1. © 2009, Prentice-Hall, Inc. Chapter 13 Properties of Solutions

2. © 2009, Prentice-Hall, Inc. Solutions Solutions are homogeneous mixtures of two or more pure substances. In a solution, the solute is dispersed uniformly throughout the solvent.

3. © 2009, Prentice-Hall, Inc. Solutions The intermolecular forces between solute and solvent particles must be strong enough to compete with those between solute particles and those between solvent particles.

4. © 2009, Prentice-Hall, Inc. How Does a Solution Form? As a solution forms, the solvent pulls solute particles apart and surrounds, or solvates, them.

5. © 2009, Prentice-Hall, Inc. How Does a Solution Form If an ionic salt is soluble in water, it is because the ion- dipole interactions are strong enough to overcome the lattice energy of the salt crystal.

6. © 2009, Prentice-Hall, Inc. Energy Changes in Solution Simply put, three processes affect the energetics of solution: – separation of solute particles, – separation of solvent particles, – new interactions between solute and solvent.

7. © 2009, Prentice-Hall, Inc. Energy Changes in Solution The enthalpy change of the overall process depends on  H for each of these steps.

8. © 2009, Prentice-Hall, Inc. Why Do Endothermic Processes Occur? Things do not tend to occur spontaneously (i.e., without outside intervention) unless the energy of the system is lowered.

9. © 2009, Prentice-Hall, Inc. Why Do Endothermic Processes Occur? Yet we know that in some processes, like the dissolution of NH 4 NO 3 in water, heat is absorbed, not released.

10. © 2009, Prentice-Hall, Inc. Enthalpy Is Only Part of the Picture The reason is that increasing the disorder or randomness (known as entropy) of a system tends to lower the energy of the system.

11. © 2009, Prentice-Hall, Inc. Enthalpy Is Only Part of the Picture So even though enthalpy may increase, the overall energy of the system can still decrease if the system becomes more disordered.

12. © 2009, Prentice-Hall, Inc. Student, Beware! Just because a substance disappears when it comes in contact with a solvent, it doesn’t mean the substance dissolved.

13. © 2009, Prentice-Hall, Inc. Student, Beware! Dissolution is a physical change — you can get back the original solute by evaporating the solvent. If you can’t, the substance didn’t dissolve, it reacted.

14. © 2009, Prentice-Hall, Inc. Types of Solutions Saturated – In a saturated solution, the solvent holds as much solute as is possible at that temperature. – Dissolved solute is in dynamic equilibrium with solid solute particles.

15. © 2009, Prentice-Hall, Inc. Types of Solutions Unsaturated – If a solution is unsaturated, less solute than is able to dissolve in the solvent at that temperature is dissolved in the solvent.

16. © 2009, Prentice-Hall, Inc. Types of Solutions Supersaturated – In supersaturated solutions, the solvent holds more solute than is normally possible at that temperature. – These solutions are unstable; crystallization can usually be stimulated by adding a “seed crystal” or scratching the side of the flask.

17. © 2009, Prentice-Hall, Inc. Factors Affecting Solubility Chemists use the axiom “like dissolves like." – Polar substances tend to dissolve in polar solvents. – Nonpolar substances tend to dissolve in nonpolar solvents.

18. © 2009, Prentice-Hall, Inc. Factors Affecting Solubility The more similar the intermolecular attractions, the more likely one substance is to be soluble in another.

19. © 2009, Prentice-Hall, Inc. Factors Affecting Solubility Glucose (which has hydrogen bonding) is very soluble in water, while cyclohexane (which only has dispersion forces) is not.

20. © 2009, Prentice-Hall, Inc. Factors Affecting Solubility Vitamin A is soluble in nonpolar compounds (like fats). Vitamin C is soluble in water.

21. © 2009, Prentice-Hall, Inc. Gases in Solution In general, the solubility of gases in water increases with increasing mass. Larger molecules have stronger dispersion forces.

22. © 2009, Prentice-Hall, Inc. Gases in Solution The solubility of liquids and solids does not change appreciably with pressure. The solubility of a gas in a liquid is directly proportional to its pressure.

23. © 2009, Prentice-Hall, Inc. Henry’s Law S g = kP g where S g is the solubility of the gas, k is the Henry’s Law constant for that gas in that solvent, and P g is the partial pressure of the gas above the liquid.

24. © 2009, Prentice-Hall, Inc. Temperature Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.

25. © 2009, Prentice-Hall, Inc. Temperature The opposite is true of gases. – Carbonated soft drinks are more “bubbly” if stored in the refrigerator. – Warm lakes have less O 2 dissolved in them than cool lakes.

26. © 2009, Prentice-Hall, Inc. Ways of Expressing Concentrations of Solutions

27. © 2009, Prentice-Hall, Inc. Mass Percentage Mass % of A = mass of A in solution total mass of solution  100

28. © 2009, Prentice-Hall, Inc. Parts per Million and Parts per Billion ppm = mass of A in solution total mass of solution  10 6 Parts per Million (ppm) Parts per Billion (ppb) ppb = mass of A in solution total mass of solution  10 9

29. © 2009, Prentice-Hall, Inc. moles of A total moles in solution X A = Mole Fraction (X) In some applications, one needs the mole fraction of solvent, not solute — make sure you find the quantity you need!

30. © 2009, Prentice-Hall, Inc. mol of solute L of solution M = Molarity (M) You will recall this concentration measure from Chapter 4. Since volume is temperature-dependent, molarity can change with temperature.

31. © 2009, Prentice-Hall, Inc. mol of solute kg of solvent m = Molality (m) Since both moles and mass do not change with temperature, molality (unlike molarity) is not temperature-dependent.

32. © 2009, Prentice-Hall, Inc. Changing Molarity to Molality If we know the density of the solution, we can calculate the molality from the molarity and vice versa.

33. Sample Exercise 13.1 Assessing Entropy Change In the process illustrated below, water vapor reacts with excess solid sodium sulfate to form the hydrated form of the salt. The chemical reaction is Essentially all of the water vapor in the closed container is consumed in this reaction. If we consider our system to consist initially of Na 2 SO 4 (s) and 10 H 2 O(g), (a) does the system become more or less ordered in this process, and (b) does the entropy of the system increase or decrease?

34. Sample Exercise 13.1 Assessing Entropy Change Does the entropy of the system increase or decrease when the stopcock is opened to allow mixing of the two gases in this apparatus? Practice Exercise

35. Sample Exercise 13.2 Predicting Solubility Patterns Predict whether each of the following substances is more likely to dissolve in the nonpolar solvent carbon tetrachloride (CCl 4 ) or in water: C 7 H 16, Na 2 SO 4, HCl, and I 2.

36. Sample Exercise 13.2 Predicting Solubility Patterns Arrange the following substances in order of increasing solubility in water: Practice Exercise

37. Sample Exercise 13.3 A Henry’s Law Calculation Calculate the concentration of CO 2 in a soft drink that is bottled with a partial pressure of CO 2 of 4.0 atm over the liquid at 25 °C. The Henry’s law constant for CO 2 in water at this temperature is 3.1 × 10 –2 mol/L-atm. Calculate the concentration of CO 2 in a soft drink after the bottle is opened and equilibrates at 25 °C under a CO 2 partial pressure of 3.0 × 10 –4 atm. Answer: 9.3 × 10 –6 M Practice Exercise Solution S CO2 = = (3.1 × 10 –2 mol/L-atm)(4.0 atm) = 0.12 mol/L = 0.12 M

38. Sample Exercise 13.4 Calculation of Mass-Related Concentrations (a) Calculate the mass percentage of NaCl in a solution containing 1.50 g of NaCl in 50.0 g of water. (b) A commercial bleaching solution contains 3.62 mass % sodium hypochlorite, NaOCl. What is the mass of NaOCl in a bottle containing 2.50 kg of bleaching solution? Answer: (a) 2.91%, (b) 90.5 g of NaOCl Practice Exercise (a) A solution is made by dissolving 13.5 g of glucose (C 6 H 12 O 6 ) in 0.100 kg of water. What is the mass percentage of solute in this solution? (b) A 2.5-g sample of groundwater was found to contain 5.4 µg of Zn 2+. What is the concentration of Zn 2+ in parts per million? Solution

39. Sample Exercise 13.5 Calculation of Molality A solution is made by dissolving 4.35 g glucose (C 6 H 12 O 6 ) in 25.0 mL of water at 25 °C. Calculate the molality of glucose in the solution. Water has a density of 1.00 g/mL. What is the molality of a solution made by dissolving 36.5 g of naphthalene (C 10 H 8 ) in 425 g of toluene (C 7 H 8 )? Answer: 0.670 m Practice Exercise Solution

40. Sample Exercise 13.6 Calculation of Mole Fraction and Molality 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. Solution

41. Sample Exercise 13.6 Calculation of Mole Fraction and Molality A commercial bleach solution contains 3.62 mass % NaOCl in water. Calculate (a) the mole fraction and (b) the molality of NaOCl in the solution. Answer: (a) 9.00 × 10 –3, (b) 0.505 m. Practice Exercise

42. Sample Exercise 13.7 Calculation of Molality Using the Density of a Solution A solution with a density of 0.876 g/mL contains 5.0 g of toluene (C 7 H 8 ) and 225 g of benzene. Calculate the molarity of the solution. Solution