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Honors chemistry ch . 14: Ions in Aqueous Solutions and Colligative Properties

Honors chemistry ch . 14: Ions in Aqueous Solutions and Colligative Properties. Dissociation Reactions. Ionic compounds that are soluble in water “dissociate” in water to give the corresponding ions. Practice writing dissociation reactions for as many ionic compounds as possible. Example:

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Honors chemistry ch . 14: Ions in Aqueous Solutions and Colligative Properties

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  1. Honors chemistry ch. 14: Ions in Aqueous Solutions and Colligative Properties

  2. Dissociation Reactions • Ionic compounds that are soluble in water “dissociate” in water to give the corresponding ions. Practice writing dissociation reactions for as many ionic compounds as possible. • Example: • Cu3(PO4)2 ------- > 3Cu+2 + 2PO4-3 • One mole of cupric phosphate on dissociation yields 3 moles of Cu+2 ions and 2 moles of PO4-3 ions. • Al(NO3)3 ------ > • AgNO3 ----- > • CuCl2 ----- > • Al2(SO4)3 ------- > • Ca3(PO4)2 ------ > • Ba(OH)2 ----- >

  3. Ionization Reactions • When a “molecular” compound dissolves in water, sometimes, it splits into ions as well. For example, most acids are molecular gases, but they dissolve readily in water to produce H+ ions. • Example • HCl ------ > H+ + Cl-1 • H+ + H2O ----- > H3O+ • H2SO4 ----- > • HNO3 ------ > • C2H4O2 ------- > • HF ------ > • H3PO4 ------ >

  4. Hydronium ion • H+ + H2O ----- > H3O+ • Because the bonds that hold water together are strong, most water molecules exist as H2O. Some of them hold onto protons (H+) to become hydronium ions (H3O+). • Hydronium ions are more common in solution than protons. • Ex. HCl ------ > H3O+ + Cl-1 H2O

  5. 11.3 Net Ionic Equations • A complete ionic equation is an equation that shows dissolved ionic compounds as dissociated free ions. • AgNO3 + NaCl AgCl is a precipitate. Therefore, it is not written as ions. Use a solubility chart to determine physical states of products.

  6. 11.3 Net Ionic Equations • An ion that appears on both sides of an equation and is not directly involved in the reaction is called a spectator ion. • The net ionic equation is an equation for a reaction in solution that shows only those particles that are directly involved in the chemical change. • A net ionic equation shows only those particles involved in the reaction and is balanced with respect to both mass and charge.

  7. 11.3 Net Ionic Equations • Sodium ions and nitrate ions are not changed during the chemical reaction of silver nitrate and sodium chloride so the net ionic equation is • AgNO3 + NaCl

  8. 11.3 Predicting the Formation of a Precipitate • You can predict the formation of a precipitate by using the general rules for solubility of ionic compounds. Will a precipitate form when a sodium carbonate solution is mixed with a barium nitrate solution? products are sodium nitrate and barium carbonate, but NaNO3 is aq so will cancel leading to net ionic equation:

  9. Writing Net Ionic equations • In chemistry honors, there are three types of reactions for which you are expected to know how to write net ionic equations. One is called a “precipitation reaction”. Here the driving force for the reaction is the formation of a solid precipitate. • Example • Write net ionic equations for the following reactions • NaCl(aq) + Pb(NO3)2(aq) ------ > • First write the balanced molecular equation • 2NaCl(aq) + Pb(NO3)2(aq) ------ > 2NaNO3(aq) + PbCl2(s) • Total ionic equation • 2Na+1 + 2Cl-1 + Pb+2 + 2NO3-1 ------- > 2Na+1 + 2NO3-1 + PbCl2(s) • Net ionic equation • Pb+2 + 2Cl-1 -------- > PbCl2(s)

  10. Practice Problems • Potassium chloride + Lead (II) nitrate  • Sodium sulfate + Barium nitrate  • Silver nitrate + Sodium chromate  • Silver nitrate + Calcium chloride  • Sodium carbonate + Iron (III) nitrate 

  11. Acid – Base Neutralization reaction • Here the driving force for the reaction is the formation of a highly stable compound like water • Example • Balanced molecular equation • HCl(aq) + NaOH(aq) ------ > NaCl(aq) + H2O(l) • Total ionic equation • H+ + Cl-1 + Na+1 + OH-1 ----- > Na+1 + Cl-1 + H2O(l) • Net ionic equation • H+ + OH-1 ------- > H2O • In general, for any acid base reaction, the net ionic equation is the formation of water • acid + base  salt + water

  12. Practice Problems • H2SO4 + Ba(OH)2 • Hydrobromic acid + Potassium hydroxide  • Phosphoric acid + Calcium hydroxide  • Nitric acid + Lithium hydroxide  • Hydrochloric acid + Aluminum hydroxide 

  13. Formation of a gas • Here, the formation of a gas is the driving force. • Example • Balanced Molecular equation • FeS(s) + 2HCl(aq) ----- > H2S(g) + FeCl2(aq) • Total ionic equation • FeS(s) + 2H+ + 2Cl- ----- > H2S(g) + Fe+2 + 2Cl- • Net ionic equation • FeS(s) + 2H+ ------- > H2S(g) + Fe+2

  14. Colligative Propertiesof Solutions Section 14.2

  15. Colligative Properties When a nonvolatile (strong forces of attraction: little tendency to become a gas) solute is added to a solvent, three physical properties of the solvent will change: Boiling point elevation. Freezing point depression. Vapor pressure.

  16. Colligative Properties Colligative properties of a solution depend only on the number of solute particles in a solution and not on the nature of those particles. Boiling point elevation. Freezing point depression. Vapor pressure .

  17. Each solvent shows a characteristic: freezing point depression constant boiling point elevation constant

  18. The vapor pressure of a liquid depends on the ease with which water molecules can escape from the liquid’s surface. H2O(l) → H2O(g)

  19. If 10% of the molecules in a solution are nonvolatile solute molecules, the vapor pressure of the solution is 10% lower than that of the pure solvent. H2O(l) → H2O(g)

  20. A liquid boils when its vapor pressure equals atmospheric pressure. A solution will have a lower vapor pressure. and consequently a higher boiling point. Vapor Pressure Curve of Pure Water and Water Solution: Boiling Point Elevation

  21. A liquid freezes when its vapor pressure equals the vapor pressure of its solid. A solution will have a lower vapor pressure. and consequently a lower freezing point. Vapor Pressure Curve of Pure Water and Water Solution: Freezing Point Depression water vapor pressure curve solution vapor pressure curve ice vapor pressure curve

  22. Molality

  23. MOLALITY: moles of solute Kg of solvent The freezing point depression and the boiling point elevation are directly proportional to the number of moles of solute per kilogram of solvent.

  24. m = molality Symbols used in the calculation of colligative properties • Δtf= freezing point depression: oC • Δtb=boiling point elevation: oC • Kf = freezing point depression constant • Kb = boiling point elevation constant

  25. Osmosis and Osmotic Pressure

  26. Osmosis is the diffusion of water, either from a dilute solution or from pure water, through a semipermeable membrane into a solution of higher concentration.

  27. In osmosis,the net transfer of solvent is always from the more concentrated to the less concentrated solution. water passes through the cellophane 14.9

  28. Osmotic pressure is the pressure that must be exerted to prevent osmosis of solvent particles through a semipermeable membrane that is separating two solutions of different solute concentrations. Osmotic pressure is a colligative property.

  29. Freezing-Point Depression and Boiling-Point Elevation • The magnitudes of the freezing-point depression and the boiling-point elevation of a solution are directly proportional to the molal concentration (m), when the solute is molecular, not ionic.

  30. Freezing-Point Depression and Boiling-Point Elevation • The constant, Kf, is the molal freezing-point depression constant, which is equal to the change in freezing point for a 1-molal solution of a nonvolatile molecular solute. • The constant, Kb, is the molal boiling-point elevation constant, which is equal to the change in boiling point for a 1-molal solution of a nonvolatile molecular solute.

  31. electrolyte vs. nonelectrolyte solute • If you have a nonvolatile electrolyte, you must consider that the electrolyte dissociates into ions • 1 mole NaCl 1 mol Na+ and 1 mol Cl- • Therefore, multiply the molality OR the freezing/boiling point change by number of ions dissociated

  32. 16.9

  33. 1. The freezing point depression caused by a given concentration of a nonvolatile molecular solute a) depends on the solute. b) depends on the solvent. c) is always the same. d) cannot be determined.

  34. 2. Compared to the freezing point depression by ethylene glycol (C2H6O2,) for a given solvent, the freezing point depression caused by the same molal concentration of CaCl2 would be a) exactly the same. b) twice as large. c) three times as large. d) four times as large

  35. 3. What are the freezing and boiling points of a 0.1m solution of CaCl2 in water? a) -0.2°C, 100.1°C b) -0.6°C, 100.1°C c) -0.6°C, 100.2°C d) -0.6°C, 99.8°C

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