Slide 1

Chapter 5: Water for Life Slide 2 Water has never lost its mystery. After at least two and a half millennia of philosophical and scientific inquiry, the most vital of the worlds substances remains surrounded by deep uncertainties. Without too much poetic license, we can reduce these questions to a single bare essential: What exactly is water? Philip Ball, in Lifes Matrix: A Biography of Water, University of California Press, Berkeley, CA, 2001, p. 115 Do you know where your drinking water comes from? Do you know if your drinking water is safe to drink? How would you know? Slide 3 5.1 Different Representations of Water Lewis structures Space-filling Slide 4 5.1 EN Values assigned by Linus Pauling, winner of TWO Nobel Prizes. Electronegativity is a measure of an atoms attraction for the electrons it shares in a covalent bond. On periodic table, EN increases Slide 5 5.1 H H O A difference in the electronegativities of the atoms in a bond creates a polar bond. Partial charges result from bond polarization. A polar covalent bond is a covalent bond in which the electrons are not equally shared, but rather displaced toward the more electronegative atom. Slide 6 5.1 H 2 has a nonpolar covalent bond. NaCl NaCl has an ionic bond look at the EN difference. Na = 1.0 Cl = 2.9 EN = 1.9 A water molecule is polar due to polar covalent bonds and the shape of the molecule. Slide 7 5.2 Polarized bonds allow hydrogen bonding to occur. Hbonds are intermolecular bonds. Covalent bonds are intramolecular bonds. A hydrogen bond is an electrostatic attraction between an atom bearing a partial positive charge in one molecule and an atom bearing a partial negative charge in a neighboring molecule. The H atom must be bonded to an O, N, or F atom. Hydrogen bonds typically are only about one-fifteenth as strong as the covalent bonds that connect atoms together within molecules. Slide 8 5.3 Water Footprint Water is necessary to produce food: Slide 9 5.3 Water Footprint Water is necessary for products: Slide 10 5.3 International Water Footprint (per capita) Slide 11 Where Does Potable (fit for consumption) Drinking Water Come From? Surface water: from lakes, rivers, reservoirs Ground water: pumped from wells drilled into underground aquifers 5.4 Slide 12 Much of our clean water comes from underground aquifers. The Ogallala Aquifer is shown in dark blue. While normally free of pollutants, groundwater can be contaminated by a number of sources: Abandoned mines Poorly constructed landfills and septic systems Runoff from fertilized fields Household chemicals poured down the drain or on the ground The average American uses almost 100 gallons of water a day. Nearly of the water entering our homes goes down the drain. 5.4 Slide 13 Access to safe drinking water varies widely across the world. Slide 14 5.5 A solution is a homogeneous mixture of uniform composition. Solutions are made up of solvents and solutes. Substances capable of dissolving other substances usually present in the greater amount. Substances dissolved in a solvent usually present in the lesser amount. When water is the solvent, you have an aqueous solution. Slide 15 5.5 Concentration Terms Parts per hundred (percent) Parts per million (ppm) Parts per billion (ppb) 20 g of NaCl in 100 g of water is a 20% NaCl solution Slide 16 Molarity (M) = moles solute liter of solution 1.0 M NaCl solution [NaCl] = 1.0 M = 1.0 mol NaCl/L solution Also this solution is 1.0 M in Na + and 1.0 M in Cl [Na + ] = 1.0 M and [Cl ] = 1.0 M [ ] = concentration of 5.5 Slide 17 How to prepare a 1.00 M NaCl solution: Note you do NOT add 58.5 g NaCl to 1.00 L of water. The 58.5 g will take up some volume, resulting in slightly more than 1.00 L of solution and the molarity would be lower. mol solute L of solution M = Slide 18 What is the concentration (in M and mass %) of the resulting solution when you add 5 grams of NaOH to 95 mL of water? 95 mL H 2 O = 95 g H 2 Omass %: 5 g NaOH/100 g solution 95 mL H 2 O =.095 L = 5% NaOH 5 g NaOH = 0.125 moles NaOH 0.125 mole NaOH/0.095 L = 1.3 M solution of NaOH 5.5 Slide 19 What is the molarity of glucose (C 6 H 12 O 6 ) in a solution containing 126 mg glucose per 100.0 mL solution? 6.99 x 10 3 M Slide 20 5.6 When ions (charged particles) are in aqueous solutions, the solutions are able to conduct electricity. (a) Pure distilled water (nonconducting) (b) Sugar dissolved in water (nonconducting): a nonelectrolyte (c) NaCl dissolved in water (conducting): an electrolyte Slide 21 Forming ions 5.6 Slide 22 Some atoms form more than one stable ion. 5.7 Slide 23 Naming simple ionic compounds is easy. Name the metallic element (cation) first, followed by the nonmetallic element (the anion) second, but with an ide suffix. 5.7 MgO Mg is the metal; O is the nonmetal. magnesium oxide NaBr Na is the metal; Br is the nonmetal. sodium bromide Slide 24 5.7 Ions that are themselves made up of more than one atom or element are called polyatomic ions. NaSO 4 (sodium sulfate) dissociates in water to form: Na + Sodium ions and Sulfate ions The sulfate group stays together in solution. Slide 25 Naming polyatomic ionic compounds is also easy. Name the cation first, followed by the anion second. 5.7 MgOH Mg + is the cation; OH is the anion. magnesium hydroxide NH 4 Br NH 4 + is the cation; Br is the anion. ammonium bromide Slide 26 5.8 Substances that will dissociate in solution are called electrolytes. Dissolution of NaCl in Water The polar water molecules stabilize the ions as they break apart (dissociate). Ions are simply charged particles atoms or groups of atoms. They may be positively charged cations. Or negatively charged anions. NaCl(s) Na + (aq) + Cl (aq) H2OH2O Slide 27 5.8 Simple generalizations about ionic compounds allow us to predict their water solubility. *Insoluble means that the compounds have extremely low solubility in water (less than 0.01 M). All ionic compounds have at least a very small solubility in water. Slide 28 5.9 Covalent molecules in solution A sucrose molecule when dissolved in water, sugar molecules interact with and become surrounded by water molecules, but the sucrose molecules do not dissociate like ionic compounds do; covalent molecules remain intact when dissolved in solution. They will not conduct electricity; they are nonelectrolytes. Slide 29 Like dissolves like 5.9 Slide 30 5.10 Maximum Contaminant Level Goal (MCLG) and Maximum Contaminant Level (MCL) Slide 31 5.10 Nitrate concentrations from California domestic groundwater wells and agricultural irrigation Slide 32 5.11 Schematic drawing of a typical municipal water treatment facility. Slide 33 5.12 Making freshwater from saltwater Desalinization a process that removes ions from saltwater Slide 34 5.12 Making freshwater from saltwater continued Distillation a separation process in which a liquid solution is heated and the vapors are condensed and collected Either perform distillation in laboratory (left) or use solar power (right). Slide 35 5.12 Making freshwater from saltwater continued Osmosis the passage of water through a semipermeable membrane from a solution that is less concentrated to a solution that is more concentrated Reverse Osmosis uses pressure to force the movement of water through a semipermeable membrane from a solution that is more concentrated to a solution that is less concentrated Slide 36 5.12 LifeStraw created for developing countries to remove bacteria, viruses, and parasites from water to use for drinking Slide 37 Water, water, every where, And all the boards did shrink; Water, water, every where, Nor any drop to drink. And every tongue, through utter drought, Was withered at the root; We could not speak, no more than if We had been choked with soot. The Rime of the Ancient Mariner, excerpt Samuel Taylor Coleridge