I. Importance. II. Properties. II. Properties. III. Dissolving in Water. II. Properties. A. Formation/Destruction. Topic 5: The Water We Drink

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1 Topic 5: The Water We Drink I. Importance Why so Important? We need it. We use a lot of it. (300 L/day per person, drink only 1-2 L) II. Properties A. Formation/Destruction H 2 + O 2 H 2 O + E water forms b/c it s more stable than H 2 and O 2 II. Properties A. Formation/Destruction Showing the same thing on reaction coordinates: H 2 O + E H 2 + O 2 II. Properties B. 5 Ways to Express Water H 2 O H-O-H H xo O ox H H xo O ox H III. Dissolving in Water Nonpolar compounds: electrons evenly distributed no partial charges anywhere. Polar compounds: regions of partial (+) charges and partial (-) charges but a net charge of 0. The δ+/ δ- on water molecules behave like other charges: 1

2 III. Dissolving in Water The δ+/ δ- on water molecules line up and the attraction holds the molecules together. This is stabilizing. Other polar or charged molecules can also be stabilized this way, therefore they dissolve in water. III. Dissolving in Water A. Nonpolar Molecules: Recognize as mostly carbon and hydrogen. No charges. No dissolving. Ex. Oil, lycopodium. III. Dissolving in Water B. Covalent molecules Recognize by a mix of atoms, especially O, N, or F. Do not separate in water remain intact. Ex. sucrose 5.9 III. Dissolving in Water C. Ionic compounds (a.k.a. salts) : 2 or more ions (full +/-) held together by attraction of opposite charges. Substances that dissociate in water are called electrolytes. Ions are simply charged particles-atoms or groups of atoms. Dissolution of NaCl in Water H NaCl(s) 2 O Na + (aq) + Cl - (aq) The polar water molecules stabilize the ions as they break apart (dissociate). 2

3 IV. Electricity & Water Pure water does not conduct electricity. IV. Electricity & Water Ions dissolved in water conduct electricity. The more ions there are, the more electricity is conducted. (a) Pure distilled water (non-conducting) (b) Sugar dissolved in water (non-conducting): a nonelectrolyte (c) NaCl dissolved in water (conducting): an electrolyte 5.7 V. IMFs Intramolecular forces- within a molecule, holds the atoms together in a molecule. Intermolecular forces (imfs)- V. IMFs Imf strength is proportional to the energy needed to pull molecules apart. Important to recognize the 4 types of imfs and relative strengths. If you recognize the type of imf present, you can predict properties. V. IMFs London dispersion forces: Dipole Dipole: Attractions between opposite V. IMFs Hydrogen bonds: very strong dipole-dipole imfs caused by LARGE partial charges. Rquires H bonded to O, N, or F (e - greedy atoms) Ionic imfs- STRONGEST. Between ions with complete +/-. 3

4 V. IMFs V. IMFs As imf strength increases, so does energy needed to move molecules apart. Temperature is proportional to amt of E needed. Ex. Rank from highest b.p. to lowest b.p. H 2 O, CH 4, CHCl 3, NaCl B. Ice Floats D water = 1.0 g/ml D ice = 0.9 g/ml B. Ice Floats D = mass volume same for ice and water 1 ml = 1 cm 3 bigger for ice than liquid B. Ice Floats Forming hydrogen bonds is stabilizing. Molecules arrange themselves to form the maximum number of H-bonds to be most stable. B. Ice Floats The arrangement that forms the most H- bonds has water molecules pretty far apart. 4

5 C. Bugs can walk on water (aka surface tension). Density determines whether something sinks or floats. D needle > D water????? Surface tension: H-bonds are stabilizing. More H-bonds = more stable. Molecules in center have more neighbors and thus more chances to form H-bonds than molecules on surface. D. Heating Water Requires Lots of energy. Specific heat- the amount of E needed to raise the temp of 1 g a substance by 1 o C. D. Heating Water Requires Lots of energy. specific heat for water = 1 cal/g o C = J/g o C To determine the amount of E needed to change temp: E= specific heat x mass x ΔT ΔT = final -initial 5

6 Ex. Find E needed to heat 1 L water from 10 o C to 50 o C. E = mass x ΔT x specific heat VII. Soap and Water Why won t water rinse out a grease stain???? How does soap help? VII. Soap and Water VII. Soap and Water Soap can also meet polarity requirements on the surface of water. Therefore it lowers the surface tension of water. 0.1% soap drops surface tension by 70% Called a surfactant. Makes water a better wetting agent. VII. Availability of Water Earth has plenty of water, but most is not safe (salt, microorganisms ). Potable VII. Availability of Water 3 Methods of water purification: Treatment plants Distillation Reverse Osmosis Ground water: pumped from wells drilled in aquifers, pools of water trapped in rock and sand underground Surface water: lakes, rivers 6

7 A. Treatment Plants Screen solids (dead bodies) filter fine particles kill microorganisms O 3 ( cheaper) HClO (from bleach, Cl 2 gas, or CaOCl) may then aerate, neutralize, or add NaF Leave high boiling point contaminates behind. (like salt) $$$- who uses it???? B. Distillation C. Reverse Osmosis Osmosis- water naturally flows through a membrane to the side with more stuff dissolved. C. Reverse Osmosis Applications of osmosis: Reverse osmosis- use high pressure to force water to move in the opposite direction (away from solute). C. Reverse Osmosis D. Other stuff you don t want in your H 2 O Hard water ( lots of Ca 2+ and Mg 2+ ) $$$ Lead 7

8 Getting the lead out: D. Other stuff you don t want in your H 2 O Pb 2+ - causes neurological problems Sources: pipes, joints, paint Maximum contaminant level goal (MCLG) of 0, set by EPA. Measured by calibration graph from spectrophotometry. Schematic of a typical spectrophotometer Using a plot of absorbance vs. concentration Calibration graph 5.12 D. Other stuff you don t want in your H 2 O D. Other stuff you don t want in your H 2 O Remedy #1: Remove Ca 2+ and Mg 2+ before use. Ion exchange- Chelator- A crab-like molecule +2 Remedy #2: Pull out Ca 2+ /Mg 2+ while using water by adding a chelator. (Calgon, Borax ) 8