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Chapter 12 Solutions

Chemistry: A Molecular Approach , 1 st Ed. Nivaldo Tro. Chapter 12 Solutions. Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA. 2008, Prentice Hall. Solutions. solute is the dissolved substance seems to “disappear” “takes on the state” of the solvent

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Chapter 12 Solutions

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  1. Chemistry: A Molecular Approach, 1st Ed.Nivaldo Tro Chapter 12Solutions Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA 2008, Prentice Hall

  2. Solutions • soluteis the dissolved substance • seems to “disappear” • “takes on the state” of the solvent • solvent is the substance solute dissolves in • does not appear to change state • when both solute and solvent have the same state, the solvent is the component present in the highest percentage • solutions in which the solvent is water are called aqueous solutions Tro, Chemistry: A Molecular Approach

  3. moles of solute liters of solution molarity = Solution ConcentrationMolarity • moles of solute per 1 liter of solution • used because it describes how many molecules of solute in each liter of solution • if a sugar solution concentration is 2.0 M, 1 liter of solution contains 2.0 moles of sugar, 2 liters = 4.0 moles sugar, 0.5 liters = 1.0 mole sugar Tro, Chemistry: A Molecular Approach

  4. Molarity and Dissociation • the molarity of the ionic compound allows you to determine the molarity of the dissolved ions • CaCl2(aq) = Ca+2(aq) + 2 Cl-1(aq) • A 1.0 M CaCl2(aq) solution contains 1.0 moles of CaCl2 in each liter of solution • 1 L = 1.0 moles CaCl2, 2 L = 2.0 moles CaCl2 • Because each CaCl2 dissociates to give one Ca+2 = 1.0 M Ca+2 • 1 L = 1.0 moles Ca+2, 2 L = 2.0 moles Ca+2 • Because each CaCl2 dissociates to give 2 Cl-1 = 2.0 M Cl-1 • 1 L = 2.0 moles Cl-1, 2 L = 4.0 moles Cl-1 Tro, Chemistry: A Molecular Approach

  5. Solution ConcentrationMolality, m • moles of solute per 1 kilogram of solvent • defined in terms of amount of solvent, not solution • like the others • does not vary with temperature • because based on masses, not volumes Tro, Chemistry: A Molecular Approach

  6. Percent • parts of solute in every 100 parts solution • mass percent = mass of solute in 100 parts solution by mass • if a solution is 0.9% by mass, then there are 0.9 grams of solute in every 100 grams of solution • or 0.9 kg solute in every 100 kg solution Tro, Chemistry: A Molecular Approach

  7. Using Concentrations asConversion Factors • concentrations show the relationship between the amount of solute and the amount of solvent • 12%(m/m) sugar(aq) means 12 g sugar  100 g solution • or 12 kg sugar  100 kg solution; or 12 lbs.  100 lbs. solution • 5.5%(m/v) Ag in Hg means 5.5 g Ag  100 mL solution • 22%(v/v) alcohol(aq) means 22 mL EtOH  100 mL solution • The concentration can then be used to convert the amount of solute into the amount of solution, or vice versa Tro, Chemistry: A Molecular Approach

  8. Preparing a Solution • need to know amount of solution and concentration of solution • calculate the mass of solute needed • start with amount of solution • use concentration as a conversion factor • 5% by mass Þ 5 g solute  100 g solution • “Dissolve the grams of solute in enough solvent to total the total amount of solution.” Tro, Chemistry: A Molecular Approach

  9. Example • How would you prepare 250.0 g of 5.00% by mass glucose solution (normal glucose)?

  10. grams solute grams solution x 106 Solution ConcentrationPPM • grams of solute per 1,000,000 g of solution • mg of solute per 1 kg of solution • 1 liter of water = 1 kg of water • for water solutions we often approximate the kg of the solution as the kg or L of water mg solute L solution mg solute kg solution

  11. Example • What volume of 10.5% by mass soda contains 78.5 g of sugar? Density of glucose is 1.04g/ml

  12. moles of components A total moles in the solution mole fraction of A = XA= Solution ConcentrationsMole Fraction, XA • the mole fractionis the fraction of the moles of one component in the total moles of all the components of the solution • total of all the mole fractions in a solution = 1 • unitless • the mole percentageis the percentage of the moles of one component in the total moles of all the components of the solution • = mole fraction x 100% Tro, Chemistry: A Molecular Approach

  13. Example • What is the molarity of a solution prepared by mixing 17.2 g of C2H6O2 with 0.500 kg of H2O to make 515 mL of solution? • What is the percent by mass of a solution prepared by mixing 17.2 g of C2H6O2 with 0.500 kg of H2O to make 515 mL of solution? • What is the mole fraction of a solution prepared by mixing 17.2 g of C2H6O2 with 0.500 kg of H2O to make 515 mL of solution?

  14. A water sample is found to contain the pollutant chlorobenzene with a concentration of 15 ppb (by mass). What volume of this water contains 5.00 x 102 mg of chlorobenzene? Assume density of 1.00 g/ml

  15. Spontaneous Mixing Tro, Chemistry: A Molecular Approach

  16. Mixing and the Solution ProcessEntropy • formation of a solution does not necessarily lower the potential energy of the system • the difference in attractive forces between atoms of two separate ideal gases vs. two mixed ideal gases is negligible • yet the gases mix spontaneously • the gases mix because the energy of the system is lowered through the release of entropy • entropy is the measure of energy dispersal throughout the system • energy has a spontaneous drive to spread out over as large a volume as it is allowed Tro, Chemistry: A Molecular Approach

  17. Intermolecular Forces and the Solution ProcessEnthalpy of Solution • energy changes in the formation of most solutions also involve differences in attractive forces between particles • must overcome solute-solute attractive forces • endothermic • must overcome some of the solvent-solvent attractive forces • endothermic • at least some of the energy to do this comes from making new solute-solvent attractions • exothermic Tro, Chemistry: A Molecular Approach

  18. Intermolecular Attractions Tro, Chemistry: A Molecular Approach

  19. Relative Interactions and Solution Formation • when the solute-to-solvent attractions are weaker than the sum of the solute-to-solute and solvent-to-solvent attractions, the solution will only form if the energy difference is small enough to be overcome by the entropy Tro, Chemistry: A Molecular Approach

  20. Will It Dissolve? • Chemist’s Rule of Thumb – Like Dissolves Like • a chemical will dissolve in a solvent if it has a similar structure to the solvent • when the solvent and solute structures are similar, the solvent molecules will attract the solute particles at least as well as the solute particles to each other Tro, Chemistry: A Molecular Approach

  21. Classifying Solvents Tro, Chemistry: A Molecular Approach

  22. Example 12.1a  predict whether the following vitamin is soluble in fat or water The 4 OH groups make the molecule highly polar and it will also H-bond to water. Vitamin C is water soluble Vitamin C Tro, Chemistry: A Molecular Approach

  23. Example 12.1b  predict whether the following vitamin is soluble in fat or water The 2 C=O groups are polar, but their geometric symmetry suggests their pulls will cancel and the molecule will be nonpolar. Vitamin K3 is fat soluble Vitamin K3 Tro, Chemistry: A Molecular Approach

  24. Energetics of Solution Formation • overcome attractions between the solute particles – endothermic • overcome some attractions between solvent molecules – endothermic • for new attractions between solute particles and solvent molecules – exothermic • the overall DH depends on the relative sizes of the DH for these 3 processes DHsol’n = DHsoluteDHsolvent + DHmix Tro, Chemistry: A Molecular Approach

  25. Heats of Hydration • for aqueous ionic solutions, the energy added to overcome the attractions between water molecules and the energy released in forming attractions between the water molecules and ions is combined into a term called the heat of hydration • attractive forces in water = H-bonds • attractive forces between ion and water = ion-dipole • DHhydration = heat released when 1 mole of gaseous ions dissolves in water Tro, Chemistry: A Molecular Approach

  26. Ion-Dipole Interactions • when ions dissolve in water they become hydrated • each ion is surrounded by water molecules Tro, Chemistry: A Molecular Approach

  27. Solution Equilibrium Tro, Chemistry: A Molecular Approach

  28. Solubility Limit • a solution that has the maximum amount of solute dissolved in it is said to be saturated • depends on the amount of solvent • depends on the temperature • and pressure of gases • a solution that has less solute than saturation is said to be unsaturated • a solution that has more solute than saturation is said to be supersaturated Tro, Chemistry: A Molecular Approach

  29. Temperature Dependence of Solubility of Solids in Water • solubility is generally given in grams of solute that will dissolve in 100 g of water • for most solids, the solubility of the solid increases as the temperature increases • when DHsolution is endothermic • solubility curves can be used to predict whether a solution with a particular amount of solute dissolved in water is saturated (on the line), unsaturated (below the line), or supersaturated (above the line) Tro, Chemistry: A Molecular Approach

  30. Temperature Dependence of Solubility of Gases in Water • solubility is generally given in moles of solute that will dissolve in 1 Liter of solution • generally lower solubility than ionic or polar covalent solids because most are nonpolar molecules • for all gases, the solubility of the gas decreases as the temperature increases • the DHsolution is exothermic because you do not need to overcome solute-solute attractions Tro, Chemistry: A Molecular Approach

  31. Soap Action • most dirt and grease is made of nonpolar molecules – making it hard for water to remove it from the surface • soap molecules form micelles around the small oil particles with the polar/ionic heads pointing out • this allows the micelle to be attracted to water and stay suspended Tro, Chemistry: A Molecular Approach

  32. The polar heads of the micelles attract them to the water, and simultaneously repel other micelles so they will not coalesce and settle out. Tro, Chemistry: A Molecular Approach

  33. Tro, Chemistry: A Molecular Approach

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