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Lecture 9: Water for Life
SCCH 100 Integrated Chemistry
Instructor: Assist. Prof. Pasit Pakawatpanurut
Department of Chemistry
Faculty of Science, Mahidol University
Email: [email protected]
Textbook:
Chemistry in Context: Applying Chemistry to Society,
7th edition, C.H.Middlecamp (ed.), 2012.
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?
Different Representations of Water
Lewis structures Space-filling
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
HH
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.
H HH2 has a nonpolar
covalent bond.
NaCl
NaCl has an ionic
bond look at the
EN difference.
Na = 1.0
Cl = 2.9
DEN = 1.9
A water molecule is polar due to
polar covalent bonds and the shape of
the molecule.
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.
The boiling point is the temperature at which the (equilibrium) vapor
pressure of a liquid is equal to the external pressure.
The normal boiling point is the temperature at which a liquid boils when
the external pressure is 1 atm.
Maximum Density
40C
Ice is less dense than water
Density of Water
Water is a Unique Substance
Hydrogen Bonding
in Water
The hydrogen-bonded lattice structure of the common form of ice. Notice the open channels
between layers of water molecules.
Water Footprint
Water is necessary to produce food:
Water Footprint
Water is necessary for products:
International Water Footprint (per capita)
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
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.
Access to safe drinking water varies widely across the world.
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.
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
2 ppb Hg2 g Hg
1109 g H 2O
210-6 g Hg
1103 g H2O
2 g Hg1 L H 2O
Molarity (M) = moles soluteliter 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
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 soluteL of solution
M =
What is the concentration (in M and mass %) of
the resulting solution when you add 5 grams of
NaOH to 95 mL of water?
What is the molarity of glucose (C6H12O6) in a solution
containing 126 mg glucose per 100.0 mL solution?
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
+ 1 e-Na Na
Na atom Na+ ion
Forming ions
+ 1 e-
Cl atom
Cl- ion
ClCl
Some atoms form more than one stable ion.
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.
MgO Mg is the metal; O is the nonmetal.
magnesium oxide
NaBr Na is the metal; Br is the nonmetal.
sodium bromide
Ions that are themselves made up of more than one
atom or element are called polyatomic ions.
NaSO4 (sodium sulfate) dissociates in water to form:
Na+
Sodium ions
and
Sulfate ions
The sulfate group
stays together in
solution.
Naming polyatomic ionic compounds is also easy.
Name the cation first, followed by the anion second.
MgOH Mg+ is the cation; OH is the anion.
magnesium
hydroxide
NH4Br NH4+ is the cation; Br is the anion.
ammonium
bromide
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)H2O
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.
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.
Maximum Contaminant Level Goal (MCLG)
and Maximum Contaminant Level (MCL)
Schematic drawing of a typical municipal water treatment facility.
Making freshwater from saltwater
Desalinization a process that removes ions from saltwater
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).
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
LifeStraw created for developing countries to remove bacteria,
viruses, and parasites from water to use for drinking
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