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Chapter 12

Chapter 12. Aqueous Solutions. Setting the Stage – Water as a Solvent. Most of the surface of the earth is covered with water with a high concentration of dissolved compounds and suspended matter. Our blood is also mostly water, with a wide variety of dissolved and suspended substances.

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Chapter 12

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  1. Chapter 12 Aqueous Solutions

  2. Setting the Stage – Water as a Solvent • Most of the surface of the earth is covered with water with a high concentration of dissolved compounds and suspended matter. • Our blood is also mostly water, with a wide variety of dissolved and suspended substances. • Water is a remarkable solvent, and is central to the understanding of chemistry. Malone and Dolter - Basic Concepts of Chemistry 9e

  3. Setting a Goal - Part ASolutions and the Quantities Involved • You will learn how water acts to dissolve specific types of compounds and how concentrations of solutes are expressed and used. Malone and Dolter - Basic Concepts of Chemistry 9e

  4. Objective for Section 12-1 • Describe the role of water in the formation of aqueous solutions of ionic compounds, strong acids, and polar compounds. Malone and Dolter - Basic Concepts of Chemistry 9e

  5. Professor’s Joke (with Apologies) One day, a biologist, a chemist and a physicist were exploring the ocean in a boat for the very first time. The physicist said, “I’m going to determine the physical laws of the ocean”. Off he went into the water, but he never returned. Later, the biologist said, “ I’m going to search the water for living organisms”. He too entered the ocean and was never seen again. The chemist waited a while and then wrote down in his note book, “Physicists and biologists are soluble in water”. Malone and Dolter - Basic Concepts of Chemistry 9e

  6. 12-1 The Nature of Aqueous Solutions • Miscible – two liquids are miscible if they mix to form a homogeneous solution. • Immiscible – two liquids are immiscible if they do not mix to form a solution (they separate into two phases like oil and water). • Our focus will be on solids dissolving to form a solution (although there are other possibilities). Malone and Dolter - Basic Concepts of Chemistry 9e

  7. Solids Dissolving in Water • Consider that when a salt such as NaCl dissolves in water • It loses Na+ - Cl- electrostatic interaction • It loses H2O - H2O interactions • It gains ion-dipole interaction (forces between the ions and the dipoles of the solvent molecules) • For dissolution to occur, the latter interaction must be stronger that the sum of the first two. Malone and Dolter - Basic Concepts of Chemistry 9e

  8. Interaction of Water and Ionic Compounds Malone and Dolter - Basic Concepts of Chemistry 9e

  9. Dissolution of Polar Compounds • Some polar compounds (e.g. strong acids) dissolve by ionization, producing dissolved ions.E.g. HCl(g) + aq  H+(aq) + Cl-(aq) or HCl(g) + H2O(l)  H3O+(aq) + Cl-(aq) Malone and Dolter - Basic Concepts of Chemistry 9e

  10. Dissolution of Polar Compounds • Many polar compounds dissolve by forming strong dipole-dipole interactions, but without ionization. E.g. methanol in water Malone and Dolter - Basic Concepts of Chemistry 9e

  11. Objectives for Section 12-2 • Define terms associated with solubility. • Predict the effects of temperature and pressure on solubility. Malone and Dolter - Basic Concepts of Chemistry 9e

  12. 12-2 The Effects of Temperature and Pressure on Solubility • The solubility of a solute is defined as the maximum amount that dissolves in a given amount of solvent at a specific temperature (usually g/100 mL at 25 ºC). • Saturated solutions are those that have the maximum amount of solute dissolved. • Unsaturated solutions are those with less than the maximum amount. Malone and Dolter - Basic Concepts of Chemistry 9e

  13. Supersaturated Solutions • Such solutions can sometimes be created where more solute is dissolved than the standard solubility. • Such solutions are metastable, and solid can crystallize out if the solution is disturbed in certain ways. Malone and Dolter - Basic Concepts of Chemistry 9e

  14. Concentration • The amount of solute present in a given amount of solvent or solution is the concentration. • Is expressed in a number of ways (one way is shown below). g of solute 100 g solvent Malone and Dolter - Basic Concepts of Chemistry 9e

  15. Temperature and Solubility • Most solids and liquids are more soluble in water at higher temperatures. • This is the basis of recrystallization – a method for purifying solids. • Gases are less soluble in water at higher temperatures (consider oxygen in water). Malone and Dolter - Basic Concepts of Chemistry 9e

  16. Effect of Pressure on Solubility • The solubility of solids and liquids is not affected to any significant extent by the external pressure. • The solubility of gases is strongly affected by pressure. Malone and Dolter - Basic Concepts of Chemistry 9e

  17. Henry’s Law • Henry’s Law states that the solubility of a gas is proportional to the partial pressure of that gas above the solution. • Solubility = kPgas The constant is known as Henry’s Law Constant. Malone and Dolter - Basic Concepts of Chemistry 9e

  18. Objective for Section 12-3 • Perform calculations of concentration involving percent by mass. Malone and Dolter - Basic Concepts of Chemistry 9e

  19. 12-3 Concentration: Percent by Mass In general, concentration can be expressed in two ways: Malone and Dolter - Basic Concepts of Chemistry 9e

  20. Percent by Mass Percent by mass expression of concentration is a very useful one. Malone and Dolter - Basic Concepts of Chemistry 9e

  21. % and Other Units by Mass • % by mass is equivalent to parts per hundred. • Extremely low concentrations are better expressed as parts per million (ppm) or parts per billion (ppb). Malone and Dolter - Basic Concepts of Chemistry 9e

  22. Parts per Million (ppm) and Parts per Billion (ppb) Sometimes it is more convenient to express concentration in ppm or ppb. Malone and Dolter - Basic Concepts of Chemistry 9e

  23. Calculating ppm and ppb Malone and Dolter - Basic Concepts of Chemistry 9e

  24. Objectives for Section 12-4 • Perform calculations of concentration involving molarity. • Perform calculations involving the dilution of concentrated solutions. Malone and Dolter - Basic Concepts of Chemistry 9e

  25. 12-4 Concentration: Molarity • Most common concentration unit used by chemists. • Number of moles of solute (n) per volume in liters (V) of solution. Malone and Dolter - Basic Concepts of Chemistry 9e

  26. Dilution of Concentrated Solutions • Materials are often shipped as concentrated solutions and diluted where it is used (think of orange juice). • We often dilute concentrated solutions in the course of being chemists. • Suppose we need to make 1.00 L (Vd)of 0.555 M (Md) HCl from the concentrated acid (11.3 M) (Mc). • What volume must we measure? Malone and Dolter - Basic Concepts of Chemistry 9e

  27. Diluting Concentrated Solutions • Number of moles of solute in dilute solution = nd = Md× Vd • Number of moles of solute in concentrated solution = nc = Mc × Vc • The number of moles of solute is constant (nd = nc), so Mc × Vc = Md × Vd Malone and Dolter - Basic Concepts of Chemistry 9e

  28. Dilution of Concentrated Solutions Solution to our dilution of concentrated HCl problem: Malone and Dolter - Basic Concepts of Chemistry 9e

  29. Dilution of Concentrated Solutions Malone and Dolter - Basic Concepts of Chemistry 9e

  30. Objective for Section 12-5 • Perform calculations involving titrations and other solution stoichiometry. Malone and Dolter - Basic Concepts of Chemistry 9e

  31. 12-5 Stoichiometry Involving Solutions • Cd(NO3)2 (aq) + Na2S(aq)  CdS(s) + 2 NaNO3 (aq) • What mass of CdS is formed when 158 mL of a 0.122 M Na2S solution reacts with excess Cd(NO3)2. ? Malone and Dolter - Basic Concepts of Chemistry 9e

  32. General Procedure: Summary Chart Malone and Dolter - Basic Concepts of Chemistry 9e

  33. Titrations • Titrations are usually volume-to-volume stoichiometry problems where a concentration is the unknown variable. • The end of the reaction is generally detected by the addition of an indicator (a substance that changes color when the reaction is just complete). Malone and Dolter - Basic Concepts of Chemistry 9e

  34. Calculating an Unknown Concentration from Titration Data • What isthe concentration of an HCl solution if 25.0 mL reacts with 38.5 mL of 0.145 M K3PO4 solution? Malone and Dolter - Basic Concepts of Chemistry 9e

  35. Malone and Dolter - Basic Concepts of Chemistry 9e

  36. Setting a Goal – Part BThe Effects of Solutes on the Properties of Water • You will learn how the physical properties of aqueous solutions differ from those of pure water. Malone and Dolter - Basic Concepts of Chemistry 9e

  37. Objective for Section 12-6 • Explain the differences between nonelectrolytes, strong electrolytes, and weak electrolytes. Malone and Dolter - Basic Concepts of Chemistry 9e

  38. 12-6 Electrical Properties of Solutions • Conductivity • Conductor – a substance that allows the flow of electrons • Insulator – a substance that resists the flow of electrons • Electrolytes – substances whose aqueous solutions conduct electricity. • It is the presence of ions in solution that allows the flow of electrons. Malone and Dolter - Basic Concepts of Chemistry 9e

  39. Physical Properties of Solutions • Nonelectrolytes – compounds whose aqueous solutions do not conduct electricity (e.g. glucose, sucrose). • Strong electrolytes – species that yield solutions that are good conductors (e.g. Na+Cl-, H2SO4). • Weak electrolytes – species that yield solutions that conduct electricity poorly. • (e.g. HF, CH3COOH) Malone and Dolter - Basic Concepts of Chemistry 9e

  40. Objectives for Section 12-7 • Explain how the presence of a solute in water affects its physical properties. • Calculate the boiling and melting points of aqueous solutions of nonelectrolytes and electrolytes. Malone and Dolter - Basic Concepts of Chemistry 9e

  41. 12-7 Colligative Properties of Solutions • Those properties that depend on the number of particles in solution are called colligative properties. • They include elevation of boiling point and depression of freezing point. • They arise due to lowering of vapor pressure. Malone and Dolter - Basic Concepts of Chemistry 9e

  42. Vapor Pressure Lowering • Vapor pressure – presence of a nonvolatile solute lowers the equilibrium vapor pressure from that of the pure solvent. • A nonvolatile solute is one that has practically no vapor pressure at the relevant temperatures. • The nonvolatile solute prevents solvent molecules from getting to the surface of the liquid. Malone and Dolter - Basic Concepts of Chemistry 9e

  43. Boiling Point Elevation • Boiling point – lowered vapor pressure of a solution results in a higher boiling point than the pure solventTb = KbmTb is number of degrees the boiling point is raised in Celsius or KelvinKb = boiling point elevation constantm = molality = mol solute/kg solvent (temperature independent concentration unit) Malone and Dolter - Basic Concepts of Chemistry 9e

  44. Freezing Point Depression • The lowered vapor pressure as a result of the solute lowers the freezing pointTf = KfmTf is the number of degrees the freezing point is loweredKf = freezing point depression constantm = molality Malone and Dolter - Basic Concepts of Chemistry 9e

  45. Calculating Molality • What is the molality of a solution of 468 g of glucose in 350 g water (molar mass of glucose = 342.5 g/mol)? • Solution Malone and Dolter - Basic Concepts of Chemistry 9e

  46. Calculating the Boiling Point of a Solution • What is the boiling point of a solution of 468 g of glucose in 350 g of water? • Solution Malone and Dolter - Basic Concepts of Chemistry 9e

  47. Osmotic Pressure • Tendency for a solvent to move through a thin porous membrane from a dilute solution to a more concentrated solution. • The membrane is said to be semipermeable in that small solvent molecules can pass through, but large hydrated species like ions cannot. Malone and Dolter - Basic Concepts of Chemistry 9e

  48. Osmotic Pressure • Solvent molecules pass through the membrane in both directions, but the rate at which they diffuse to the more dilute side of the membrane is lower. • The solutes hold back the solvent due to the solute-solvent interactions. • This uneven passage of water molecules causes the water level to rise on the concentrated side and drop on the dilute side. Malone and Dolter - Basic Concepts of Chemistry 9e

  49. Osmosis • The difference in height of solution caused by osmosis creates a pressure on the solute side, which eventually counterbalances the osmosis until equilibrium is established. • The extra pressure required to establish the equilibrium is the osmotic pressure. Malone and Dolter - Basic Concepts of Chemistry 9e

  50. Colligative Properties: Summary • Electrolytes yield a stronger effect than nonelectrolytes, since colligative properties depend on the number of particles. • For example, NaCl yields two particles when it dissolves, while glucose (a nonelectrolyte) yields only one particle. • Hence, NaCl is twice as effective as, say, glucose, at the same molality. Malone and Dolter - Basic Concepts of Chemistry 9e

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