Chapter 13

1. Chapter 13 Ions in aqueous Solutions And Colligative Properties

2. Compounds in Aqueous Solution • Dissociation • The separation of ions that occurs when an ionic compound dissolves • H2O NaCl(s) Na+ (aq) + Cl- (aq) H2O MgCl2 (s)  Mg2+ (aq) + 2 Cl- (aq)

3. Assuming 100% dissociation 1 mole NaCl in water -> 1 mole Na+ and 1 mole Cl- =2 mole of solute ions 1 mole CaCl2 -> 1 mole of Ca and 2 mol of Cl- = 3 mole of solute ions

4. Do practice page 436

5. Remember must represent the facts when writing an equation • Some substances do not remain soluble in solution. • Use table 437 and 860 to determine solubility

6. Net Ionic Equations • Show the compounds and ions that undergo a chemical change in a reaction in an aq soln. • To create an ionic equation, must convert a balanced chemical equation into an ionic eq. • ALL soluble ionic compounds are shown as dissociated ions in soln.

7. Ionic equations • 2HCl (aq) + Na2O (aq) -> 2NaCl (aq) + H20 (l) • 2H+ (aq) + 2Cl- (aq) + 2Na+ (aq) + O2- (aq)  2 Na+ (aq) + 2 Cl- (aq) + H20 (l)

8. Spectator Ions • Ions that do not take part in a chemical reaction and are found in solution both before and after the reaction • To convert an ionic equation into a net ionic equation, remove the spectator ions from both sides of the equation • The ions and compounds left after the spectator ions are removed is called the net ionic equation

9. See practice page 440

10. Ionization • Molecular compounds can also form ions in solution • Ionization means creating ions where there were none H2O • HCl(g) - H+ (aq) + Cl- (aq)

11. Hydronium Ion • Molecular compounds often contain Hydrogen bonded by a polar covalent bond. • Some of the compounds ionize in an aq. Solution to release H+ , the H+ ion has such a high reactivity that it attaches itself to water to form H3O+ or the hydronium ion • HCl(g) + H2O H3O+ (aq) + Cl- (aq)

12. Electrolytes • Strong electrolytes • Any compound whose dilute aqueous solutions conduct electricity well • Caused by all (almost all) solute existing as ions in solution • Weak electrolytes • Any compound whose dilute aqueous solutions conduct electricity poorly • Caused by very small amounts of solute existing as ions in solution

13. Do section review page 443 • Homework: page 458 #2, 8-13

14. Colligative Properties of Soln. • Properties that depend on the concentration of solute particles but not on their identity • These include: freezing point, boiling point, vapor pressure, osmotic pressure,

15. Raoult's Law and Vapor Pressure Lowering • Bp and Fp of soln differ from those of pure solvents • When Nonvolatile solutes (a substance that has little tendency to become a gas under existing conditions) added to a liquid to form a solution, the vapor pressure above that solution decreases.

16. Liquid molecules at the surface of a liquid can escape to the gas phase when they have a sufficient amount of energy to break free of the liquid's intermolecular forces. • That vaporization process is reversible. Gaseous molecules coming into contact with the surface of a liquid can be trapped by intermolecular forces in the liquid. • Eventually the rate of escape will equal the rate of capture to establish a constant, equilibrium vapor pressure above the pure liquid.

17. If add a nonvolatile solute to that liquid, the amount of surface area available for the escaping solvent molecules is reduced because some of that area is occupied by solute particles. • the solvent molecules will have a lower probability to escape the solution than the pure solvent.

18. The Vapor Pressure of a Solution is Lower than that of the Pure Solvent

19. The French chemist Francois Raoult discovered the law that mathematically describes the vapor pressure lowering phenomenon. • Raoult's law states: • the vapor pressure of an ideal solution is dependent on the vapor pressure of each chemical component and the mole fraction of the component present in the solution. • Once the components in the solution have reached equilibrium, the total vapor pressure p of the solution is:

20. and the individual vapor pressure for each component is: • Where p* is the vapor pressure of the pure component x is the mole fraction of the component in solution

21. Solutions that obey Raoult's law are called ideal solutions because they behave exactly as we would predict. • Solutions that show a deviation from Raoult's law are called non-ideal solutions because they deviate from the expected behavior. • Very few solutions actually approach ideality, but Raoult's law for the ideal solution is a good enough approximation for the non- ideal solutions

22. Freezing Point Depression • describes the phenomenon that the freezing point of a liquid (a solvent) is depressed when another compound is added, meaning that a solution has a lower freezing point than a pure solvent. This happens whenever a solute is added to a pure solvent, such as water. • The phenomenon may be observed in sea water, which due to its salt content remains liquid at temperatures below 0°C, the freezing point of pure water. • The freezing point depression happens both when the solute is an electrolyte, such as various salts, and a nonelectrolyte

23. ΔTf = Kf · mB • ΔTf, the freezing point depression, is defined as Tf (pure solvent) − Tf (solution), the difference between the freezing point of the pure solvent and the solution. It is defined to assume positive values when the freezing point depression takes place. • Kf, the cryoscopic constant, which is dependent on the properties of the solvent. It can be calculated as Kf = RTm2M/ΔHf, where R is the gas constant, Tm is the melting point of the pure solvent in kelvin, M is the molar mass of the solvent, and ΔHf is the heat of fusion per mole of the solvent. (LOOK THESE UP) • mB is the molality of the solution, van 't Hoff factor i as mB = msolute · i. The factor i accounts for the number of individual particles (typically ions) formed by a compound in solution. • Examples: • i = 1 for sugar in water • i = 2 for sodium chloride in water, due to dissociation of NaCl into Na+ and Cl- • i = 3 for calcium chloride in water, due to dissociation of CaCl2 into Ca2+ and 2 Cl- • i = 2 for hydrogen chloride in water, due to complete dissociation of HCl into H+ and Cl-

24. Kf of nonelectrolytes is -1.86 *C/m • *C is celsius degree • m is mol solute/kg solvent • Kf of electrolytes may be looked up • See page 448 for examples

25. Boiling Point Elevation • Nonvolatile solutes elevate the bp of the solvent • Molal boiling point constant is the boiling point elevation of the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte solute • Water is .51 *C/m

26. ΔTb = Kb · mB • ΔTb, the boiling point elevation, is defined as Tb (solution) - Tb (pure solvent). • Kb, the ebullioscopic constant, which is dependent on the properties of the solvent. It can be calculated as Kb = RTb2M/ΔHv, where R is the gas constant, and Tb is the boiling temperature of the pure solvent, M is the molar mass of the solvent, and ΔHv is the heat of vaporization per mole of the solvent. • mB is the molality of the solution, calculated by taking dissociation into account since the boiling point elevation is a colligative property, dependent on the number of particles in solution. This is most easily done by using the van 't Hoff factor i as mB = msolute · i. The factor i accounts for the number of individual particles (typically ions) formed by a compound in solution. • Examples: • i = 1 for sugar in water • i = 2 for sodium chloride in water, due to the full dissociation of NaCl into Na+ and Cl- • i = 3 for calcium chloride in water, due to dissociation of CaCl2 into Ca2+ and 2Cl-

27. Do practice page 450 and 451