Chapter 12 Solutions

1. Chemistry: A Molecular Approach, 2nd Ed.Nivaldo Tro Chapter 12Solutions Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA

2. Thirsty Seawater • Drinking seawater can cause you to dehydrate • Seawater is a homogeneous mixture of salts with water • Seawater contains high concentrations of salts • higher than the salt content of your cells • As seawater passes through your body, it pulls water out of your cells; due mainly to nature’s tendency toward spontaneous mixing • This reduces your cells’ water level and usually results in diarrhea as this extra liquid flows out with the seawater Tro: Chemistry: A Molecular Approach, 2/e

3. Seawater Tro: Chemistry: A Molecular Approach, 2/e

4. Seawater • Drinking seawater will dehydrate you and give you diarrhea • The cell wall acts as a barrier to solute moving so the only way for the seawater and the cell solution to have uniform mixing is for water to flow out of the cells of your intestine and into your digestive tract Tro: Chemistry: A Molecular Approach, 2/e

5. Solutions • Homogeneous mixtures • composition may vary from one sample to another • appears to be one substance, though really contains multiple materials • Most homogeneous materials we encounter are actually solutions • e.g., air and seawater • Nature has a tendency toward spontaneous mixing • generally, uniform mixing is more energetically favorable Tro: Chemistry: A Molecular Approach, 2/e

6. Solutions • When table salt is mixed with water, it seems to disappear, or become a liquid – the mixture is homogeneous • the salt is still there, as you can tell from the taste, or simply boiling away the water • Homogeneous mixtures are called solutions • The component of the solution that changes state is called the solute • The component that keeps its state is called the solvent • if both components start in the same state, the major component is the solvent Tro: Chemistry: A Molecular Approach, 2/e

7. Examples of Solutions Tro: Chemistry: A Molecular Approach, 2/e

8. Solutions that contain Hg and some other metal are called amalgams Solutions that contain metal solutes and a metal solvent are called alloys Common Types of Solution Tro: Chemistry: A Molecular Approach, 2/e

9. Brass Tro: Chemistry: A Molecular Approach, 2/e

10. Solubility • When one substance (solute) dissolves in another (solvent) it is said to be soluble • salt is soluble in water • bromine is soluble in methylene chloride • When one substance does not dissolve in another it is said to be insoluble • oil is insoluble in water • The solubility of one substance in another depends on two factors – nature’s tendency toward mixing, and the types of intermolecular attractive forces Tro: Chemistry: A Molecular Approach, 2/e

11. Spontaneous Mixing When solutions with different solute concentrations come in contact, they spontaneously mix to result in a uniform distribution of solute throughout the solution Tro: Chemistry: A Molecular Approach, 2/e

12. Mixing and the Solution Process:Entropy • Most processes occur because the end result has less potential energy • But formation of a solution does not necessarily lower the potential energy of the system • When two ideal gases are put into the same container, they spontaneously mix • even though the difference in attractive forces is negligible • The gases mix because the energy of the system is lowered through the release of entropy Tro: Chemistry: A Molecular Approach, 2/e

13. Mixing and the Solution ProcessEntropy • 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 • By each gas expanding to fill the container, it spreads its energy out and lowers its entropy Tro: Chemistry: A Molecular Approach, 2/e

14. Intermolecular Forces and the Solution Process • Energy changes in the formation of most solutions also involve differences in attractive forces between the particles • For the solvent and solute to mix you must overcome 1. all of the solute–solute attractive forces 2. some of the solvent–solvent attractive forces • both processes are endothermic • At least some of the energy to do this comes from making new solute–solvent attractions • which is exothermic Tro: Chemistry: A Molecular Approach, 2/e

15. Intermolecular Attractions Tro: Chemistry: A Molecular Approach, 2/e

16. Solution Interactions Tro: Chemistry: A Molecular Approach, 2/e

17. 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 increase in entropy from mixing Tro: Chemistry: A Molecular Approach, 2/e

18. Solubility • There is usually a limit to the solubility of one substance in another • gases are alwayssoluble in each other • two liquids that are mutually soluble are said to be miscible • alcohol and water are miscible • oil and water are immiscible • The maximum amount of solute that can be dissolved in a given amount of solvent is called the solubility • The solubility of one substance in another varies with temperature and pressure Tro: Chemistry: A Molecular Approach, 2/e

19. 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 are attracted to each other • Polar molecules and ionic compounds will be more soluble in polar solvents • Nonpolar molecules will be more soluble in nonpolar solvents Tro: Chemistry: A Molecular Approach, 2/e

20. Classifying Solvents Tro: Chemistry: A Molecular Approach, 2/e

21. Example 12.1a: Predict whether the following vitamin is soluble in fat or water Water is a polar solvent. Fat is mostly made of nonpolar molecules. The four 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, 2/e

22. Example 12.1b: Predict whether the following vitamin is soluble in fat or water Water is a polar solvent. Fat is mostly made of nonpolar molecules. The two 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, 2/e

23. Practice – Decide if the following are more soluble in hexane, C6H14, or water nonpolar molecule more soluble in C6H14 polar molecule more soluble in H2O nonpolar part dominant more soluble in C6H14 Tro: Chemistry: A Molecular Approach, 2/e

24. Practice – Explain the solubility trends seen in the table below Tro: Chemistry: A Molecular Approach, 2/e

25. Practice – Explain the solubility trends seen in the table below These alcohols all have a polar OH part and a nonpolar CHn part. As we go down the table the nonpolar part gets larger, but the amount of OH stays the same. We therefore expect that the solubility in water (polar solvent) should decrease and the solubility in hexane (nonpolar solvent) should increase, and it does. Tro: Chemistry: A Molecular Approach, 2/e

26. Heat of Solution • When some compounds, such as NaOH, dissolve in water, a lot of heat is released • the container gets hot • When other compounds, such as NH4NO3, dissolve in water, heat is absorbed from the surroundings • the container gets cold • Why is this? Tro: Chemistry: A Molecular Approach, 2/e

27. Energetics of Solution Formation: the Enthalpy of Solution • To make a solution you must 1. overcome all attractions between the solute particles; therefore DHsolute is endothermic 2. overcome some attractions between solvent molecules; therefore DHsolvent is endothermic 3. form new attractions between solute particles and solvent molecules; therefore DHmix is exothermic • The overall DH for making a solution depends on the relative sizes of the DH for these three processes DHsol’n = DHsolute + DHsolvent + DHmix Tro: Chemistry: A Molecular Approach, 2/e

28. 1. Add energy in to overcome all solute–solute attractions 3. Form new solute–solvent attractions, releasing energy 2. Add energy in to overcome some solvent–solvent attractions Solution Process Tro: Chemistry: A Molecular Approach, 2/e

29. If the total energy cost for breaking attractions between particles in the pure solute and pure solvent is less than the energy released in making the new attractions between the solute and solvent, the overall process will be exothermic If the total energy cost for breaking attractions between particles in the pure solute and pure solvent is greater than the energy released in making the new attractions between the solute and solvent, the overall process will be endothermic Energetics of Solution Formation Tro: Chemistry: A Molecular Approach, 2/e

30. Heats of Hydration • For aqueous solutions of ionic compounds, 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 between ions = lattice energy • DHsolute = −DHlattice energy • 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 = DHsolvent + DHmix Tro: Chemistry: A Molecular Approach, 2/e

31. Ion-Dipole Interactions • When ions dissolve in water they become hydrated • each ion is surrounded by water molecules • The formation of these ion-dipole attractions causes the heat of hydration to be very exothermic Tro: Chemistry: A Molecular Approach, 2/e

32. Heats of Solution for Ionic Compounds • For an aqueous solution of an ionic compound, the DHsolution is the difference between the Heat of Hydration and the Lattice Energy DHsolution = DHsolute+ DHsolvent + DHmix DHsolution = −DHlattice energy+ DHsolvent + DHmix DHsolution = DHhydration−DHlattice energy DHsolution = −DHlattice energy + DHhydration Tro: Chemistry: A Molecular Approach, 2/e

33. Heat of Hydration DHsolution = DHhydration−DHlattice energy Tro: Chemistry: A Molecular Approach, 2/e

34. Comparing Heat of Solution to Heat of Hydration • Because the lattice energy is always exothermic, the size and sign on the DHsol’n tells us something about DHhydration • If the heat of solution is large and endothermic, then the amount of energy it costs to separate the ions is more than the energy released from hydrating the ions • DHhydration < DHlattice when DHsol’n is (+) • If the heat of solution is large and exothermic, then the amount of energy it costs to separate the ions is less than the energy released from hydrating the ions • DHhydration > DHlattice when DHsol’n is (−) Tro: Chemistry: A Molecular Approach, 2/e

35. Practice – What is the lattice energy of KI if DHsol’n = +21.5 kJ/mol and the DHhydration = −583 kJ/mol? Tro: Chemistry: A Molecular Approach, 2/e

36. DHsol’n, DHhydration DHlattice Practice – What is the lattice energy of KI if DHsol’n = +21.5 kJ/mol and the DHhydration = −583 kJ/mol? Given: Find: DHsol’n = +21.5 kJ/mol, DHhydration = −583 kJ/mol DHlattice, kJ/mol Conceptual Plan: Relationships: DHsol’n = DHhydration− DHlattice Solve: Check: the unit is correct, the lattice energy being exothermic is correct Tro: Chemistry: A Molecular Approach, 2/e

37. Solution Equilibrium • The dissolution of a solute in a solvent is an equilibrium process • Initially, when there is no dissolved solute, the only process possible is dissolution • Shortly after some solute is dissolved, solute particles can start to recombine to reform solute molecules – but the rate of dissolution >> rate of deposition and the solute continues to dissolve • Eventually, the rate of dissolution = the rate of deposition – the solution is saturated with solute and no more solute will dissolve Tro: Chemistry: A Molecular Approach, 2/e

38. Solution Equilibrium Tro: Chemistry: A Molecular Approach, 2/e

39. Solubility Limit • A solution that has the solute and solvent in dynamic equilibrium is said to be saturated • if you add more solute it will not dissolve • the saturation concentration depends on the temperature • and pressure of gases • A solution that has less solute than saturation is said to be unsaturated • more solute will dissolve at this temperature • A solution that has more solute than saturation is said to be supersaturated Tro: Chemistry: A Molecular Approach, 2/e

40. How Can You Make a Solvent Hold More Solute Than It Is Able To? • Solutions can be made saturated at non-room conditions – then allowed to come to room conditions slowly • For some solutes, instead of coming out of solution when the conditions change, they get stuck in-between the solvent molecules and the solution becomes supersaturated • Supersaturated solutions are unstable and lose all the solute above saturation when disturbed • e.g. shaking a carbonated beverage Tro: Chemistry: A Molecular Approach, 2/e

41. Adding a Crystal of NaC2H3O2 to a Supersaturated Solution Tro: Chemistry: A Molecular Approach, 2/e

42. 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, 2/e

43. Solubility Curves Tro: Chemistry: A Molecular Approach, 2/e

44. Temperature Dependence of Solid Solubility in Water (g/100 g H2O) Tro: Chemistry: A Molecular Approach, 2/e

45. Purification by Recrystallization • One of the common operations performed by a chemist is removing impurities from a solid compound • One method of purification involves dissolving a solid in a hot solvent until the solution is saturated • As the solution slowly cools, the solid crystallizes out, leaving impurities behind Tro: Chemistry: A Molecular Approach, 2/e

46. Recrystallization of KNO3 • KNO3 can be purified by dissolving a little less then 106 g in 100 g of water at 60 ºC then allowing it to cool slowly • When it cools to 0 ºC only 13.9 g will remain in solution, the rest will precipitate out Tro: Chemistry: A Molecular Approach, 2/e

47. Practice – Decide if each of the following solutions is saturated, unsaturated, or supersaturated 50 g KNO3 in 100 g H2O @ 34 ºC saturated 50 g KNO3 in 100 g H2O @ 50 ºC unsaturated 50 g KNO3 in 50 g H2O @ 50 ºC supersaturated 100 g NH4Cl in 200 g H2O @ 70 ºC unsaturated 100 g NH4Cl in 150 g H2O @ 50 ºC supersaturated Tro: Chemistry: A Molecular Approach, 2/e

48. Temperature Dependence of Solubility of Gases in Water • Gases generally have lower solubility in water than ionic or polar covalent solids because most are nonpolar molecules • gases with high solubility usually are actually reacting with water • 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, 2/e

49. Temperature Dependence of Gas Solubility in Water (g/100 g H2O) Tro: Chemistry: A Molecular Approach, 2/e

50. Tro: Chemistry: A Molecular Approach, 2/e