Chapter 14

1. Chapter 14 Solutions

2. “how things dissolve” • Solutions – homogeneous mixtures of 2 or more substances • Solvent – does the dissolving (H2O is the universal solvent) • Solute – what gets dissolved 3 things happen when dissolving occurs: 1. solvent molecules split up 2. solute units split up 3. Form solvent/solute attractions

3. Rules for solutions “likes dissolve like” • ~ water is highly polar, dissolves polar solutes and ionic solutes fairly well • Polar – solvent/solute attraction H2O NaCl(s)  Na+(aq) + Cl-(aq) • Strong electrolytes dissociate into ions

4. nonpolar – molecules split up b/c of the weak attraction into smallest molecule unit H2O C12H22O11 C12H22O11 Non-electrolytes undergo molecular solvation Ex. oil/water vs. oil/benzene Nonpolar/polar nonpolar/nonpolar

5. ~ liquid in liquid – often volumes are not additive, called miscible. Why? intermolecular attractions 50 mL H2O + 50 mL methanol ≈ 97 mL total gas in liquid – same attraction • gases tend to be less soluble at higher temps. • The higher the pressure, the more soluble, b/c the gas is forced into the solution more. If pressure decreases the gas is less soluble

6. Finely divided solids dissolve much more rapidly than large crystals • Granulated sugar vs. sugar cubes • Unsaturated sol’n – a sol’n that is capable of dissolving more solute. (ex. 15.0g NaCl in 100g of H2O at room temp.) There are no un-dissolved solutes remaining.

7. Saturated sol’n – a sol’n has its maximum amount of solute dissolved in it. You always see un-dissolved solute. (ex. 40.0g NaCl in room temp. water) When a sol’n is saturated, the solute is still dissolving – it’s just that the solute is also re-crystallizing out of the sol’n at the same rate, so there is no apparent change. • Supersaturated sol’n – by heating a saturated sol’n and carefully cooling it, you can get more solute dissolved then theoretically possible. It is an unstable condition that is the basis for rock candy.

8. Spontaneity of the Dissolution Process • Assume solvent is a liquid • Major factors that affect dissolution of solutes (dissolving of solutes) • change of energy content, DHsolution • exothermic favors dissolution • endothermic does not favor dissolution • change in disorder, or randomness, DSmixing • increase in disorder favors dissolution • increase in order does not favor dissolution • Best conditions for dissolution • exothermic & disordered

9. Molarity: M= n/V n = moles of solute V = Liters of solution • Molality: m = n/kg n = same kg = mass of solvent • Mole fraction (x) number of moles of one part per moles of all parts of the solution xa = naxb = nb etc… ntotal ntotal

10. Colligative properties • properties of solutions that depend only on the number of particles dissolved and not the kinds of particles dissolved. 4 types • Vapor Pressure • Boiling Point Elevation • Freezing Point Depression • Osmotic Pressure

11. 1. Vapor pressure -- Pvap goes down with increase in concentration. Water vs. water + sugar *more sugar higher Pvap vs. lower Pvap In solns, fewer molecules have the chance to turn into vapor Raoult’s Law – the vapor pressure of an ideal solution is directly proportional to the mole fraction of the solvent in the solution Pvap = (Pvapo)(xsolvent) ~ ideal gases obey PV = nRT very well ~ Raoult’s Law is close to ideal when the solute and solvent have very similar I.F.

12. Effect of Pressure on Solubility • Pressure changes have little or no effect on solubility of liquids and solids in liquids • Pressure changes have large effects on the solubility of gases in liquids • why carbonated drinks fizz when opened • cause of several scuba diving related problems including the “bends”

13. 2. & 3. Boiling Point Elevation & Freezing Point Depression • solutions exist as liquids over a wider temp. range • solutions move the triple point of a substance further down the curve.   pg. 563 & next slide show a phase diagram comparison between pure solvent and a solution • Why? Higher B.P. – more attractions need to be overcome before boiling can occur. Lower F.P. – ions get in the way of freezing.

14. Phase Diagram

15. Dissociation of Electrolytes & Colligative Properties • Electrolytes have larger effects on boiling point elevation & freezing point depression than soluble nonelectrolytes • one mole of sugar dissolves in water to produce one mole of aqueous sugar molecules • one mole of NaCl dissolves in water to produce 1 mole of Na+ & 1 mole of Cl- ions • colligative properties depend on number of dissolved particles • expect twice the effect for NaCl than for sugar

16. Ex. for F. P & B.P. • Salt in water for pasta raises the B.P. • Which of the following raises the B.P. more? • 0.1 M HF vs. 0.1 M HCl • Weak electrolyte vs. strong electrolyte (s.e. dissociates thus higher B.P.) • 0.1 M CH3OH vs. 0.1 M CH3COOH • Nonelectrolytevs. weak electrolyte (w.e. slightly soluble so higher B.P.) • Salt on the road ~ often CaCl2 (3 particles dissolved) is chosen over NaCl (2 particles dissolved) CaCl2 lowers F.P. more • Homemade ice-cream ~ Yummy!

17. B.P & F.P. Equations • Boiling point elevation ∆Tb = imkb ∆T = change in temp. m = molality k = constant i = van’thofffactor ~ measures the extent of ionization (number of dissolved particles) • Freezing point depression ∆Tf = imkf pg 564. table 14-2

18. 4. Osmotic pressure • the pressure produced on the surface of a semipermeable membrane by osmosis. • Osmosis – net flow of solvent between two solns separated by a semipermeable membrane • Solvent passes from lower concentration soln. into higher concentration soln. • Ex. of semipermeable membranes ~ skin, cell membranes, cellophane, and saran wrap • π= MRT π= symbol for osmotic pressure M = molarity T = temp R = 0.082057 L atm/mol K

19. Example problems Ex. 1) What is the molality and mole fraction of 50.0 g potassium chloride in 425 g water? Ex. 2) 15.0 g of ethanol is dissolved in 750. g of formic acid. The freezing point of the solution is 7.20oC. The freezing point of pure formic acid is 8.40oC. What is kf for formic acid? Ex. 3) What is the molar mass (grams/mole) of 30.0 mL of an unknown substance if 0.300 g of that substance has an osmotic pressure of 0.400kPa at room temperature?

20. Colloids • Mixtures that have particle sizes between those of true solutions and suspensions (suspensions have particles that are very large, they will settle out unless the mixture is constantly stirred). • Fog, smoke, paint, milk • The Tyndall Effect – colloids scatter light when its shined on them.

21. Hydrophilic & Hydrophobic Colloids • Hydrophilic – water loving colloids • Blood plasma, some biological proteins • Hydrophobic – water hating colloids, require emulsifying agents to stabilize in water (emulsion helps keep two things together that normally would not mix) • Milk – emulsion of fat and proteins mixed in water, casein is the emulsifier • Mayonnaise – oil and eggs with water, lecithin from egg yolk is the emulsifier • Soaps and detergents are excellent emulsifying agents ~ hard water reacts with soap anions and precipitates making bathroom scum

22. Hydrophilic & Hydrophobic Colloids

23. Medicines that are injected into humans such as shots must be at the same concentration as the existing chemical in our blood. These solutions are called isotonic. For example, if the medicine contains potassium ions, they must be the same concentration as the potassium ions in our blood. Why do medicines have to be made that way?