October 8, chapter 5

Happy Columbus Day

Happy Thanksgiving, eh ?

• in Canada

Question: What United States President designated Thursday, Late Nov. as a national day of Thanksgiving? (see end of notes.)

Chapter 5-- Water

• properties of water
• water in our environment
• drinking water

Water is an unusual small molecule

• compare H₂O, NH₃ , CH₄, HF
• nearly same size, mass
• only water is a liquid
  • under usual circumstances
  • the others are gases

Liquids-- molecules come together

• molecules hold together
• force, attraction between molecules
• strong enough to overcome usual motion
• must be stronger forces between water molecules

Table: (species, melts, boils)

• Can see an explanation in a Table of Electronegativity
• Electronegativity is tendency of an atom to acquire another electron
  • Periodic table arranged this way
  • large values for halogens
  • pretty large values for oxygen, sulfur
  • modest values for nitrogen and carbon
  • small values on the left for Na, K
    • rule... gets larger to right of table
      • (except for noble/inert gases)
    • rule... gets larger going up the table

• If two elements differ greatly in electronegativity
  • they react to form ionic compounds
    • NaF (0.9/ 4.0)
    • MgCl₂ (1.2 / 3.0)
• When their electronegativities are closer
  • they react to form covalent bonds
  • share electrons
    • but not necessarily evenly
    • it's even in O₂, N₂, F₂ of course
• Water: H (2.1) and O (3.5)
  • forms covalent bonds
  • a simple bent molecule H-O-H
  • but O wants the electrons more

• shared electrons are, on average, closer to oxygen
  • electrons are negatively charged
  • so O is negative, H positive
    • modest charge, less than that of ions
    • instead of charged, we say bond is polar
  • molecule with polar bonds = polar molecule

• electrical charges--
  • like charges repel
  • unlike charges attract
    • so H (on one molecule) is attracted to an O on other molecule
    • as molecules go, fairly strong attraction

• Compare NH₃ and CH₄ and HF
  • N(3.0) and H (2.1) difference
    • less pronounced than water
    • still relatively polar
  • C(2.5) and H (2.1) pretty close
    • nearly uncharged
    • nearly nonpolar
  • F(4.0) and H (2.1) stronger than water
    • (similar, but H₂O has two sites to bond)

• Compare three solvents (liquids at room temp.)
  • water
    • quite polar
  • ethyl alcohol, ethanol
    • C₂H₅-O-H
    • polar, less so than water
  • hexane,
    • nonpolar

• Water is a good solvent (dissolves other species)
  • for ionic compounds, NaCl, MgCl₂, NaOH, KNO₃
  • for other polar compounds alcohols
• Water is a moderate solvent
  • for some smaller, nonpolar compounds
  • O₂ (fish couldn't live otherwise)
  • N₂ (who cares? divers can get the bends because of this)
  • CO₂ (algae need this, carbonated beverage)
    • CO₂ is more complicated, discuss later
• Water is a terrible solvent
  • for nonpolar compounds
    • like hexane (forms layers)
      • with liquids we say immiscible
    • like fats and oils
• Hexane is a terrible solvent
  • for ionic compounds, polar molecules
  • good solvent for nonpolar molecules
    • dissolves waxes, grease, some plastics
• Alcohol-- a bit of both worlds
  • one end is polar and a bit water like
  • the other end is nonpolar

We can see clear difference between dissolved ionic and nonionic compounds

• water is electrical insulator
• water with NaCl is an electrical conductor

• Solid NaCl is an electrical insulator
  • no electrons free to move
  • (metals conduct, have free electrons)
  • charged species (ions) locked in crystal lattice
    • they are not free to move
• Melt the salt and it is electrical conductor
  • not because electrons travel
  • but ions (+ and -) move across solution
• Dissolve and salt conducts
  • must be free ions
  • (polar is not enough-- water doesn't conduct)
  • NaCl (water) ----> free Na+ and free Cl- ions

• a. tendency to dissolve in water)
• b. number: amount that will dissolve in water

• Express solution concentration
  • % (typically wt of solute / wt of solvent) x100
    • solvent is the liquid
    • solute is what we dissolve (solid usually)
      • think... %=parts per hundred
    • ppm (parts per million)
      • 1 ppm = 1 mg/liter of water
  • chemists like Molarity
  • moles of soluteper liter of solution
    • (nearly per liter of solvent)
  • measure 0.1 liter of 1.0 Molar solution get 0.1 mole
  • measure 0.257 liters, get 0.257 moles
  • for a 0.25 M solution, 50 milliliters
    • 50 ml = 50 x 10-3 liters
    • 0.050 liters
    • #moles = 0.25 x 0.050
      • moles/liter x volume (liters)
      • units follow rules of arithmetic too
      • (1/liters) x liters cancels, divides

Saturated solution

• one that can not dissolve any more solute
  • (often changes with temperature)
  • most (not all) are more soluble hot
  • don't confuse-- everything dissolves faster when hot
• Amounts vary widely
  • NaCl 360 g/liter mw 58.45
  • sugar 1800 g/liter cold
  • MgCl₂ 540 g/l
  • MgF₂ .008 g/liter,virtually insoluble in water
    • ionic, but stronger forces in lattice than with water

• The word "dissolves" also has other meanings
  • an iron nail will dissolve in Coke (Pepsi at BG)
    • actually reacts with phosphoric acid
    • product is Fe⁺² ion and H₂ gas
    • product dissolves
  • might look similar to NaCl dissolving
  • difference:
    • evaporate solvent, get NaCl back
    • evaporate, won't get back Fe metal

• One special case (show up later)
  • weak acids (and weak bases)
  • literally react with water
  • HF = polar molecule
    • HF + H₂O --> H₃O⁺ + F^-
  • in water, becomes H⁺ (really H₃O⁺) and F^-
    • solution is ionic, conducts electricity
  • remove water, get back HF
  • reversible process
    • really a chemical reaction with water
    • but it is reversible (Na, Fe in acid is not)

• mentioned CO₂ earlier
  • mostly dissolves, reacts with water
  • becomes HCO₃- and H⁺ ions
  • a little becomes H₂CO₃
    • bicarbonate ion, carbonic acid

Water:

• how much
• where found on planet
• planet's water cycle
  • evaporates
  • condenses (rain, snow)
  • rivers flow to lakes, sea
    • gradually dissolve minerals enroute
  • sea gets saltier (salts don't evaporate)
• a few processes involving ice
  • snow, sleet, hail
  • glaciers flow (finally melt)
  • ice can evaporate (sublimation)
  • water vapor can freeze directly (frost)

Drinking Water

• sources of fresh water
  • wells, springs
  • surface waters (rivers, lakes)
  • rain and cisterns (store)
• salty water (oceans, salt lakes)
  • unusable for drinking
  • can desalinate
    • process is expensive

Desalination:

1. distillation

• boil water and evaporate it
  • (costly-- energy requirements)
  • cool and condense the vapor
    • (costly, cooling process)
  • most impurities do not vaporize
  • nearly pure distilled water
    • main impurities: O₂, N₂, CO₂
  • costly: boiler gets caked with minerals, needs cleaning

2. freezing

• when aqueous solution freezes
  • aqueous = solution with water as the solvent
• it's pure ice that forms
  • ice with no impurities
  • liquid keeps the impurities
    • (applejack -- a way to concentrate alcohol in hard cider)
  • desalination by freezing does work, costly

3. Ion Exchange

• water softeners, Britta filters
  • Britta, etc. mostly charcoal treatment
• chemical solids that exchange ions
  • cation exchange: remove Ca²⁺, Mg²⁺, return Na+
    • clears up hard water, but leaves salt
    • other resins: remove Na+ too, return H+
  • anion resin, swap Cl-, SO₄2- for OH-
• H+ and OH- react, form water
• 4. Reverse osmosis

Normal Osmosis

• membrane (thin plastic layer, biological)
• some will transmit water, not other species
  • semipermeable membranes
  • occurs in biology: cell walls
• Osmosis seems to be wrong direction for us
  • water passes through membrane
  • to even out salt concentration
  • pure water/ salt water
    • pure water goes into salt water region
• • osmotic pressure
• slowly pressure builds up across membrane
  • water flow stops at this pressure
  • resumes if we decrease the pressure
  • reverses if we increase the pressure

Reverse Osmosis

• add enough pressure to make
  • water move from salty side to pure side
• gradually produce more pure water
• also produce saltier water
  • best to discard, get more original water, not to use increasingly pressures to purify this
• Reverse Osmosis
• purify sea water
• (need relatively high pressures, 25 atm.)
• purify drinking water
• (relatively low pressures)
• rarely use "distilled water' now
• our labs have RO units, stores sell RO water
• RO water tastes "blah"-- some minerals add taste

What's routinely found in wells, surface waters?

• species and amounts
• is it a problem?
• sources
• preserving safe water resources
• treating water
  • treating sewage is part of the picture
  • industrial discharge is part of the picture

• John Hanson, of course.
• Who?
• The first US President
  • George Washington was the 8th President
  • First under the Constitution, but the nation existed for a number of years under the Articles of Confederation.
  • Hanson's most difficult job was preventing Washington's troops from rioting and taking control, since they had not been paid for years and until they were paid they were a serious threat.