Download
chapter 17 water and aqueous systems n.
Skip this Video
Loading SlideShow in 5 Seconds..
Chapter 17 “Water and Aqueous Systems” PowerPoint Presentation
Download Presentation
Chapter 17 “Water and Aqueous Systems”

Chapter 17 “Water and Aqueous Systems”

561 Vues Download Presentation
Télécharger la présentation

Chapter 17 “Water and Aqueous Systems”

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

  1. Chapter 17“Water and Aqueous Systems”

  2. Section 17.1Liquid Water and it’s Properties • OBJECTIVES: • Describe the hydrogen bonding that occurs in water.

  3. Section 17.1Liquid Water and it’s Properties • OBJECTIVES: • Explain the high surface tension and low vapor pressure of water in terms of hydrogen bonding.

  4. The Water Molecule • Water is a simple triatomic molecule. • Each O-H bond is highly polar, because of the high electronegativity of the oxygen • bond angle = 105 o • due to the bent shape, the O-H bond polarities do not cancel. This means water as a whole is polar. • Fig. 17.2, p.475

  5. The Water Molecule • Water’s bent shape and ability tohydrogen bond gives water many special properties! • Water molecules are attracted to one another. • This gives water: high surface tension, low vapor pressure, high specific heat, high heat of vaporization, and high boiling point

  6. High Surface Tension • liquid water acts like it has a skin • glass of water bulges over the top • Water forms round drops • spray water on greasy surface • All because water hydrogen bonds. • Fig. 17.4, p.476

  7. O H H O H H Surface Tension d- • One water molecule hydrogen bonds to another. • Also, hydrogen bonding occurs to other molecules all around. d+ d+ d- d+ d+

  8. Surface Tension • A water molecule in the middle of solution is pulled in all directions.

  9. Surface Tension • Not true at the surface. • Only pulled down and to each side. • Holds the molecules together. • Causes surface tension.

  10. Surface Tension • Water drops are round, because all molecules on the edge are pulled to the middle- not to the air!

  11. Surface Tension • Glass has polar molecules. • Glass can hydrogen bond. • Attracts the water molecules. • Some of the pull is up a cylinder.

  12. Meniscus • Water curves up along the side of glass. • This makes the meniscus, as in a graduated cylinder • Plastics are non-wetting; no attraction

  13. Meniscus In Plastic In Glass

  14. Surface tension • All liquids have surface tension • water is higher than most others • How to decrease surface tension? • Use a surfactant - surface active agent • a wetting agent, like detergent or soap • interferes with hydrogen bonding

  15. Low vapor pressure • Fig. 17.6, p.477 • Hydrogen bonding also explains water’s unusually low vapor pressure. • Holds water molecules together, so they do not escape • good thing- lakes and oceans would evaporate very quickly!

  16. Specific Heat Capacity • Water has a high heat capacity (also called specific heat). • It absorbs 4.18 J/gºC, while iron absorbs only 0.447 J/gºC. • Remember: SH = heat Mass x DT • If we calculate the heat need to raise the temperature of both iron and water by 75ºC - water is almost 10 x more!

  17. Section 17.2Water Vapor and Ice • OBJECTIVES: • Account for the high heat of vaporization and the high boiling point of water, in terms of hydrogen bonding.

  18. Section 17.2Water Vapor and Ice • OBJECTIVES: • Explain why ice floats in water.

  19. Evaporation and Condensation • Because of the strong hydrogen bonds, it takes a large amount of energy to change water from a liquid to a vapor. • 2,260 J/g is the heat of vaporization. • This much energy to boil 1 gram water • You get this much energy back when it condenses. • Steam burns, but heats things well.

  20. Ice • Most liquids contract (get smaller) as they are cooled. • They get more dense. • When they change to solid, they are more dense than the liquid. • Solid metals sink in liquid metal. • But, ice floats in water. • Why?

  21. Ice • Water becomes more dense as it cools until it reaches 4ºC. • Then it becomes less dense. • As the molecules slow down, they arrange themselves into honeycomb shaped crystals. • These are held together by hydrogen bonds. (Fig. 17.9, p.481)

  22. H H H O O H H H H H O O H O H O H H H H O H O O H H H H H H O O O H H H Liquid Solid

  23. Ice • 10% greater volume than water. • Water freezes from the top down. • The layer of ice on a pond acts as an insulator for water below • It takes a great deal of energy to turn solid water to liquid water. • Heat of fusion is: 334 J/g.

  24. Section 17.3Aqueous Solutions • OBJECTIVES: • Explain the significance of the statement “like dissolves like”.

  25. Section 17.3Aqueous Solutions • OBJECTIVES: • Distinguish among strong electrolytes, weak electrolytes, and nonelectrolytes, giving examples of each.

  26. Solvents and Solutes • Solution - a homogenous mixture, that is mixed molecule by molecule. • Solvent- the dissolving medium • Solute -the dissolved particles • Aqueous solution- a solution with water as the solvent. • Particle size about 1 nm; cannot be separated by filtration!

  27. Aqueous Solutions • Water dissolves ionic compounds and polar covalent molecules best. • The rule is: “like dissolves like” • Polar dissolves polar. • Nonpolar dissolves nonpolar. • Oil is nonpolar. • Oil and water don’t mix. • Salt is ionic- makes salt water.

  28. How Ionic solids dissolve • Called solvation. • Water breaks the + and - charged pieces apart and surrounds them. • Fig. 17.12, p. 483 • In some ionic compounds, the attraction between ions is greater than the attraction exerted by water • Barium sulfate and calcium carbonate

  29. H H H H O O O H H H H O O H H O O H H H H H H O H O H How Ionic solids dissolve

  30. Solids will dissolve if the attractive force of the water molecules is stronger than the attractive force of the crystal. • If not, the solids are insoluble. • Water doesn’t dissolve nonpolar molecules because the water molecules can’t hold onto them. • The water molecules hold onto each other, and separate from the nonpolar molecules. • Nonpolars? No repulsion between them

  31. Electrolytes and Nonelectrolytes • Electrolytes- compounds that conduct an electric current in aqueous solution, or in the molten state • all ionic compounds are electrolytes (they are also salts) • barium sulfate- will conduct when molten, but is insoluble in water!

  32. Electrolytes and Nonelectrolytes • Do not conduct? Nonelectrolytes. • Many molecular materials, because they do not have ions • Not all electrolytes conduct to the same degree • there are weak electrolytes, and strong electrolytes • depends on: degree of ionization

  33. Electrolytes and Nonelectrolytes • Table 17.3, p.485 lists some common electrolytes and nonelectrolytes • How do you know if it is strong or weak? Rules on handout sheet.

  34. Electrolyte Summary • Substances that conduct electricity when dissolved in water, or molten. • Must have charged particles that can move. • Ionic compounds break into charged ions: NaCl ® Na1+and Cl1- • These ions can conduct electricity.

  35. Nonelectrolytes do not conduct electricity when dissolved in water or molten • Polar covalent molecules such as methanol (CH3OH) don’t fall apart into ions when they dissolve. • Weak electrolytes don’t fall completely apart into ions. • Strong electrolytes do ionize completely.

  36. + heat - heat Water of Hydration(or Water of Crystallization) • Water molecules chemically bonded to solid salt molecules (not in solution) • These compounds have fixed amounts of water. • The water can be driven off by heating: • CuSO4.5H2O CuSO4 + 5H2O • Called copper(II)sulfate pentahydrate.

  37. Hydrates • Table 17.4, p.486 list some familiar hydrates • Since heat can drive off the water, the forces holding it are weak • If a hydrate has a vapor pressure higher than that of water vapor in air, the hydrate will effloresce by losing the water of hydration

  38. Hydrates • Some hydrates that have a low vapor pressure remove water from the air to form higher hydrates- called hygroscopic • used as drying agents, or dessicants • packaged with products to absorb moisture

  39. Hydrates • Some compounds are so hygroscopic, they become wet when exposed to normally moist air- called deliquescent • remove sufficient water to dissolve completely and form solutions • Fig. 17.17, p.487 • Sample Problem 17-1, p.488 for percent composition

  40. Section 17.4Heterogeneous Aqueous Systems • OBJECTIVES: • Explain how colloids and suspensions differ from solutions.

  41. Section 17.4Heterogeneous Aqueous Systems • OBJECTIVES: • Describe the Tyndall effect.

  42. Mixtures that are NOT Solutions • Suspensions: mixtures that slowly settle upon standing. • Particles of a suspension are greater in diameter than 100 nm. • Can be separated by filtering (p.490) • Colloids: heterogeneous mixtures with particles between size of suspensions and true solutions (1-100 nm)

  43. Mixtures that are NOT Solutions • The small particles are the dispersedphase, and are spread throughout the dispersion medium • The first colloids were glues. Others include mixtures such as gelatin, paint, aerosol sprays, and smoke • Table 17.5, p.491 list some common colloidal systems and examples

  44. Mixtures that are NOT Solutions • Many colloids are cloudy or milky in appearance when concentrated, but almost clear when dilute • do not settle out • cannot be filtered out • Colloids exhibit the Tyndall effect- the scattering of visible light in all directions. • suspensions also show Tyndall effect

  45. Mixtures that are NOT Solutions • Flashes of light are seen when colloids are studied under a microscope- light is reflecting- called Brownian motiontodescribe the chaotic movement of the particles • Table 17.6, p.492 summarizes the properties of solutions, colloids, and suspensions

  46. Mixtures that are NOT Solutions • Emulsions- colloids dispersions of liquids in liquids • an emulsifying agent is essential for maintaining stability • oil and water not soluble; but with soap or detergent, they will be. • Oil and vinegar dressing? • Mayonnaise? Margarine?