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Unit 15

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Unit 15

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  1. Unit 15 Section 1 – Homogenous and Heterogeneous Solutions Section 2 – Concentration of Solutions Section 3 – Properties of Solutions

  2. Objectives Section 1 – Homogenous and Heterogeneous Solutions • To understand the process of dissolving • To learn why certain substances dissolve in water • To learn qualitative terms describing the concentration of a solution • To understand the factors that affect the rate at which a solid dissolves

  3. A sinkhole forms when the roof of a cave weakens from being dissolved by groundwater and suddenly collapses. One recorded sinkhole swallowed a house, several other buildings, five cars, and a swimming pool! You will learn how the solution process occurs and the factors that influence the process.

  4. What is a solution? • Solution – homogeneous mixture • Solvent – substance present in largest amount • Solutes – other substances in the solution • Aqueous solution – solution with water as the solvent

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

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

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

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

  9. 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.

  10. 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”

  11. 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

  12. 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

  13. Solution Composition: An Introduction • A supersaturated solution is clear before a seed crystal is added.

  14. Solution Composition: An Introduction • Crystals begin to form in the solution immediately after the addition of a seed crystal.

  15. Solution Composition: An Introduction • Excess solute crystallizes rapidly.

  16. 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

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

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

  19. Factors Affecting the Rate of Dissolving • Surface area • Stirring • Temperature

  20. Factors Affecting the Rate of Dissolving • A cube of sugar in cold tea dissolves slowly.

  21. Factors Affecting the Rate of Dissolving • Granulated sugar dissolves in cold water more quickly than a sugar cube, especially with stirring.

  22. Factors Affecting the Rate of Dissolving • Granulated sugar dissolves very quickly in hot tea.

  23. Factors Affecting the Rate of Dissolving • Stirring and Solution Formation • Stirring speeds up the dissolving process because fresh solvent (the water in tea) is continually brought into contact with the surface of the solute (sugar).

  24. Factors Affecting the Rate of Dissolving • Temperature and Solution Formation • At higher temperatures, the kinetic energy of water molecules is greater than at lower temperatures, so they move faster. As a result, the solvent molecules collide with the surface of the sugar crystals more frequently and with more force.

  25. Factors Affecting the Rate of Dissolving • Particle Size and Solution Formation • A spoonful of granulated sugar dissolves more quickly than a sugar cube because the smaller particles in granulated sugar expose a much greater surface area to the colliding water molecules.

  26. Objectives Section 2 – Concentration of Solutions • To understand mass percent and how to calculate it • To understand and use molarity • To learn to calculate the concentration of a solution made by diluting a stock solution

  27. Solution Composition: Mass Percent

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

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

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

  31. 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.

  32. Molarity • To make a 0.5-molar (0.5M) solution, first add 0.5 mol of solute to a 1-L volumetric flask half filled with distilled water.

  33. Molarity • Swirl the flask carefully to dissolve the solute.

  34. Molarity • Fill the flask with water exactly to the 1-L mark.

  35. 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.

  36. 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.

  37. Making Dilutions • The total number of moles of solute remains unchanged upon dilution, so you can write this equation. • M1 and V1 are the molarity and volume of the initial solution, and M2 and V2 are the molarity and volume of the diluted solution.

  38. Making Dilutions • Making a Dilute Solution

  39. Making Dilutions • To prepare 100 ml of 0.40M MgSO4 from a stock solution of 2.0M MgSO4, a student first measures 20 mL of the stock solution with a 20-mL pipet.

  40. Making Dilutions • She then transfers the 20 mL to a 100-mL volumetric flask.

  41. Making Dilutions • Finally she carefully adds water to the mark to make 100 mL of solution.

  42. Making Dilutions • Volume-Measuring Devices

  43. Objectives Section 3 – Properties of Solutions • To learn to solve stoichiometric problems involving solution reactions • To do calculations involving acid-base reactions • To learn about normality and equivalent weight • To use normality in stoichiometric calculations • To understand the effect of a solute on solution properties   

  44. Stoichiometry of Solution Reactions

  45. Neutralization Reactions • An acid-base reaction is called a neutralization reaction. • Steps to solve these problems are the same as before.

  46. Normality • Unit of concentration • One equivalent of acid – amount of acid that furnishes 1 mol of H+ ions • One equivalent of base – amount of base that furnishes 1 mol of OH ions • Equivalent weight – mass in grams of 1 equivalent of acid or base

  47. Normality

  48. Normality

  49. Normality • To find number of equivalents