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Ionic Solutions and Colligative Properties

This chapter covers the dissociation equations of various compounds, classification of electrolytes and nonelectrolytes, and the colligative properties of solutions.

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Ionic Solutions and Colligative Properties

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  1. CHAPTER 14 Ions in Aqueous Solutions and Colligative Properties

  2. Dissociation Equations NaCl(s)  Na+(aq) + Cl-(aq) AgNO3(s)  Ag+(aq) + NO3-(aq) MgCl2(s)  Mg2+(aq) + 2 Cl-(aq) Na2SO4(s)  2 Na+(aq) + SO42-(aq) AlCl3(s)  Al3+(aq) + 3 Cl-(aq)

  3. Dissolution of sodium Chloride

  4. Double replacement forming a precipitate… Double replacement (ionic) equation Pb(NO3)2(aq) + 2KI(aq)  PbI2(s) + 2KNO3(aq) Complete ionic equation shows compounds as aqueous ions Pb2+(aq) + 2 NO3-(aq) + 2 K+(aq) +2 I-(aq)  PbI2(s) + 2K+(aq) + 2 NO3-(aq) Net ionic equation eliminates the spectator ions Pb2+(aq) + 2 I-(aq)  PbI2(s)

  5. Solubility of Ionic Compounds

  6. The ability of a solution to conduct an electric current can be measured with a simple device. The ammeter measures the flow of electrons (current) through the circuit. If the ammeter measures a current, and the bulb glows, then the solution conducts. If the ammeter fails to measure a current, and the bulb does not glow, the solution is non-conducting.

  7. Definition of Electrolytes and Nonelectrolytes An electrolyte is: • A substance whose aqueous solution conducts • an electric current. A nonelectrolyte is: • A substance whose aqueous solution does not • conduct an electric current. Try to classify the following substances as electrolytes or nonelectrolytes…

  8. Electrolytes? • Pure water • Tap water • Sugar solution • Sodium chloride solution • Hydrochloric acid solution • Lactic acid solution • Ethyl alcohol solution • Pure sodium chloride

  9. Answers to Electrolytes ELECTROLYTES: NONELECTROLYTES: • Tap water (weak) • NaCl solution • HCl solution • Lactate solution (weak) • Pure water • Sugar solution • Ethanol solution • Pure NaCl But why do some compounds conduct electricity in solution while others do not…?

  10. Ions tend to stay in solution where they canconduct a current rather than re-forming a solid. The reason for this is the polar nature of the water molecule… • Positive ions associate with the negative • end of the water dipole (oxygen). • Negative ions associate with the positive • end of the water dipole (hydrogen).

  11. Some covalent compounds IONIZE in solution Covalent acids form ions in solution, with the help of the water molecules. For instance, hydrogen chloride molecules, which are polar, give up their hydrogens to water, forming chloride ions (Cl-) and hydronium ions (H3O+).

  12. Ionization of HCl makes it a strong electrolyte

  13. Strong acids such as HCl are completelyionized in solution. Other examples of strong acids include: • Sulfuric acid, H2SO4 • Nitric acid, HNO3 • Hydriodic acid, HI • Perchloric acid, HClO4

  14. Weak acids such as lactic acid usually ionize less than 5% of the time. Many of these weaker acids are “organic” acids that contain a “carboxyl” group. The carboxyl group does not easily give up its hydrogen.

  15. Because of the carboxyl group, organic acids aresometimes called “carboxylic acids”. Other organic acids and their sources include: • Citric acid – citrus fruit • Malic acid – apples • Butyric acid – rancid butter • Amino acids – protein • Nucleic acids – DNA and RNA • Ascorbic acid – Vitamin C This is an enormous group of compounds, these are only a few examples.

  16. However, most covalent compounds do not ionizeat all in solution. Sugar (sucrose – C12H22O11), and ethanol (ethyl alcohol – C2H5OH) do not ionize - That is why they are nonelectrolytes!

  17. Colligative Properties Colligative properties are those that depend on the concentration of particles in a solution, not upon the identity of those properties. • Boiling Point Elevation • Freezing Point Depression • Osmotic Pressure

  18. Freezing Point Depression Each mole of solute particles lowers the freezing point of 1 kilogram of water by 1.86 degrees Celsius. Kf = 1.86 C  kilogram/mol

  19. Boiling Point Elevation Each mole of solute particles raises the boiling point of 1 kilogram of water by 0.51 degrees Celsius. Kb = 0.51 C  kilogram/mol

  20. Freezing Point Depression and Boiling Point Elevation Constants

  21. The van’t Hoff Factor, i Electrolytes may have two, three or more times the effect on boiling point and freezing point, depending on its dissociation. T = i K  m

  22. Dissociation Equations i = 2 NaCl(s)  Na+(aq) + Cl-(aq) i = 2 AgNO3(s)  Ag+(aq) + NO3-(aq) i = 3 MgCl2(s)  Mg2+(aq) + 2 Cl-(aq) i = 3 Na2SO4(s)  2 Na+(aq) + SO42-(aq) AlCl3(s)  Al3+(aq) + 3 Cl-(aq) i = 4

  23. Preventing icing of roads using CaCl2

  24. Ideal vs. Real van’t Hoff Factor The ideal van’t Hoff Factor is only achieved in VERY DILUTE solution.

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