Chapter 5: The water we drink

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Chapter 5 The water we drink
Chem 100 Lecture 9 February
22, 2005
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Reaction of Na and Cl
http//www.visionlearning.com/library/module_viewe
r.php?mid55
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Covalent bonding

• In covalent bonds, the electrons are shared.

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Polyatomic Ions

• Polyatomic ions are ions made up of more than one
  atom or element.
• The atoms making up the ion are covalently
  bonded, but the ion itself is involved in the
  ionic bond
• Example Sulfate, SO4
• 6 4x(6) 2 32
• S O charge octet
• 
  rule
• 
  satisfied

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Some Common Polyatomic Ions
Another polyatomic ion chlorate, ClO3-
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• EXAMPLES OF FORMULAS FOR IONIC COMPOUNDS
  CONTAINING POLYATOMIC IONS
• Compound containing K and ClO3- KClO3
• Compound containing Ca2 and ClO3- Ca(ClO3)2
• Compound containing Ca2 and PO43- Ca3(PO4)2
• NAMING IONIC COMPOUNDS THAT CONTAIN A POLYATOMIC
  ION
• The names of ionic compounds that contain a
  polyatomic ion are obtained using the following
  pattern name name of metal name of
  polyatomic ion
• Examples KClO3 is named potassium chlorate
• Ca(ClO3)2 is named calcium chlorate Ca3(PO4)2 is
  named calcium phosphate CaHPO4 is named calcium
  hydrogen phosphate.

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Most ionic compounds are soluble in water

• Why?
• Because to be soluble, the attractive forces
  between the solvent and the ions must be greater
  than the attractive forces of the ionic bonds

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Water solutions of ionic compounds

• Recall that water molecules are polar

• The partial negatively charged O is attracted to
  the cations and the partially positively charged
  H is attracted to the anions

Dissolution of NaCl animation
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• Reaction for dissolution of NaCl in water
• NaCl (s) H2O(l) ? Na(aq) Cl-(aq)
• When compounds containing polyatomic ions
  dissolve in water, the polyatomic ions remain
  intact. Example
• Na2SO4(s) H2O(l) ? 2Na(aq) SO42-(aq)

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Sucrose dissolves, but it doesnt conduct
electricitywhy?

• It doesnt form ions
• Sucrose C12H22O11
• There are no atoms at the extreme ends of the
  periodic table (not likely to form ions)
• This molecule consists solely of COVALENT BONDS

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Why does sucrose dissolve at all?

• There must be strong forces between the H2O
  molecules and the sucrose
• Sucrose has many OH groups these can form
  Hydrogen bonds with H2O molecules.

The attraction between sucrose and many H2Os will
be greater than between sucrose molecules
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Solubility of covalent compounds

• Not usually as soluble as ionic compounds
• Covalent compounds that do dissolve well are
  those with the ability to hydrogen bond with
  water
• Examples

Sucrose (sugar)
Antifreeze
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Simple solubility rule

• In general, Like dissolves like
• Polar compounds dissolve polar compounds
• But polar and nonpolar compounds do not dissolve
  in each other
• Oil and water dont mix!

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Why isnt oil soluble?

• Oil is made up of hydrocarbons
• There is only a small electronegativity
  difference H 2.1 C 2.5 Difference 0.4
• Bonds are not very polar at all
• Because it is effectively non-polar, it does not
  dissolve in water

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Protecting drinking water

• Maximum Contaminant Level Goal (MCLG) The level
  at which a person weighing 70 kg (154 lb) could
  drink 2 L of water containing the contaminant
  every day for 70 years with no ill effect
• Not a legal limit
• All known carcinogens have an MCLG of 0
• Maximum Contaminant Level (MCL)
• The legal concentration limit allowed for a
  contaminant

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Table of MCLGs and MCL
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How is drinking water treated?
You want to remove bacteria, organic matter,
suspended solids, etc.
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Screening
http//www.chemistry.wustl.edu/edudev/LabTutorial
s/Water/PublicWaterSupply/PublicWaterSupply.html
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Sedimentation
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Coagulation
The water is then filtered and disinfected
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Disinfection

• Disinfectants kill bacteria that can cause
  disease in humans
• Cryptosporidium, Giardia, fecal colifrom
• Chlorine most common disinfectant in U.S.
• Taste, odor
• Chlorination has the potential to form
  undesirable by-products
• Trihalomethanes (THMs) such as Chloroform,
  (CHCl3)
• Ozone More powerful disinfectant than chlorine
  so less is needed
• More expensive
• Decomposes quickly
• Also forms byproducts

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Hard water

• Hard water is water that contains certain ions
  from dissolved minerals
• Ca2 , Mg2 , CO32- , HCO3-, SO42-
• Soft water contains few of these ions
• Measure of hardness is ppm CaCO3
• This is NOT to say that a sample actually
  contains this amount of solid CaCO3
• Rather, this specifies the amount of CaCO3 that
  could be formed from the Ca2 provided there is
  also enough CO32- present.
• Ca2 (aq) CO32- (aq) ? CaCO3 (s)
• Why is hardness measured this way? Ca2 is
  largest contributor

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Source

• Source of hard water is limestone rock
• Composed of CaCO3 and MgCO3
• When water flows over limestone, these minerals
  dissolve
• In US, water hardness varies from nearly 0 ppm to
  over 400 ppm in Midwest

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How soap works

• Soap

• Because of polar end, soap dissolves in water
• Greases, oil, and dirt are nonpolar, so are not
  soluble in water

• Dirt, grease etc. will dissolve in the non-polar
  part of of the soap ion. Many soap ions surround
  the dirt particle, with their polar ends facing
  water.
• The negatively charged grease molecule can now
  be dissolved in water.

animation
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Soap and hard water
Ca2 , Mg2 , etc ions form ionic compounds with
soap that are less soluble than the ionic
compound containing Na

• Consequences of this lower solubility
• It precipitates out of solution (soap scum!)
• More soap is required to get the same amount of
  cleaning power

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What to do about hard water

• Soften it! (Remove the Ca2 and Mg2)
• How?
• Add Na2CO3
• Ion exchange column

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Na2CO3

• Na2CO3 (s)?2 Na (aq) CO32- (aq)
• Ca2 (aq) CO32- (aq) ? CaCO3 (s)
• The CaCO3 is an insoluble mineral that can be
  rinsed away.

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2) Ion exchange

• Typically contains zeolite claylike mineral
  made up of Al, Si, O
• Surface is negatively charged
• Normally balanced by Na ions

-- Ions with greater charge will be more
strongly bound to the zeolite surface, and will
replace the Na ions
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Regeneration of ion exchangers

• Eventually the zeolite gets full of Ca2, Mg2,
  ions.
• Can add salt (NaCl) to regenerate the zeolite
  back so it is bonded to Na ions instead

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Desalination

• Desalination- A broad term describing any process
  that removes ions from salty water
• Ways to desalinate water
• Distillation
• Reverse osmosis

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Distillation

• DistillationA liquid is evaporated and then
  condensed
• Based on the fact that as the water vaporizes, it
  leaves behind most of the dissolved impurities
• Passes through a condenser where it cools and
  reverts back to a liquidimpurity free!
• Distilled water is produced by this process

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Energy involved in distillation

• Remember that water has a high boiling point and
  high specific heat
• This means that it requires a large amount of
  energy to get the liquid into the gas phase.
• This process will only be cost effective for
  countries with abundant and cheap energy

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Osmosis

• Osmosis The natural tendency for a solvent to
  move through a membrane from a region of higher
  solvent concentration to a region of lower
  solvent concentration.
• animation

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Reverse Osmosis

• Reverse osmosis
• Can reverse this process if you provide enough
  pressure to the salt water side