Characteristics of Water
Chemistry in Context: Chapter 3:
The Water We Drink
All the problems with blue codes
or answers on Page 528-529.
„ Color
„ Odor
„ Taste
„ Source
„ Costs
Bottled Water
Sales are about $35 billion worldwide and $6
billion in the U. S.
„ Tap H2O is 0.1⊄ per quart; bottled water may
cost up to $1.00 per quart.
„ Annual sales growth of bottled water is >10
% ;best selling “soft drink” in Europe.
„ Certain brands are more expensive than milk,
soft drink, and West Texas crude oil.
„ Advertisement conjures up nature, purity, and
pristine beauty.
„
Page 203
Comparison of Mineral Content of Water
Page 203
Page 204
Page 203
Sources of Water
„ Surface
waters: Oceans, lakes,
rivers, snow, and glaciers.
„ Underground: Soil moisture and
aquifers, which are pools of water
trapped in sand and gravel; 50 to 500
feet underground.
„ Atmosphere: Water vapor and rain
clouds.
Figure 5.27 page 235
Figure 5.4 page 201
Ogallala Aquifer on Page 201
stretches across 8 states
Solution
„ Solution
is a homogeneous mixture of
two or more substances.
„ Solvent is the substance present in the
greatest amount.
„ Solute (s) are substances that dissolve
in a solvent.
„ Water as solvent = aqueous solution
„ Alloys = solid solution
Sources of Drinking Water
„ Surface
waters such as lakes, rivers, and
reservoirs are used in large scale water
supply systems; contains substances that
must be removed before use as drinking
water.
„ Ground water, which is usually free of
contaminants, can be pumped from wells
drilled into underground aquifers; more
common in small towns and rural areas.
Concentrations
„ Concentration:
ratio of the amount of
solutes to the amount of solvent.
„ Three types of ratios based on weight and
volume (w/v, v/v, w/w)
„ Percentages, %, parts per 100
„ 1 ppm = 1 mg solute / 1 L H2O
„ 1 ppb = 1 µg solute / 1 L H2O
„ 1 L = Volume occupied by 1000 g of water
at 4 °C
Molarity (M)
„ Molarity
is a useful concentration
unit for showing the number of
molecules or ions in a given volume
of an aqueous solution.
„ M is defined as moles of solute per
liter solution
„ 1 M NaCl = 1 mole NaCl (58.5 g) in
1 liter of solution
Properties of Water
„ Universal
solvent
„ Liquid at room temperature
„ Molar mass = 18 g / mole
„ High boiling point, 100 °C
„ Freezing Point, 0 °C; expands when
frozen.
Figure 5.5
page 206
Table 5.4
Table 5.4 page 207
Structure of Water
Electronegativity (EN)
EN is a measure of an atom’s attraction for
the electrons it shares in a covalent bond
„ Linus Pauling developed an EN scale
ranging from 0 to 4.
„
„Bent;
polar bonds
„O
attracts electrons more
strongly than H
O-H Bond
„ Polar
covalent bond
„ O is the negative end
„ H is the positive end
„ Greater difference in electronegativity
of bonded atoms leads to a more polar
bond. (HF>HCl> HBr>HI &
H2O>H2S)
Hydrogen Bonds
Strongest intermolecular attractive force
between molecules; raises the boiling point or
lowers the vapor pressure of substances;
∴H2O-liquid but H2S-gas
„ H-bonding occurs between H atoms and O, N,
or F atoms of adjacent molecules.
„ Molecules with H-bonding include NH3
(ammonia), HF (hydrogen fluoride), ethanol
and other alcohols, carbohydrates, proteins, and
nucleic acids.
„
1 H2O can hydrogen bond to 4 adjacent H2O
molecules via the attraction between the negative
end of O and positive end of H.
°C; implies
a lot of heat energy
needed to break
intermolecular
hydrogen bonds !
„BP=100
„Need 2250 J per
gram to convert
water to steam
„Without hydrogen
bonds, BP of H2O
would be -75 °C
Hydrogen bonding allows
a crystalline structure to
be formed in ice with
empty spaces in the
hexagonal channels.
Density of liquid H2O is
greater than density of
solid H2O (∴
(∴ ice floats!).
Figure 5.9 page 209
Density
Mass per unit volume; how much matter (in
grams) is packed into a specific volume, usually
a cm3.
„ Density usually decreases from solid to liquid to
gas.
„ Mercury (13.6 g/cm3) Iron (7.8 g/cm3), Nickel
(8.9 g/cm3), and SiO2 (2.6 g/cm3).
„ Core of Earth contains denser substances!
„
Figure 5.10 page 210
Specific Heat
„ Quantity
of heat energy that must be
absorbed to increase the temperature of 1
gram of a substance by 1 °C.
„ Water has one of the highest specific heat
at 4.18 J/ g °C among common liquids and
solids.
„ Absorbs AND retains heat! Regulates
climate and body temperature via specific
heat capacity and heat of vaporization of
water.
Water and Heat
„ Absorbs
more heat energy from the sun than
the ground in summer.
„ Retains heat and provides warmth to
surrounding areas in winter, ∴moderates
climate in islands and coasts.
„ Heat is absorbed when water evaporates
from oceans but is released when water
condenses as snow or rainwater.
Water or sucrose solution Figure 5.11 page 212
Electrolytes and Non-electrolytes
„ Electrolytes
form ions or charged
particles in solution
„ Examples are acids (HCl and H2SO4),
bases (NaOH and Mg(OH)2, and salts
(NaCl)
„ Electrolytes conducts electricity
„ Non-electrolytes such as ethanol,
Freons, and sand are non-conductors.
NaCl solution is a conductor Fig. 5.11 Pg. 212
Ions
Ions are electrically charged particles.
„ Metallic atoms form positive cations by losing
electrons.
„ Non-metallic atoms form negative anions by
gaining electrons.
„ Electrostatic attraction between cations and
anions forms strong ionic bonds that accounts
for the high melting and boiling points of salts
or ionic compounds.
„
Page 213
Page 214
Ions of NaCl in Solution
Crystalline lattice of Ionic Compounds
„
„
„
Electrons are
transferred from a
metal (Na) to a nonnonmetal (Cl
Cl)
)
(
Involves elements on
opposite ends of the
periodic table
NaCl structure on
Fig. 5.12; Page 213
„
„
„
Na+ and Cl- ions are
solvated by water
molecules.
Negative oxygen
ends point toward
Na+ and positive
hydrogen ends
point toward Cl-.
See Figure 5.14
page 217
Naming Ionic Compounds
“Staircase” in Periodic Table separates metals
(left) and non-metals (right) that are combined
in ionic compounds.
„ Name the metals first followed by non-metals
with the suffix “ide”.
„ Formulas must show a balance of + and charges by putting correct numerical subscripts
on each element of a compound.
„ 5.16 and 5.17: Your Turn on page 216
„
Table 5.8
Table 5.8 page 218
Polyatomic Ions Common in Ionic Compounds
Table 5.7 page 216
Put parenthesis around the polyatomic ion formula if more than 1 unit are present
in the ionic compound [i.e. Ca(HCO3)2, (NH4)3PO4, Al2(SO4)3]
Table 5.9
5.5 page 219
203
Table 5.9
Covalent Compounds
Share electrons in covalent bonds to achieve
octet structure.
„ Two types of covalent compounds
„ Hydrogen and non-metals (H2O, HCl)
„ Non-metals and non-metals (CS2, CO2)
„ do not form ions; ∴ no electrostatic attraction
like most ionic solids
„ most covalent compounds exist as gas or
liquids via dipole interactions (δ- and δ+).
„
Figure 5.16 page 220
Fig. 5.15
Figure 5.15 page 219
8 OH bonds account for the
solid state of sucrose
Sucrose C12H22O11
Figure 5.17 page 220
δ+
δ+
δδ+
Ethanol and ethylene glycol are liquids with 1 and 2
OH bonds, respectively; ethylene (C2H4) and ethane
(C2H6) are gases due to the lack of OH bonds
δ-
Toxicity of Water Pollutants
„
„
„
„
Figure 5.28 on Page 236
„
Toxic pesticides and polychlorinated biphenyls
(PCBs) are nonnon-polar and accumulate in fatty tissues
of animals.
Chromium and benzene are carcinogenic.
Lead is related to learning disability and hyperactivity
in children.
Nitrate and nitrite limits the ability of blood to carry
oxygen.
Microbes such as Giardia and coliform bacteria can
cause diarrhea, cramps, and nausea.
Safe Drinking Water Act
Enacted in 1974 and amended in 1996; regulated
by EPA.
„ Public health protection by setting drinking
water standards with
„
„
„
„
maximum contaminant levels (MCLs
(MCLs)) – legal limit
maximum contaminant level goals (MCLGs
(MCLGs)) – ideal levels for a 70
kg person to drink 2 liters of water per day for 70 years without
without
suffering any ill effects.
More than 80 contaminants regulated in the categories
of metals (e.g. Cd,
Cd, Cr, Cu, Hg, Pb),
Pb), nonnon-metals (F &
As), pesticides, solvents, organic toxicants (benzene),
and radioactive elements (Rn
).
(Rn).
Table 5.10 page 221
Clean Water Act
„ Control
pollution of rivers, lakes, streams,
and coastal areas.
„ National Pollutant Discharge Elimination
System (NPDES) regulates the industrial
discharge of wastewaters by requiring
testing and treatment of water prior to
issuance of permits.
„ Great Lakes pollutants include mercury
and PCBs.
Water Hardness
„
„
“Hard” water refers to the presence of Ca2+, Mg2+, and
Fe2+,3+ in water in the form of their compounds with
Cl-, CO32-, HCO3-, and SO4-.
Responsible for scale or deposit formation.
„
„
Ca2+ (aq)
aq) + CO32- (aq)
aq) → CaCO3 (s)
Water hardness is measured by titration with
ethylened
thylenediaminet
iaminetetraa
etraacetate (EDTA).
„
„
Ca2+ (aq)
aq) + EDTA2- (aq)
aq) → CaEDTA (s)
Dropwise addition of EDTA2- continues until all Ca2+ is reacted as
indicated by a color change. Hence, the known volume and
concentration of the EDTA titrant at the end point together with
the 1:1 mole ratio of Ca2+:EDTA2- can be used to calculate the
concentration of Ca2+ or water hardness.
hardness
Disinfection of Water
„
Water disinfection for killing disease-causing
microbes can be achieved by
Chlorination with chlorine gas, Cl2
Sodium hypochlorite, NaOCl, used in bleach
„ Calcium hypochlorite, CaOCl, disinfect swimming
pools.
„
„
HOCl, hypochlorous acid is generated as
antibacterial agent at 0.075-0.600 ppm.
„ Other methods such as ozonation and UV
radiation offer no protection once water leaves
the treatment plant; ∴ needs trace Cl2.
„
Fig. 5.29 on
Page 237
Distillation
•Water migrates across
a membrane.
•Used in Saudi Arabia
and Florida for water
desalination plants.
•Expensive; used in car
wash and by boaters.
Fig. 5.30 on
Page 238
Reverse
Osmosis
Water as a solvent
„ “Like
dissolves like” principle
„ Polar
solvents dissolve polar solutes more
effectively (e.g. ethanol in water).
„ Non-polar solvents dissolve non-polar solutes
more effectively (e.g. benzene and octanes in
gasoline).
„ Basis
for the cleaning action of
detergents involves surfactants with both
a polar and a non-polar ends.
Figure 5.21 page 227
Hydrophobic or non-polar ends of soap
or detergent molecules dissolve in nonpolar “grease” molecules and the polar
negative or hydrophilic ends orient
toward the positive hydrogen ends of
water molecules to allow grease to be
rinsed away.
In the presence of hard water, Ca2+ or
Mg2+ combine with the stearate anion to
form an insoluble precipitate or “soap
scum” to make washing less efficient.
Therefore, hard water is usually softened
by ion exchange with a zeolite to replace
Ca2+ or Mg2+ with Na+ as shown.
Na2Z(s) + Ca2+ (aq) → CaZ (s) + 2 Na+ (aq)
Zeolite(Hard water) Zeolite (Soft water)
Common Water Pollutants
Metals (lead, chromium, and mercury)
„ Trihalomethanes (chloroform [CHCl3], carbon
tetrachloride [CCl4])
„ Pesticides (chlorpyrifos, atrazine, & DDT)
„ Polychlorinated biphenyls (PCBs)
„ Dioxins (2,3,7,8-tetrachlorodibenzodioxane)
„ Industrial solvents (CCl4, C2Cl4, acetone, and
isopropyl alcohol)
„ Phosphates and nitrates lead to eutrophication.
„ Microbes (Coliform, Giardia, Crytosporidium)
„
Fig. 5.23 on Page 230
Schematic of a spectrometer
Solutes or pollutants in water absorb light or photons of a
particular wavelength and the reduction in light intensity is
measured by a detector as optical absorbance that is
proportional to the concentrations of analytes.
Gas Chromatograph (GC)
Chromatogram of THMs
Mixtures of organic or
hydrocarbon compounds are
separated by differential
absorption/retention of the
compounds in a stream of helium
carrier gas on the polymer
coating inside the capillary
columns (1-100 meter) at
elevated temperatures.
Thus, weakly absorbed
compounds are detected earlier
than the strongly absorbed
compounds according to their
retention times. The
chromatogram shows a plot of
retention times (X-axis) versus
the concentrations (Y-axis)
Fig. 5.26 on Page 233
Fig. 5.24 on Page 231: Calibration Plot
Concentration of Fe in a
water is obtained by
interpolation of the plot as
shown by the dotted lines