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Chapter 11

Chapter 11. Solutions and Their Colligative Properties. solution  a homogeneous mixture of two or more substances components of a solution 1. solute  2. solvent . Types of Solutions. solutions may exist in any of the three states of matter

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Chapter 11

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  1. Chapter 11 Solutions and Their Colligative Properties

  2. solution  a homogeneous mixture of two or more substances • components of a solution 1. solute  2. solvent 

  3. Types of Solutions • solutions may exist in any of the three states of matter • gaseous solutions  nonreactive gases can mix in all proportions to give a gaseous mixture  the air we breathe • liquid solutions  most obtained by dissolving a gas, liquid or solid in some liquid  most common type of solution  ethanol in water  CO2 dissolved in water = club soda

  4. solid solutions  solute and solvent are both solids • solid solutions include alloys mixtures of metals  brass is an alloy of copper and zinc  bronze is an alloy of copper and tin  pewter is an alloy of zinc and tin

  5. Solution Formation • energy changes in the formation of most solutions involve differences in the attractive forces between particles  must overcome solute-solute interactions  must overcome some of the solvent- solvent interactions  new solute-solvent interactions are made

  6. depending on the exact nature of the forces in the solute and solvent, these forces may promote the formation of a solution or prevent it

  7. Vaporization • molecules in a liquid are constantly in motion • the average kinetic energy is proportional to temperature • some molecules may have more kinetic energy than the average - if these molecules are at the surface of the liquid, they may have enough energy to escape the attractive forces vaporization

  8. Open vs. Closed Container • in an open container vaporization will continue until the liquid is gone • What about a closed container? in a closed container, once the rates of vaporization and condensation are equal, the total amount of vapor and liquid does not change

  9. Dynamic Equilibrium • evaporation and condensation are still occurring, but because they are opposite processes, there is no net gain or loss of either vapor or liquid • dynamic equilibrium  occurs when 2 opposite processes reach the same rate so that there is no net gain or loss of material equilibrium does not mean there are equal amounts of vapor & liquid – they are just changing by equal amounts

  10. Vapor Pressure • vapor pressure  the pressure exerted by the vapor when it is in dynamic equilibrium with its liquid • the weaker the attractive forces between the molecules, the more molecules will be in the vapor  weaker attractive forces  higher vapor pressure  a more volatile liquid

  11. VP is a characteristic property of a liquid at a specific temperature • increasing the temperature increases the number of molecules to escape the liquid and therefore increases the VP • small changes in temperature can make big changes in VP – the rate of growth depends on the strength of the IMFs

  12. Boiling Point • when the temperature of a liquid reaches a point where its vapor pressure is the same as the external pressure, vapor bubbles form in the liquid • this phenomena is what is called boiling and the temperature required to have the vapor pressure equal the external pressure is the boiling point

  13. because the external pressure on a liquid can vary, so can the boiling point • the lower the external pressure, the lower the boiling point of the liquid • normal boiling point  the temperature at which the vapor pressure of the liquid equals 1 atm

  14. Clausius - Clapeyron Equation • the vapor pressure of a substance depends on temperature • by assuming that the vapor behaves like an ideal gas, can derive an equation that relates the vapor pressure of a liquid and the Kelvin temperature Clausius - Clapeyron equation

  15. ?? Diethyl ether is a volatile, highly flammable liquid that is used mainly as a solvent. The vapor pressure of diethyl ether is 401 mm Hg at 18 C. Calculate its vapor pressure at 32 C. The Hvap for diethyl ether is 26.0 kJ/mol.

  16. ?? A certain substance has a vapor pressure of 350 mm Hg at 21.7 C and a vapor pressure of 610 mm Hg at 38.9 C. What is the heat of vaporization, in kJ/mol?

  17. ?? The vapor pressure of water at 25 °C is 23.8 torrand the heat of vaporization of water is 43.9 kJ/mol. If the vapor pressure is increased to 93.7 torr, calculate the temperature this would correspond to.

  18. ?? The vapor pressure of benzene, C6H6, is 40.1 mm Hg at 7.6 C. What is its vapor pressure at 60.6 C? The heat of vaporization of benzene is 31.0 kJ/mol.

  19. ?? A certain substance has a normal boiling point of 71.2C and it has a vapor pressure of 598 mm Hg at 60.8C. What is the heat of vaporization, in kJ/mol?

  20. ?? A certain substance has a heat of vaporization of 23.8 kJ/mol. If it has a vapor pressure of 541 mm Hg at 23.1 C, what vapor pressure will it have at 36.2 C?

  21. ?? The normal boiling point of acetone is 56.5 ° C. At an elevation of 5300 ft the atmospheric pressure is 630 torr. What would be the boiling point of acetone at this elevation? The heat of vaporization of acetone is 32.0 kJ/mol.

  22. ?? The vapor pressure of ethanol is 100 mm Hg at 34.9 C. What is its vapor pressure at 63.5 C? The Hvap is 39.3 kJ/mol for ethanol.

  23. ?? An unknown liquid has a vapor pressure of 88 mm Hg at 45 oC and 39 mm Hg at 25 oC. What is its heat of vaporization?

  24. ?? Carbon tetrachloride, CCl4, has a vapor pressure of 213 torr at 40 °C and 836 torr at 80 °C. Find the Hvap and the normal boiling point of CCl4?

  25. ?? Estimate the heat of vaporization of a liquid whose vapor pressure doubles when the temperature is raised from 85 C to 95 C.

  26. Expressing Solution Concentration • solutions have variable composition • to describe a solution, need to describe components and relative amounts • concentration  the amount of solute in a given amount of solution • solution concentration be expressed qualitatively with dilute vs. concentrated

  27. Quantitative Expression of Concentration • there are many ways to quantitatively express solution concentration molarity molality mole fraction

  28. Molarity • molarity the amount of solute (in moles) per liter of solution • convenient because it describes how many moles of solute are in each liter of solution 2 M NaCl = 2 moles NaCl in 1 liter of solution so… 0.5 L of solution would contain 1 mole NaCl and 2 L of solution would contain 4 moles NaCl

  29. a 5 M glucose solution has the same number of molecules as a 5 M ethanol solution  even though the mass of ethanol and the mass of glucose used to make each solution are different • molarity is useful for chemists because they sometimes want to know how many particles of solute are available to react

  30. Molality • molality is defined in terms of the amount of solvent, not solution • molality moles of solute per kilogram of solvent • does not vary with temperature

  31. Mole Fraction • useful for applications in which the ratio of solute to solvent can vary widely • mole fraction  a ratio of moles of solute to total moles in the solution • mole fractions sum to 1 if all components are accounted for

  32. ?? Calculate the mole fraction of methanol (CH3OH) in a solution containing 8.50 g methanol in 224 g water.

  33. ?? Calculate the molality of a solution formed by dissolving 1.80 moles KCl in 16.0 moles water.

  34. ?? Calculate the molarity of a solution made by dissolving 35 g NaClO4 in enough water to make 250 mL of solution.

  35. ?? In a mixture of pentane, hexane and heptane, the mole fraction of pentane is 0.18 and the mole fraction of hexane is 0.36. What is the mole fraction of heptane?

  36. ?? Calculate the molarity of 2.17 L of a solution containing 15.3 g KBr.

  37. ?? What is the molality of a solution made by dissolving 1.48 g of table sugar (sucrose, C12H22O11) in 30.0 mL of water?

  38. ?? Calculate the number of grams of MgCl2 in 3.00 L of a 2.00 M magnesium chloride solution.

  39. ?? Calculate the molarity of a solution made by dissolving 22.4 g LiClO4• 2 H2O in enough water to make 125 mL of solution.

  40. ?? How many grams of sulfur, S8, must be dissolved in 100.0 g napthalene (C10H8) to make a 0.12 m solution?

  41. ?? A solution is made by dissolving 0.385 g cholesterol, C27H46O, in 40.0 g chloroform, CHCl3. (a) Calculate the molality of the solution.

  42. (b) Calculate the mole fraction of cholesterol.

  43. ?? An aqueous sulfuric acid solution containing 571.6 g H2SO4per liter of solution has a density of 1.329 g/mL.Calculate: (a) the mole fraction of H2SO4

  44. (b) the molarity of H2SO4 (c) the molality of H2SO4

  45. Colligative Properties of Nonelectrolyte Solutions • colligative properties  properties that depend only on the number of solute particles dissolved in solution and not on the nature of the solute particles vapor - pressure lowering boiling - point elevation freezing - point depression osmotic pressure

  46. Vapor – Pressure Lowering • the vapor pressure over a liquid results from a dynamic equilibrium between the liquid and vapor • the addition of a nonvolatile solute to the liquid inhibits liquid molecules from moving out of the liquid phase • eventually, equilibrium is reestablished, but with fewer molecules in the gas phase

  47. adding a nonvolatile solute to a solvent lowers the vapor pressure • the extent of vapor pressure lowering is proportional to concentration  Raoult’s Law the vapor pressure of a solution containing nonvolatile solutes is proportional to the mole fraction of the solvent

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