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Chapter 13

Chapter 13. Properties of solutions. Solution Formation. How do solutions form?. Why do some things (solutes) dissolve in water (solvent) and yet others don’t?. Sweet’n’Low vs. Equal in water Factors affecting solubility? Assume 3 distinct steps for solution formation:

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Chapter 13

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  1. Chapter 13 Properties of solutions

  2. Solution Formation

  3. How do solutions form? • Why do some things (solutes) dissolve in water (solvent) and yet others don’t?

  4. Sweet’n’Low vs. Equal in water Factors affecting solubility? Assume 3 distinct steps for solution formation: Expanding the solute (H1) - endothermic Expanding the solvent (H2) - endothermic Interaction of solute and solvent (H3) - exothermic Hsoln = H1 + H2 + H3 Oil and water NaCl and water Energetics in solution formation

  5. Soluble or not?

  6. Spontaneity and disorder • Usually, spontaneous processes are exothermic (from CHEM 1211) • Exothermic means….. • Take a look at a solution of ammonium nitrate in water. How does this feel when you touch it? • Not all spontaneous processes are exothermic; some are…. • Why? An increase in disorder, or Entropy of the system (in this case the solution).

  7. Assessing Entropy changes • Na2SO4 (s) + 10 H2O (g)  Na2SO4.10H2O (s) (a) Does the system more or less ordered in this process? (b) Does the entropy of the system increase or decrease?

  8. Assessing Entropy changes • Does the entropy of this system increase or decrease when the stopcock is opened to allow mixing of the gases?

  9. Assessing Entropy changes • Silver chloride (AgCl) is essentially insoluble in water. Would you expect a significant change in the entropy of the system when 10g of AgCl is added to 500 mL of water? • Can you draw a picture that would represent AgCl in water?

  10. Solution Concentrations • Unsaturated • Saturated • Supersaturated

  11. Which solution type is it? • The solubility of Cr(NO3)3.9H2O in water is 208 g per 100 g of water at 15 C. A solution of Cr(NO3)3.9H2O in water at 35 C is formed by dissolving 324 g in 100 g of water. When this solution is slowly cooled to 15 C, no precipitate forms. • Is this solution unsaturated, saturated or supersaturated? • How might crystallization of this solution be initiated?

  12. Factors affecting solubility • For a high solubility, you want ________ interactions between solute and solvent. • Like dissolves like (nonpolar vs. polar) • Hydrogen bonds for aqueous solubility

  13. Predicting solubility patterns • Predict whether each of the following is more likely to dissolve in carbon tetrachloride (CCl4) or in water: C7H16, Na2SO4, HCl, and I2. • Which of the following 2 vitamins would be more soluble in water? Vitamin A Vitamin C

  14. Soda – bottled under high pressure of CO2 (high conc. of dissolved CO2 in liquid) Why does soda ‘fizz’ when opened? Relationship between gas pressure and conc. of gas dissolved gas: Henry’s Law; C=kP (William Henry – 1801) C = Concentration k = Henry’s Law Constant (different for each solute/solvent pair) P = Partial pressure Amount of gas dissolved in a solution  pressure of gas above the solution Pressure Effects

  15. Bottle of ‘Jolt’ cola at 25 °C, has CO2 gas present at 7.5 atm above the solution. Assuming that PCO2 in atmosphere is 4.0 x 10-4 atm, calculate equilibrium concentrations of CO2 in cola both before and after bottle is opened. Henry’s Law Calculation

  16. Solids in water – Gases in water – Temperature Effects

  17. Quantitative terms: Concentrated, dilute Qualitative expressions: Molarity (M); Units: moles / L Mass percent (weight percent) Mass of solute x 100 Mass of solution Mole fraction (); Mixture of A, B and C: A = nA / (nA + nB + nC) Molality (m); Units: moles of solute / kg of solvent Solution composition

  18. 10.0g of ethanol (C2H5OH), mixed with 100.0g of water, to give a final volume of 110 mL. Calculate: (i) Molarity, (ii) mass percent, (iii) mole fraction and (iv) molality of ethanol in solution. Composition calculations

  19. Solution composition - ppm • An aqueous solution contains 0.00023g of SO2 per 1000g of solution. What is the concentration of SO2 in ppm?

  20. Converting between concentrations • An aqueous solution of hydrochloric acid contains 36% HCl by mass. • (a) Calculate the mole fraction of HCl in the solution. • (b) Calculate the molality of HCl in the solution.

  21. A property of a solution that depends only on the quantity of solute particles present, not on their chemical identity. Look at several of these: Boiling point elevation Freezing point depression Vapor pressure lowering Osmosis and Osmotic Pressure Colligative properties

  22. Impurities lower freezing points and raise boiling points of liquids T = Kb .msolute T = Kf .msolute Sugar in water to make candy antifreeze in radiator salt on icy roads Freezing Point Depression / BPE

  23. What mass of ethylene glycol (C2H6O2), the main component of antifreeze, must be added to 10.0L of water to produce a solution for use in a car’s radiator that freezes at –23.3 °C? Density of water = 1g/mL. Kf (H2O) = 1.86 °C.kg/mol Freezing point depression

  24. Determining molecular weight • FPD (and most colligative properties, in fact) can be used to determine the molecular weight of a solute: • A sample of a human hormone weighing 0.546g was dissolved in 15.0g benzene. The fpd was determined to be 0.240 C. What’s the molecular weight of the hormone? Kf(benzene) = 5.20 °C.kg/mol

  25. Pressure of gas (vapor) above solvent/solution (higher for more volatile solvents) Liquid solutions – different properties to pure liquids Antifreeze (ethylene glycol) Salt on icy roads Solutes change properties of pure solvent (H2O) Nonvolatile solute reduces vapor pressure of a solvent. Vapor Pressure Lowering

  26. Osmosis • Semipermeable membrane: permits passage of some components of a solution (cell membranes) • Osmosis: the movement of a solvent from low solute concentration to high solute concentration. • There is movement in both directions across a semipermeable membrane. • As solvent moves across the membrane, the fluid levels in the arms becomes uneven.

  27. Osmosis calculations • Osmotic pressure () •  = MRT • The average osmotic pressure of blood is 7.7 atm at 25 C. What concentration of glucose will be isotonic with blood?

  28. Left cylinder contains concentrated NaCl solution. Right cylinder contains pure water. What’s going on in each case? Left carrot has shrunk. Right carrot has expanded. Osmosis and carrots?

  29. Why is osmosis important to human beings? Analogous to situation with carrots we just saw. Red blood cells have a certain osmotic pressure; their surrounding medium (plasma) needs to have the same osmotic pressure, or nasty things happen….. Solutions with equal osmotic pressures known as isotonic. Thus, contents of red blood cells and surrounding plasma are isotonic. When somebody needs intravenous injections, some solution is needed. This is routinely 0.89% sodium chloride in water, or saline solution. This is isotonic with red blood cells, thus safe. Osmosis and red blood cells

  30. Osmosis and red blood cells Hypertonic solution Hypotonic solution

  31. Osmotic pressure calculation • Osmotic pressure () •  = MRT • The average osmotic pressure of blood is 7.7 atm at 25 C. What concentration of glucose will be isotonic with blood? • If 0.5 M glucose and 0.25 M MgCl2 were placed into compartments A and B of an osmosis chamber, in which compartment would the solution level rise?

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