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Chapter 13

Chapter 13. Aqueous Solutions. Aqueous solutions: A solution in which the solvent is water. Solvent: Dissolving medium. Solute: Dissolved particles (can be liquid, solid, or gas). Solutions are homogeneous mixtures, and stable – can’t be filtered because the solute particles are too small.

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Chapter 13

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  1. Chapter 13

  2. Aqueous Solutions • Aqueous solutions: A solution in which the solvent is water. • Solvent: Dissolving medium. • Solute: Dissolved particles (can be liquid, solid, or gas). • Solutions are homogeneous mixtures, and stable – can’t be filtered because the solute particles are too small.

  3. Solvation • Solvation: Occurs when a solute dissolves. Individual ions or molecules become surrounded by solvent molecules. • Water is good at dissolving polar molecules and ionic compounds, because of the attraction of hydrogen bonds and the polarity of water itself.

  4. Some ionic compounds have stronger inter-molecular bonds than the attractions exerted by water. They will not dissolve, and so are called insoluble. • Grease dissolves in gasoline because they are both nonpolar attractions. They mix. • Rule: “likes dissolves like”

  5. Electrolyte and Nonelectrolyte • Electrolyte: Compounds that conduct an electric current in an aqueous solution or the molten state. • All ionic compounds are electrolytes. • Those that don’t dissolve in water like BaSO4, conduct in their molten state. • Nonelectrolytes: Don’t conduct. Ex: Most carbon elements. • Weak and strong electrolytes differ in the amount of the compound that ionizes in solution.

  6. Suspensions and Colloids • Suspensions: Mixtures from which some of the particles will settle slowly upon standing. Suspensions are heterogeneous , and can be filtered. • Colloids: Mixtures containing particles that are intermediate in size between suspensions and solutions. Examples: glass, gelatins, paints, aerosol sprays, smoke.

  7. Colloids are cloudy/milky in appearance but look clear when they are very dilute. • Colloids can’t be filtered and don’t settle out. • Colloidal particles exhibit the Tyndall effect, which is the scattering of visible light in all directions. http://www.youtube.com/watch?v=E2ULbn7Uxsk

  8. Colloidal particles move in chaotic movement called Brownian motion, which causes the light scattering. • Emulsion: Colloidal dispersions of liquids in liquids (paints).

  9. Water of Hydration • Water forms an integral part of the crystal structure of many substances – called the water of hydration. • We’ve studied several hydrates, such as CuSO4•5H20. The dot indicates that water is part of the crystal structure, although loosely bound. • Hygroscopic material can remove water from the air to become higher hydrates. • Many salts do this.

  10. Solution Formation • Things that help the dissolution of a solute (solvation) are: adding agitation, increasing temperature (in most cases), decreasing particle size. • Solvation is a surface phenomenon.

  11. Solubility • A saturated solution contains the maximum amount of solute for a given amount of solvent at a constant temperature. • There is solid at the bottom of the container, and the solid at the bottom is in dynamic equilibrium with the solid in the solution. • Solubility is the amount of a substance that dissolves in a given quantity of a solvent at a given temperature to produce a saturated solution. • Solubility is expressed in grams solute/100 grams solvent.

  12. A solution that contains less solute than a saturated solution is unsaturated. • A solution which contains more solute that it can theoretically hold at a given temperature is a supersaturated solution. • There is no undissolved solid, so therefore there is no dynamic equilibrium going on.

  13. You can initiate the crystallization of a supersaturated solution by seeding it with a tiny crystal. This is how they seed rain clouds. http://www.youtube.com/watch?v=nwonVY_cNS4

  14. Two liquids that dissolve in each other are miscible. • Two liquids that are insoluble in one another are immiscible.

  15. Factors Affecting Solubility • The solubility of most substances increases with increasing temperature, but some substances are exceptions (Na2SO4). • The solubility of gases is greater in cold water than hot water, because at higher temperatures, the gases escape to the atmosphere (they have more kinetic energy).

  16. As the partial pressure of a gas above a solution increases, the solubility of that gas increases (that’s how they bottle soda, under high pressure. When you open a soda, the pressure equalizes with the atmosphere and the gas begins to escape in tiny bubbles. Eventually, a soda left out will go flat).

  17. Henry’s Law states that at a given temperature, the solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid: S1/P1 = S2/P2

  18. Molarity • A dilute solution has a small amount of solute in a large amount of solvent. • A concentrated solution has a large amount of solute in the solvent. • Dilute and concentrated are relative terms, but a measurement of concentration is molarity. M = moles of solute/liters of solution

  19. Molality and Mole Fraction • These are ways to express the ratio of solute particles to solvent particles: molality(m) = moles of solute/kg of solvent • The big difference with molarity and molality is that molarity uses volume of solution in the denominator and molality uses mass of solvent in the denominator.

  20. Mole Fraction • The ratio of moles: XA = nA and XB = nB nA + nBnA+ nB

  21. Calculating Boiling Point and Freezing Point Changes • The change in b.p. can be determined and the molality of the solute that causes the change can be calculated: ΔTb = Kbmi, where Kb is a constant of the solvent, i is the van’t Hoff factor, which indicates the number of particles that each mole of solute produces in solution (i.e., i= 2 for NaCl and 3 for CaCl2).

  22. The van't Hoff factor, symboli, expresses how many ions and particles are formed (on an average) in a solution from one formula unit of solute. Examples: • One formula unit of NaCl will create two particles in solution, a Na+ ion and a Cl- ion. • One formula unit of CaCl2 will create three particles in solution, a Ca+ ion and two Cl- ions.

  23. One formula unit (molecule) of methanol CH3OH will create only one particle in solution, since methanol is a non electrolyte and does not dissolve as ions. • One formula unit (molecule) of HF will form one particle in solution, since it is a weak electrolyte (weak acid). • One formula unit (molecule) of NH3 will form one particle in solution, since it is a weak electrolyte (weak base). • The same equation can be used for f.p.: ΔTf = Kfmi

  24. Molecular Mass Determination • We can use the b.p. elevation or f.p. depression to determine the mass of an unknown solute. You must use a solvent with a known Kb or Kf, and you must measure the b.p. elevation or f.p. depression very carefully.

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