Solutions (& Their Properties), Chapter 11

Transcription

1 Solutions (& Their Properties), Chapter 11 We ll spend little time on properties. I. Solutions Why should you care? What does heparin do? Go to the Medlineplus encyclopedia and search heparin? A. Concepts/Terms 1. Pure substances cannot be broken down into smaller units by physical means. (Example: plants & H2O.) 2. Components of pure substances occur in fixed ratios. 3. What fraction of the matter in here is in pure substances? 1

2 B. Mixtures 1. Component of mixtures can be separated by physical means. Example: 2. Components of mixtures usually can occur in a continuum of values. 3. What fraction of matter in Sm 319 is in mixtures? 4. Mixtures may be: a) homogeneous, e.g. b) heterogeneous, e.g. 2

3 C. A homogeneous mixture is a solution 1. Imagine looking into a unit volume of a sample with a very powerful microscope. If one unit volume looks much like another, the mixture is called homogeneous. 2. This gets complicated depending on volumes sizes. (Cheese pizza vs. pepperoni pizza?) 3. For our purposes the component of a solution present in the greatest amount is the solvent. All other components are solutes. Should we do the tequila story? Fig 11-2 Burn this into your frontal lobes. Then animate it. 3

4 II. Energy Changes & Solution Processes A. For mixing 2 liquid phases, consider only 2 states: 1. Some liquid pairs are incompatible; the left state is favored. (Ex.?) 2. Some pairs are totally miscible, yielding the state on right. (Ex.?) 3. Some solvent pairs exhibit partially solubility. Can you draw a picture for that? 4

5 Best theoretical approach for making predictions: ÄG = ÄH TÄS Have we defined the variables yet? ÄG ÄH ÄS T If our initial state is the left one shown above and the final state is the right one, can you draw a reaction coordinate diagram for two completely miscible solvents? 5

6 You can draw these for physical processes, too. Which state should have the higher plateau? (Hill height?????) 6

7 Be careful with this. Some students tend to focus completely on ÄH issues and ignore ÄS issues. This can lead one astray, particularly for non-polar substances dissolving in water. Can you think of biological examples? To look at some ÄH components see Fig. 11.4, p B. How does water dissolve ionic compounds (and why is water such an interesting solvent?) 1. See Fig of NaCl, p ÄH? 7

8 2. We really need to animate this. Useful web sites: go to animations, then dissolve.html 3. Compare initial & final states, all components re. for ÄG = ÄH TÄS. If initial is separate solute & soln.: a) If substance dissolves to a significant extent, ÄG for that process has a sign. Rxn. Progress diagram? b) The ÄH term relates to bonding interactions (ex. Can you identify some of types?) c) For some solutes dissolving in water, ÄS term dominates. i) Macroscopic entropy issues always favor solution as opposed to separate phases. Fig. 11.2, next page. ii) Microscopic entropy issues may not favor the dissolved state. (The Hydrophobic Effect & biological membrane formation.) Can you track all of the ÄH TÄS components? 8

9 Does Table 11-2 bother you? 1. Did you expect all +ÄH values? (Bonds breaking?) 2. Can you imagine ÄS values? (Increased order?) d) The Hydrophobic Effect predicts that solubility of a solute in water is inversely proportional to the solute s non-polar surface area. 2 i) Often microscopic entropy (ÄS) driven ii) Relates to H O forming ordered cages around nonpolar surfaces to maintain Hydrogen Bonding. Try Probs & 11.2, p Aside: It helps to be able to picture physical & chemical processes in your mind. Download Boltzmann 3D & use it. Thanks, Dr. R.B. Shirts! 9

10 III. Units of Concentration A. Molarity (M) = mole solute L solution. 1. Chemists like this a lot. (Stoichiometry issues!) 2. Volume measurements are often convenient. B. Mole Fraction ( ) = mole component total moles soln. 1. Physical chemists like this. Not affected by T. 2. Good for gases. 10

11 C. Mass percent (%w/w) = Variations: (mass component total mass) x Parts per million (ppm) is x 10 instead of x Parts per billion (ppb) is 1 & 2 are often used in environmental or toxicological studies where concentrations of solutes are very low. 3. Biochemists and biologist often use: All vol. are ml volume % (%v/v) = (volume component total vol) x 100 & % wt. to vol. (%w/v) = (mass component (g) total vol) x

12 D. Molality (m) = mole solute kg solvent 1. Notice that this has solvent, not soln in denominator. 2. Physical chemists like this one: it is T independent. Aside: The volume of one mole of H2 O (l) does change as you heat it from 0 C to 100 C. Does the number of moles change? Does the mass? Problems to try: 11.3, 11.4, 11.5, 11.6, 12

13 IV. Some Factors Affecting Solubility A. Solubility Limits. 1. Saturation (Is there a limit to how sweet tea can be?) 2. Superstaturation (system not at equilibrium) 3. Brief discussion of Fig. 11.5, p B. Effect of Temperature 1. Increasing T generally increases solubility of solid solutes in water. (Fig. 11.6, p. 401) 2. Increasing T generally decreases the solubility of gaseous solutes in water. (Fig. 11.7, p. 401) We can discuss this more thoroughly in Chapter

14 C. Effect of Pressure 1. Henry s Law: solubility = k P P = pressure k = Henry s Law constant, specific to a given gas and solvent some values from R. Sander: substance k (M/atm) H These k H values for H2O as solvent. N O We ll use one of these shortly. 2 He Xe

15 2. Physiological Relevance: a) The Bends Laundering $$$??? (Licence to Kill ) b) Acute altitude sickness (mountain climbers). What does Henry s Law say will happen to the [O 2(aq)] as you go to higher in the atmosphere? Your biochemical capacity to adapt to changes in altitude is a thing of beauty. Prob & 11.12, p

16 Aside: Human respiratory physiology & hemoglobin (Hb). Structure is from pdb 1a3n. Note: Not directly from textbook. The logic with this chemistry helps you understand why (& how much) Hb in your blood is useful, and why being anemic (define?) is disadvantageous. You can enhance your knowledge on this topic by solving the following problem. 16

17 Calculate the [O ] on a normal weather day in 2 total Charleston, SC in molar units for: (By [O ] I 2 total mean both the O dissolved in solution and the O bound to Hb.) 1. water arterial blood with [Hb] = M. Think about the problem for a second. What information/logic do you need to solve this problem? 3 17

18 For part 1 you need: a) normal atmospheric pressure at sea level, & b) what fraction of the air is O 2 (so you can use Dalton s Law {p. 330} of partial pressures), then Henry s Law.. For part 2 you need: a) what fraction of the Hb would have O 2 bound to it in arterial blood & b) stoichiometry of O 2 binding to Hb. For the first item, assume 0.98 (reasonable for a healthy person at rest). For the second item, use the following equation: 4 O 2(aq) + Hb(aq) Hb (O ) 2 4(aq) Does this Stoichiometry seem reasonable based on the structure of Hb shown on p. 22? Finally, how many times more O 2 can be carried in your arterial blood than could be carried in blood devoid of Hb? 18

19 We may come back to some of these (see below) topics later in the semester, but do not focus on them just yet. V. Physical Behavior: Colligative Properties VI. Vapor-Pressure Lowering: Raoult s Law VII. Boiling Point Elevation, Freezing Point Depression VIII. Osmosis & Osmotic Pressure IX. Uses of Colligative Properties 19