Solutions (Concentration)

Transcription

1 Solutions (Concentration) I. INTRODUCTION A. Types of matter (see figure in Compounds) 1. Pure substances: diamond is pure really clean water is pure table salt is pure??? 1

2 2. Mixtures (reminder?) a) heterogeneous (not same all over, ex.: pepperoni piazza ) b) homogeneous (same all over, but view? example: Koolaide, sweet tea?) c) colloids between hetero- & homogeneous in chunk size B. omogeneous mixture: solution (soln.), important in medicine. 2

3 II. SOLUTIONS (phases & definitions) A. We often think of solns. as liquids, other phases are possible. Ex?? B. It is useful to define 2 components of a soln.: 1. The solvent is (usually) the component present in greater amount. 2. The solute(s) is component present in lesser amount. There may be more than one solute. 3

4 III. SOLUTION CARACTERISTICS A. Distribution of components is uniform. B. Components do not settle on standing. C. Components cannot be separated by filtration. D. Solute-solvent ratios: continuous range. (Ex: strong vs. weak coffee.) E. Solns. are clear. Do not scatter light. Clear: opposite of cloudy. F. Solns. can be separated into pure components by physical means. Ex.: NaCl and 2 O in sea water. 4

5 IV. SOLUBILITY (usually has limits) A. Solubility has a number of meanings: 1. general process of solute dissolving in solvent 2. quantity of solute that will dissolve in solvent: a) Max. amount of solute that can dissolve (sweet tea?)??? b) When the max. amount is dissolved, soln. is saturated. c) A soln. can be supersaturated for short periods of time (Coke that explodes when bumped), but a supersaturated soln. is not at equilibrium., etc.) d) Comment on completely miscible liquids. 5

6 B. Nature of Solute and Solvent 1. Like dissolves like! 2. Usual focus is on polarity, but consider ydrogen Bonding too. 3. You are really try to compare 2 possible outcomes. Then, which has lower Gibbs Free Energy (G value) (i.e., stability). ΔG = Δ TΔS If ΔG is solubility is high. 6

7 Imagine salt dissolving in water: 2 O NaCl(s) Na + (aq) + Cl (aq) Remember what Δ means: Δ = final initial final = all bonding, heat, etc. interactions in the final state initial = all bonding, heat, etc. interactions in the initial state 7

8 C. Temperature Effects on Solubility 1. Solids & liquids dissolving in liquid: solubility increases as temp. increases (sugar/tea). 2. For gases dissolving, solubility often decreases as T increases: a) Pepsi, Coke, etc. b) Thermal pollution by industrial heat sources. 8

9 D. Pressure 1. As pressure increases, gas solubility increases. 2. Quantitatively, enry s Law: k C dissolved gas molecules = P of gas above liquid C = concentration P = pressure k = enry s Law constant, which is specific for a given gas solute and a given solvent at a specific temp 9

10 solute k (atm M) for 2 O 25 C O N CO Examples at water depth = 99 ft? We ll return to this after we learn what M is. 3. Applications: a) yperbaric O 2 : gangrene, CO poisoning, etc. b) bends: N 2 solubility, scuba and the bends diving re. Boyle!!! c) human respiratory (biochemical) adaptation to high altitude 10

11 V. CONCENTRATION: UNITS & DILUTION A. This is tremendously important when we apply solutions to life. 1. Examples: a) ow strong do you like your coffee (tea)? b) Effective drug dose vs. overdose 2. All concentration units are some type of ratio, for us: quantity of solute quantity of solution 11

12 B. Percent Concentration (nice for making solns.) Definition of %: ratio to 100: part whole = % 100 Rearrange to get % = (part whole) x % weight: solute wt to volume soln. (% w v) % (w v) = (g solute ml of solution) x 100 Problem: Calculate % (w/v) of soln containing g NaCl in volume of L. 12

13 2. percent volume solute to volume soln. (Alcohol in drinks) % (v v) = (ml solute ml of solution) x percent weight solute to weight soln. (% w w) % (w w) = (g solute g of solution) x

14 C. Molarity (chemists like molarity!), abbrev.: M 1. Definition: molarity = mole solute L solution 2. Use mol L instead of M. elps w/ cancelling. Problem: Calculate the conc in M of soln whose vol is 855 ml that contains g of KCl. D. Parts per million (ppm) Ratio to 10 6, usually by wt. Compare to %, that was ratio to 100. Used in environmental toxicity settings. 14

15 E. Dilution (like with concentrated orange juice?) M 1 V 1 = M 2 V 2 where M = conc. V= volume 1 = initial 2 = final Problem: If you dilute 65.0 ml of 8.0 M Cl, to a final volume of L, what will be the concentration of the diluted solution. 15

16 VI. Solutions & Water as a Solvent 1 st see 2 O as a liquid: Molecular Motion in Water: A. ow does water dissolve ionic compounds? 1. Animation: Draw a picture of a cation dissolved in 2 O 16

17 2. Compare initial & final states, all components re. ΔG = Δ TΔS a) If substance dissolves readily, ΔG is (sign). b) Δ term relates to bonding interactions (ex. Bonding) c) For some substances dissolving in water, the ΔS term dominates (The ydrophobic Effect). B. Electrolytes 1. Strong, ex.: NaCl falls apart completely to give Na + & Cl ) 2. Weak, ex.: acetic acid (C 3 COO) falls apart partially) 17

18 C. ow water dissolves covalent compounds 1. Draw picture of ethanol(aq) (C 2 5 O). Compare dissolved vs. two separate phases. a) ydrogen Bonds do form. b) Is ydrogen Bond formation between 2 O & C 2 5 O new ydrogen Bond formation (which would contribute favorably to Δ), or are we just swapping one Bond for another (i.e., no net gain in Δ)? c) Ethanol is so soluble in water because it has a small nonpolar surface area. Identify the non-polar part C 2 5 O. Is C 6 13 O more or less soluble in water than C 2 5 O? 18

19 C C O C C C C C C O 19

20 Alcohol Chain Length: Water Solubility &Boiling Point Name Structure solubility in 2 O (g/100 ml) b.p. (C o ) 1-propanol C C C O completely miscible butanol C C C C O 7.4 ~ pentanol C C C C C O hexanol C C C C C C O All values from the 10th edition of the Merck Index except for the solubility of 1-hexanol. That value is from: 20

21 Based on the table on p. 20: Is non-polar surface area or ydrogen Bonding more important in determining the solubility of organic alcohols in water? Circle one non-polar surface area ydrogen Bonding 21

22 1. What about dissolving large non-polar molecules? The ydrophobic Effect. Does gasoline dissolve in water? (Use octane (C 8 18 ) to represent gasoline.) a) Tricky, because it can t be viewed strictly as bonding. b) Relates to the ordering of 2 O molecules into cage-like structures around non-polar solutes. Let s look at an animation: ydration shell dynamics of a hydrophobic particle: 22

23 c) Is formation of ordered structures inherently favorable? d) Draw pictures of the two states: C 8 18 & water as separate phases C 8 18 dissolved in water Which state has more favorable Δ and ΔS terms? Try making a list for each state! Closing note: We will look at colligative properties, particularly osmosis, after we understand more about biological membranes. 23