Chemistry 103 Spring 2010

Transcription

1 Today 1. Factors affecting solubility. 2. Colligative properties. Announcements 1. OWL homework for Ch. 14 due next week. 2. DROP deadline on Apr. 6, Tuesday. 3. ADD deadline on Apr. 12, Monday. 4. First midterm exam on Apr. 19 or CSULA closure on May 21 (furlough). 6. CSULA closure on May 31 (holiday). 7. Bring textbook and calculator to each lecture.

2 Factors affecting solubility Substances can mix to form solutions. The solubility of one substance in another substance depends on intermolecular forces (IMF s). Solutions have three categories of interactions: 1) solvent-solvent IMF s, 2) solute-solute IMF s, and 3) solute-solvent IMF s. Example: methanol CH 3 OH dissolving in water 2

3 The relative strengths of these various IMF s determine the overall enthalpy change (energy change) when the substances mix. Fig a) solvent-solvent IMF s => ΔH solvent b) solute-solute IMF s => ΔH solute c) solute-solvent IMF s => ΔH solute-solvent ΔH solution = ΔH a + ΔH b + ΔH c = ΔH soln ΔH < 0 means ΔH > 0 means Another important factor in forming solutions is the entropy change (ΔS) when the substances mix. entropy: a measure of the extent to which energy is dispersed throughout a system; a quantitative (numerical) measure of disorder. 3

4 Entropy is given the symbol S, and at absolute zero (0 K, C), all substances have zero entropy (no disorder): S = 0. In brief, processes that increase entropy (ΔS > 0) create more disorder and are favored, while processes that decrease entropy (ΔS < 0) create more order and are not favored. Examples: 1. Things fall apart over time. (constant energy input is required to maintain things) 2. Spilling water from a glass is much easier than getting the spilled water back into the glass. 4

5 When substances mix, there is a large entropy increase. The highly ordered pure solvent and pure solute both become disordered as they mix and become contaminated by each other. In some solutions, the large entropy increase (ΔS >> 0) overcomes an unfavorable energy change (ΔH soln > 0), i.e., IMF s within each pure substance are stronger than IMF s between solvent and solute. More entropy in Chapter 17. 5

6 Colligative properties of solutions colligative property: a property of a solution that depends only on the concentration of solute particles, not the type of particles. 1. Vapor pressure lowering P substance = X substance P substance (Raoult s Law) new vapor pressure = mole fraction of substance x vapor pressure of pure substance 2. Boiling point elevation (ΔT b ) ΔT b = K b m solute (this K is not an equil. constant) change in bp = bp constant x molality of soln 3. Freezing point lowering (ΔT f ) ΔT f = K f m solute (this K is not an equil. constant) change in fp = fp constant x molality of soln Remember, molality = moles of solute = mol = m kg of solvent kg 6

7 Example: Problem

8 Practice: Problem

9 Important: colligative properties are affected by the total amount of dissolved particles. 1 mole of NaCl dissolves to produce => 1 mole of Na + ions + 1 mole of Cl- ions total dissolved particles = 2 moles! i NaCl = 2 = i solute = number of particles per formula unit of the solute (van t Hoff) The molality solute in these calculations must account for the actual number of particles. Example: 3.22 moles of NaCl are dissolved in 5.00 L of water. What is the freezing point of this solution? 9

10 Osmotic pressure (Π) Figs and Chemistry 103 Spring 2010 Solute particles cannot move through the membrane but solvent particles can. (semipermeable) Entropy favors greater mixing (less purity), so solvent moves toward the higher concentration. The solvent flow is faster into areas with more solute and slower out of areas with more solute. osmosis: the net flow of solvent through a semipermeable membrane from lower concentration solution (more pure solvent) to higher concentration solution (less pure solvent). 10

11 osmotic pressure (Π): the pressure needed to equalize the rates of solvent flow; the pressure applied to the solution to stop osmosis. Mathematically, Π = crti where Π = osmotic pressure c = molarity of solution (M) R = gas constant (pressure) T = temperature (Kelvin) i = # particles/formula unit In summary, solutes lower the vapor pressure, raise the boiling point, lower the freezing point, and can lead to osmotic pressure in the presence of a semipermeable membrane. 11

12 Example: Problem

13 Announcements 1. OWL homework for Ch. 14 due next week. 2. DROP deadline on Apr. 6, Tuesday. 3. ADD deadline on Apr. 12, Monday. 4. First midterm exam on Apr. 19 or CSULA closure on May 21 (furlough). 6. CSULA closure on May 31 (holiday). 7. Bring textbook and calculator to each lecture. Before next class, 1. Study Chapter 14, esp Skim , skip reverse osmosis. 2. Start on OWL HW. 3. Read Chapter 15.1, 15.4, and