GPS: SC7 a: Explain the process of dissolving in terms of solute/solvent interactions:

1. GPS: SC7 a: Explain the process of dissolving in terms of solute/solvent interactions: Observe factors that affect the rate at which a solute dissolves in a specific solvent. Express concentrations as molarity and molality. Prepare and properly label solutions of specified molar concentration. Relate molality to colligative properties. Essential Questions: Are all solutions liquid? What are solutions, and how do we describe them chemically? How are solutions measured? How does a solute affect the physical properties of a solution? How do you prepare a solution at a specific concentration?

2. Performance Tasks: • Observe factors that affect the rate at which a solute dissolves in a specific solvent. • Students will identify factors affecting the solubility of a solute. • Students will analyze solubility curves to identify areas of saturation, unsaturation and supersaturation.

3. What is a solution? • Solution – homogeneous mixture • Solvent – substance present in largest amount (Does the dissolving) • Solutes – other substances in the solution (Gets Dissolved) • Aqueous solution – solution with water as the solvent

4. Solutions • Solutions are homogeneous mixtures of two or more pure substances. • In a solution, the solute is dispersed uniformly throughout the solvent.

5. 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.

7. B. Solution Composition: An Introduction • The solubility of a solute is limited. • Saturated solution – contains as much solute as will dissolve at that temperature • Unsaturated solution – has not reached the limit of solute that will dissolve

8. B. Solution Composition: An Introduction • Supersaturated solution – occurs when a solution is saturated at an elevated temperature and then allowed to cool but all of the solid remains dissolved • Contains more dissolved solid than a saturated solution at that temperature • Unstable – adding a crystal causes precipitation

9. The Energy Terms for Various Types of Solutes and Solvents

10. B. Solution Composition: An Introduction • Solutions are mixtures. • Amounts of substances can vary in different solutions. • Specify the amounts of solvent and solutes • Qualitative measures of concentration • concentrated – relatively large amount of solute • dilute – relatively small amount of solute

11. B. Solution Composition: An Introduction • Which solution is more concentrated?

12. B. Solution Composition: An Introduction • Which solution is more concentrated?

13. C. Factors Affecting the Rate of Dissolving (Solubility) • Surface area – increased SA = increased solubility • Stirring – more stirring increases solubility • Temperature – higher temp. usually increases solubility • Pressure - The solubility of a gas in a liquid is directly proportional to its pressure. • Identity of solute & solvent - Both polar or nonpolar?

14. A. Solubility • How Substances Dissolve • A “hole” must be made in the water structure for each solute particle. • The lost water-water interactions must be replaced by water-solute interactions. • “like dissolves like” • Polar substances dissolve other polar substances • Nonpolar substances dissolve other nonpolar substances.

15. A. Solubility • Solubility of Ionic Substances • Ionic substances breakup into individual cations and anions.

16. A. Solubility • Solubility of Ionic Substances • Polar water molecules interact with the positive and negative ions of a salt.

17. A. Solubility • Solubility of Polar Substances • Ethanol is soluble in water because of the polar OH bond.

18. A. Solubility • Solubility of Polar Substances • Why is solid sugar soluble in water?

19. A. Solubility • Substances Insoluble in Water • Nonpolar oil does not interact with polar water. • Water-water hydrogen bonds keep the water from mixing with the nonpolar molecules.

20. 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.

21. Factors Affecting Solubility The more similar the intermolecular attractions, the more likely one substance is to be soluble in another.

22. Factors Affecting Solubility Glucose (which has hydrogen bonding) is very soluble in water, while cyclohexane (which only has dispersion forces) is not.

23. Gases in Solution • In general, the solubility of gases in water increases with increasing mass. • Larger molecules have stronger dispersion forces.

24. 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.

25. Henry’s Law Sg = kPg where • Sg is the solubility of the gas, • k is the Henry’s Law constant for that gas in that solvent, and • Pg is the partial pressure of the gas above the liquid.

26. Temperature Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.

27. Temperature • The opposite is true of gases. • Carbonated soft drinks are more “bubbly” if stored in the refrigerator. • Warm lakes have less O2 dissolved in them than cool lakes.

28. Student, Beware! Just because a substance disappears when it comes in contact with a solvent, it doesn’t mean the substance dissolved.

29. 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.

30. Ways of Expressing Concentrations of Solutions

31. Objectives • Calculate the concentration of various solutions in terms of molarity and molality. • Relate molality to colligative properties by calculating boiling point elevation, freezing point depression, and vapor pressure lowering for various solutions.

32. B. Solution Composition: Molarity • Concentration of a solution is the amount of solute in a given volume of solution.

33. B. Solution Composition: Molarity • Consider both the amount of solute and the volume to find concentration.

34. B. Solution Composition: Molarity • To find the moles of solute in a given volume of solution of known molarity use the definition of molarity.

35. B. Solution Composition: Molarity • Standard solution - a solution whose concentration is accurately known • To make a standard solution • Weigh out a sample of solute. • Transfer to a volumetric flask. • Add enough solvent to mark on flask.

36. C. Dilution • Water can be added to an aqueous solution to dilute the solution to a lower concentration. • Only water is added in the dilution – the amount of solute is the same in both the original and final solution.

37. D. Dilution • Diluting a solution • Transfer a measured amount of original solution to a flask containing some water. • Add water to the flask to the mark (with swirling) and mix by inverting the flask. • Dilution Calculation: • M1V1 = M2V2

38. 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.

39. Colligative Properties • Changes in colligative properties depend only on the number of solute particles present, not on the identity of the solute particles. • Among colligative properties are • Vapor pressure lowering • Boiling point elevation • Freezing point depression • Osmotic pressure

40. Vapor Pressure Because of solute-solvent intermolecular attraction, higher concentrations of nonvolatile solutes make it harder for solvent to escape to the vapor phase.

41. Vapor Pressure Therefore, the vapor pressure of a solution is lower than that of the pure solvent.

42. Vapor Pressure Lowering: Liquid/Vapor Equilibrium

43. Vapor Pressure Lowering: Addition of a Solute

44. Vapor Pressure Lowering: Solution/Vapor Equilibrium

45. Note that in both equations, T does not depend on what the solute is, but only on how many particles are dissolved. Tb = Kb  m Tf = Kf  m Boiling Point Elevation and Freezing Point Depression

46. Boiling Point Elevation and Freezing Point Depression Nonvolatile solute-solvent interactions also cause solutions to have higher boiling points and lower freezing points than the pure solvent.

47. Boiling Point Elevation • The change in boiling point is proportional to the molality of the solution: Tb = Kb  m where Kb is the molal boiling point elevation constant, a property of the solvent. Tb is added to the normal boiling point of the solvent.

48. Freesing Point Depression • The change in freezing point can be found similarly: Tf = Kf  m • Here Kf is the molal freezing point depression constant of the solvent. Tf is subtracted from the normal boiling point of the solvent.

49. D. Boiling Point and Freezing Point • The presence of solute “particles” causes the liquid range to become wider. • Boiling point increases • Freezing point decreases

50. D. Boiling Point and Freezing Point • Why does the boiling point of a solution increase? • Forming a bubble in a solution • Solute particles block some of the water molecules trying to enter the bubble. • Need higher pressure to maintain the bubble.