Kb depends only on solvent

19. Colligative Properties Freezing-Point Depression • The solution freezes at a lower temperature (Tf) than the pure solvent. • Decrease in freezing point (Tf) is directly proportional to molality (Kfis the molal freezing-point-depression constant): Kf depends only on solvent

20. ΔTb =Kbm ΔTf=Kfm

21. Colligative Properties Can be used to determine molecular mass of solute. Problem: Soln of 5.00 g lauryl alcohol (solute) dissolved in 100 g benzene (solvent) freezes at 4.1oC. Calculate molar mass of lauryl alcohol. ΔTf = 4.1oC ΔTf=5.5-4.1=1.4oC Use ΔTf= Kfm; Kf = 5.12oC/m, fp = 5.5° for benzene m= ΔTf/Kf = 1.4oC/(5.12oC/m) = 0.27 m molality is mol solute/kg solvent

22. Semipermeable membrane OSMOSIS more concentrated solution less concentrated solution H2O H2O Particles of solute H2O Particles of solute H2O Water less concentrated more concentrated

23. Osmotic pressure causes a difference in levels of water

25. Colligative Properties Osmosis • Osmotic pressure, , is the pressure required to stop osmosis: • Isotonic solutions: two solutions with the same separated by a semipermeable membrane. • Hypotonic solutions: a solution of lower (less concentrated) than a hypertonic solution (more concentrated). • Red blood cell walls are semipermeable membranes.

26. Colligative Properties Salty food causes retention of water and swelling of tissues (edema) Osmosis • Crenation and Hemolysis: (Cell in hypertonic solution) (Cell in hypotonic solution) Crenation (shrivels up) Hemolysis (bursts)

27. TOTAL NUMBER OF PARTICLES Colligative properties depend on total numbers of particles, and salts give more than 1 mole of particles per mole of compound. A 1.0 m solution of NaCl provides 1.0 mol of Na+and 1.0 mol of Cl- Thus, 1.0 mol of NaCl provides 2.0 mol of ions (particles) The colligative property is enhanced by a factor of 2. Other examples: 1.0 mol of Na2SO4 provides 3.0 mol of ions. 1.0 mol of (NH4)3PO4 provides 4.0 mol of ions. Covalent molecules like sucrose or ethylene glycol do not ionize.

28. Colloids Colloids are huge molecules which create suspensions with properties between those of mixtures and solutions. Examples: smoke, fog, mayonnaise, marshmallows, emulsions, colored glass. The Tyndall effect is used to characterize liquid colloids in water: a beam of light through a colloidal suspension can be seen from the side (because of the scattered light); a beam of light through a true solution is not visible from the side.