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Chapter 17

Chapter 17. Evaporation. Evaporation. Occurs when high kinetic energy particles at the surface of a liquid break free from the attractive forces of the neighboring particles and become vapor. Vapor.

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Chapter 17

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  1. Chapter 17

  2. Evaporation

  3. Evaporation • Occurs when high kinetic energy particles at the surface of a liquid break free from the attractive forces of the neighboring particles and become vapor.

  4. Vapor • A vapor is the gaseous state of a substance that is liquid or solid under ordinary conditions.

  5. Evaporation • Evaporation is a cooling process.

  6. Why doesn’t evaporating water freeze?

  7. Evaporation is a cooling process

  8. Evaporation is a cooling process

  9. Heat Index

  10. Evaporation is a cooling process

  11. Saharan Desert

  12. Saharan Oasis

  13. Sahara was once fertile grasslands

  14. H2O(l)  H2O(g) • Evaporation and condensation are reversible processes. • An equilibrium occurs when two opposing processes occur at the same rate.

  15. EquilibriumH2O(l)  H2O(g)

  16. H2O(l)  H2O(g) • Evaporation and condensation are reversible processes. • An equilibrium occurs when two opposing processes occur at the same rate. • At equilibrium the concentrations of the substances involved in the reaction will remain constant. • [ ] = concentration.

  17. EquilibriumH2O(l)  H2O(g)

  18. Vapor Pressure • Suppose we conduct an experiment in which we place a quantity of ethanol (C2H5OH) in an evacuated, closed container (a). • The ethanol will quickly begin to evaporate. • The pressure exerted by the vapor will begin to increase. • After a short time the pressure will attain a constant value, called the vapor pressure of ethanol (b).

  19. Vapor Pressure • At any given temperature, for a particular substance, there is a pressure at which the gas of that substance is in equilibrium with its liquid or solid form. This is the vapor pressure of that substance at that temperature. • The equilibrium vapor pressure is an indication of a liquid's evaporation rate. • A substance with a high vapor pressure at normal temperatures has a high rate of evaporation and is often referred to as volatile.

  20. Attractive Forces Two types of attractive forces are: • Intramolecular forces • Intermolecular forces

  21. Intramolecular Forces • Intramolecular forces = attractive forces that hold particles together using ionic, covalent, or metallic bonds • Intra means “within” • Intramolecular forces = bonds

  22. Intermolecular Forces(Van der Waals forces) • Inter means “between” • Intermolecular forces are forces between molecules • Larger polar molecules have greater Van der Waals forces.

  23. Intermolecular forces vs. Intramolecular forces

  24. What happens when the water evaporates? • Intermolecular attractions (van der Waals forces) are broken. • Intramolecular attractions (bonds) are not broken.

  25. alcohol vs. water • How do the vapor pressures, rates of evaporation, and van der Waals forces compare? • How would the boiling point of alcohol compare to water?

  26. Why is it called rubbing alcohol?

  27. Substances with weaker Van der Waals Forces • Are easier to evaporate • Have higher vapor pressure • Are more volatile • Have lower boiling points

  28. Equilibrium • A state of equilibrium is the most stable state for a reversible system.

  29. LeChatlier’s Principle • If a stress is placed on a system in equilibrium the system will tend to readjust so that the stress is reduced. • 3 Stresses are changing the: • Concentration • Temperature • Pressure • LeChatlier’s Principle = “Do the opposite”

  30. I smell gasolineC8H18(l) ↔ C8H18(g)

  31. 2NO2(g) ↔N2O4(g)browncolorless How does applying LeChatlier’s Principle explain that this reaction is exothermic? Rule: An increase in temperature will always shift a reaction in the endothermic direction. What other rule could we use? Note: Apply the rule, it accounts for doing the opposite. So don’t do the opposite of the rule.

  32. H2O(l)  H2O(g) Is the reaction above exothermic or endothermic?

  33. Predict the effect of the following changes on the reaction: 2SO3(g) ↔ 2SO2 (g) + O2 (g) ∆H = +197.78 kJ (a) Increasing the temperature of the reaction.

  34. Predict the effect of the following changes on the reaction: 2SO3(g) ↔ 2SO2 (g) + O2 (g) ∆H = +197.78 kJ (b) Increasing the pressure on the reaction.

  35. Predict the effect of the following changes on the reaction: 2SO3(g) ↔ 2SO2 (g) + O2 (g) ∆H = +197.78 kJ (c) Adding more O2.

  36. Predict the effect of the following changes on the reaction: 2SO3(g) ↔ 2SO2 (g) + O2 (g) ∆H = +197.78 kJ (d) Removing O2.

  37. Homework • Worksheet 1 Chapter 17.

  38. Solid-Vapor Equilibrium • Sublimation is a process in which molecules go directly from the solid into the vapor phase (deposition is the reverse process).

  39. Sublimation Iodine and dry ice are substances that commonly undergo sublimation. Which of these can reach equilibrium?

  40. Deposition • The formation of frost.

  41. Boiling • A substance boils when the vapor pressure of the liquid is equal to the atmospheric pressure.

  42. How could I boil this water? AA A

  43. What is temperature of the water? AA A

  44. Why does water boil at 100°C?

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