Chem 1151: Ch. 7

1. Chem 1151: Ch. 7 Solutions and Colloids

2. Physical States of Solutions Solutions: Homogeneous mixtures of two or more substances Particles uniformly distributed and transparent (clear) Particles in constant motion (KE), don’t settle under influence of gravity Solution = Solvent + Dissolved Solute Solvent is most abundant substance comprising solution Solute(s) is/are other substance(s) that are not the solvent Solutions usually liquids but may be gases or solids Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011; http://en.wikipedia.org/; http://www.medfinity.com/index.php?cPath=704_737

3. Solubility • Soluble substances: Dissolve completely in solvent • Insoluble substances: Do not dissolve in solvent • Immiscible: Liquid solute that does not dissolve in a liquid solvent • The extent of solubility can vary • Isopropyl alcohol and water are completely soluble in any proportion • Limit on how much sugar will dissolve • Oil and water form distinct, separate layers • Saturated solution: Solution with max amount of dissolved solute • Supersaturated solution: Solutions where amount of solute dissolved is greater than solute solubility Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

4. Calcium carbonate Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

5. Supersaturation • Preparation of a Supersaturated solution • Form a nearly saturated solution at high temp (higher solubility) • Cool solution to lower temperature (lower solubility) • “Seed” the solution with some solid solute  excess solute (above saturation) crystallizes • The remaining solution is saturated Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011 ;http://www.myrecipes.com/recipe/rock-candy-10000001176193/

6. Effects of Pressure and Temperature on Gas Solubility • Solubility of gases in water decreases as temp increases • CO2 is less soluble in warm soda than cold soda • Gas solubility is directly proportional to gas pressure • If gas pressure is doubled, solubility is doubled • Ex.1 Why your champagne is bubbly • Cold champagne (wine) is saturated with CO2, sealed under pressure • Pressure is relieved when bottle opened • Ex.2 The bends • Divers breath air under pressure that causes N2 to be more soluble in blood than occurs at 1 atm • If divers swims to surface too fast, N2 bubbles form in blood and joints • Joint pain • Paralysis • Death http://www.knightsbridgeinvest.com/realestateblog/scuba-diving-panama/

7. Formation of Solutions • When solid, ionic compounds placed in water, polar water molecules orient themselves by charge along surface of solid • Produces a shielding effect, allows water to remove ions from matrix (i.e., ions become hydrated) • Hydrated ions become evenly distributed throughout solution • Process continues until number of ions in solution results in saturation. • Ions begin to reform solid  equilibrium. • Similar results are observed with polar, non-ionic solids (e.g., sugar) Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

8. Formation of a Solution • Two reasons why solute won’t dissolve in solvent: • Forces between solute particles stronger than solvent particles • Solvent particles more attractive to each other than solute particles • “Like dissolves like” • Polar molecules dissolve in polar solvents (e.g., water) • Non-polar molecules dissolve in non-polar solvents (e.g., CCl4) Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

9. Predicting Solubility Predict the solubility of the following: NH3 in water O2 in water Ca(NO3)2 in water Mg3(PO4)2 in water Paraffin wax (nonpolar) in CCl4 BaCO3 in water Li3PO4 in water Soluble, NH3 is polar Insoluble, O2 is nonpolar Soluble Insoluble Soluble Insoluble Soluble Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

10. Rate of Solubility Insoluble compounds may be soluble over long periods of time (chemical weathering) Normally soluble solutes may dissolve slowly (rock candy dissolves slower than granulated sugar) Dissolving rate may be increased Crushing or grinding solute (increases surface area exposure) Heating solvent (molecules move faster, more collisions) Stirring or agitating Heat of Solubility • Heat is usually absorbed or released when solute dissolves in solvent • In endothermic processes, heat is absorbed by interaction between solute and solvent molecules • Removes heat from bulk solvent • In exothermic processes, heat is release by interaction between solute and solvent molecules • Adds heat to bulk solvent Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

11. Solution Concentrations Concentration: How much solute is in a solvent Molarity Percentage % w/w % w/v % v/v

12. Calculating Solution Concentrations (Molarity) Molarity 2.00 L of solution contains 1.50 mol of solute. 150 mL of solution contains 0.210 mol of solute. 315 mL of solution contains 10.3 g of C3H7OH.

13. Calculating Solution Concentrations A solution contains 100 g of water and 1.20 g of solute. What is the %(w/w) concentration? % w/w A solution is made by mixing 90.0 mL of alcohol with enough water to give 250.0 mL of solution. What is the %(v/v) concentration of alcohol in the solution?

14. Calculating Solution Concentrations A 150.0 mL sample of saltwater is evaporated to dryness. A residue of salt weighing 27.9 g is left behind. Calculate the % (w/v) of the original saltwater. A solution is made by dissolving 0.900 g of salt in 100.0 mL of water. Assume that 1.0 mL of water weighs 1.00 g, and the final solution volume is 100.0 mL. Calculate the %(w/w) and %(w/v) for the solution.

15. Solution Preparation Method 1: Mix solute and solvent Method 2: Dilute a concentrated solution with solvent Moles/L %(w/w) %(v/v) %(w/v) Measurements may be volumetric, gravimetric or both, depending on your concentration objective Mass may be converted by volume based on density, so that volume can be measured, not mass Volume not as accurate as mass D = m/V

16. Solution Preparation Describe how to prepare 1.00 L of 1.50 M CoCl2 solution Describe how to prepare 1.50 L of 0.50 M CoCl2 solution Describe how to prepare 200 mL of 0.200 M CoCl2 solution Describe how to prepare 500 mL of 1.6 × 10-4 M Pb(NO3)2 solution

17. Solution Preparation Describe how to prepare 250 mL of 0.900% (w/v) NaCl solution Describe how to prepare 100 mL of 12.0% (w/v) MgCl2 solution

18. Solution Preparation - Dilution This equation works for any form of concentration based on volume Describe how to prepare 250 mL of 0.100 M NaCl solution from 2.00 M NaCl Describe how to prepare 500 mL of 0.250 M NaOH solution from 6.00 M NaOH

19. Solution Stoichiometry • Stoichiometry deals with relative quantities of reactants and products in balanced chemical equation • We can now apply everything learned about mass, moles, concentrations and balancing equations to solutions Ex. 01: What volume of 0.200 M NaOH solution is needed to exactly react with 0.150 moles of HCl? We need 0.150 moles of NaOH for 0.150 moles of HCl (1:1 stoichiometry) NaOH solution is 0.200 M (moles/L)

20. Solution Stoichiometry Ex. 01: What volume of 0.185 M NaOH solution is needed to exactly react with 25.0 mL of 0.255 M HCl? We need to figure out moles in 25.0 mL of 0.255 M HCl Now we can find volume of of 0.185 M NaOH from moles HCl and stoichiometry

21. Solution Properties – Electrical Conductivity • Electrolytes: Solutes that form water solutions capable of conducting electricity • Solutes dissociate to form ions Strong Electrolytes (e.g., HCl) dissociate completely, strong conductors Weak Electrolytes (e.g., Acetic acid) dissociate slightly, weak conductors Electrolytes maintain precise osmotic gradients at the cellular level. These regulate body hydration and blood pH, and are critical for nerve and muscle function. http://healthic.net/supplements/electrolytes-drink-enabling-transmission-of-nutrients-in-body/

22. Colligative Properties • Depend only on concentration of solute particles present, not identity of solute • Solutions containing 1 mol sugar and 1 mol salt would have identical colligative properties • Closely-related colligative properties include: • Vapor pressure • Boiling point (BP) • Freezing point (FP) • Colligative properties differ between solutions and pure solvents: • Vapor pressure of water (solvent) above solution is lower than vapor pressure of pure water • Causes higher BP and lower FP of solutions, compared with pure solvent • These differences can be calculated with the following equations: m = solution molality (same as molarity for dilute solutions) K = Derived constant (°C/M) related to BP or FP of solvent n = number solute moles in solution when 1 mol solute dissolves

23. Colligative Property: Osmotic Pressure • Water can pass through membrane but sugar cannot • Net flow into sugar side to increase vapor pressure • Continues until osmotic pressure balances tendency of water to flow to sugar side (until equilibrium) • In general, net flow of solvent through semipermeable membranes is always from more dilute solution into more concentrated solution • Osmotic pressure can be calculated with the following: Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

24. Colloids • Similar to solutions • Homogeneous mixtures of 2 or more components • More of one component than others • Colloids defined by: • Dispersing medium: Like solvent • Dispersed phase: Like solute • Main differences: • Colloids have much larger particles (up to 10X larger) than solutions • Gives them cloudy appearance • Colloids: • Glues • Gels (shaving cream) • Cheese