Chapter Overview:
water use
composition of water
polar covalent bonds
hydrogen bonding
density of water and ice
solutions in water / electrolytes and nonelectrolytes
ionic bonding
solubility in water
water and energy
water sources and water purification
Water Usage
U. S. average per person is about 300 liters per day (about 80 gallons)
only ~ 1 gallon is consumed, rest is for hygiene (washing, toilets, etc.)
U. S. industrial use is 57% of total, agricultural 34%
Composition of Water
electrolysis breaks water into hydrogen and oxygen
twice as much hydrogen as oxygen is formed
energy (electrical) must be input in order to split water (endothermic)
H 2 O (l) + 239 kJ ----> H 2 (g) + 1/2 O 2 (g)
conversely, hydrogen is an excellent fuel when it burns (exothermic)
H 2 (g) + 1/2 O 2 (g) ----> H 2 O (l) + 239 kJ
(239 kJ/mole corresponds to 120 kJ/g for hydrogen)
Polar Covalent Bonds
molecular structure of water includes two covalent O-H bonds and two lone pairs on O
the electron pairs in the covalent bonds are not shared equally
oxygen has a greater tendency to hold electrons (called electronegativity )
the O-H bonds are called polar because there is a charge separation
one end has a partial positive (+) charge (the H end)
and the other end has a partial negative ( - ) charge (the O end)
Hydrogen Bonds
polar bonds tend to align with one another because opposite charges attract
in water, a network of hydrogen bonding occurs between O-H bonds and O lone pairs
the strength of typical H-bonds is about 20 - 40 kJ/mole
compare with covalent single bonds that range from 200 - 500 kJ/mole
nevertheless, the number of H-bonds makes the total effect very significant
DNA double strand is held together by H-bonding (like Velcro)
high degree of H-bonding in water causes a number of special properties:
relatively high boiling point (liquid at normal temperatures)
highly ordered structure of ice
high amounts of energy required to break apart water molecules from one another
e.g., melting of ice, boiling of water
Density of Water
density is defined as the weight per unit volume of a given substance
typical units are grams per cubic centimeter ( 1 cm 3 = 1 cc = 1 mL )
water at 4·C d = 1.00000 g/mL (international standard of weight)
water at 25·C d = 0.99707 g/mL (typical decrease at higher temperature)
water at 0·C d = 0.99987 g/mL (atypical decrease at lower temperature)
ice at 0·C d = 0.917 g/mL (atypical decrease upon freezing)
the decrease as temperature is lowered below 4·C is unusual for a liquid
the lower value for ice means that ice floats on water (also unusual for a liquid)
Solutions in Water
solution - a mixture consisting of a dissolved substance (solute) in a liquid (solvent)
e.g., sugar water or salt water
electrolyte solutions - solutions that conduct electricity (like salt water)
nonelectrolyte solutions - solutions that don't conduct electricity (like sugar water)
Ionic Bonding
some molecules are formed by transfer (rather than sharing) of electrons
e.g., NaCl is composed of Na + and Cl- , not Na-Cl bonds
ions - atoms or molecules with a net electrical charge (like Na + and Cl- )
fulfillment of the octet rule for Na is most easily done by giving away one electron
fulfillment of the octet rule for Cl is easily done by acquiring one extra electron
Na (s) + 1/2 Cl 2 (g) -----> NaCl (s)
one electron is transferred from Na (with one valence electron)
to Cl (with 7 valence electrons) - each ends up with a full shell
the result is a pair of ions - Na + and Cl- , which attract one another
the solid structure of NaCl is a crystal of alternating ions, not individual molecules
oxidation - loss of electrons
reduction - gain of electrons
consider ionic bonding similar to polar covalent bonding, but the extreme case
ionic bonding is favored between elements of very different electronegativity
one element has a strong tendency to hold electrons, another a weak tendency
e.g., metals on left side of periodic table and nonmetallic elements on right side
nonpolar covalent bonds polar covalent bonds ionic bonds
electron pairs electron pairs electrons
shared equally shared unequally transferred
e.g., CH 4 e.g., H 2 O , HCl e.g., Li + Br -
Solubility in Water (Aqueous Solutions)
ionic compounds generally dissolve well in water
negative ions stabilized by H-bonding (positive end of O-H bond points towards ion)
positive ions stabilized by lone pairs of O pointing towards ion
common elemental ions - metals: Li + , Na + , K + , Ca +2 , Mg +2 , Fe +2 , etc.
nonmetals: F- , Cl- , Br- , I- , O- 2 , S- 2 , etc.
common polyatomic ions - sulfate (SO 4-2 ) , nitrate (NO 3 -) , phosphate (PO 4 - 3 ),
carbonate (CO 3-2 ) , bicarbonate (HCO 3 -), hydroxide (OH-), ammonium (NH 4+ )
polar covalent compounds also generally dissolve well in water
as long as H-bonding can occur
e.g., alcohols have an O-H bond for good H-bonding
sugars typically have lots of O-H bonds
nonpolar covalent compounds generally do not dissolve well in water
if they dissolved, they would break up the strong H-bonding in water
general rule is "like dissolves like"
Water and Energy
it takes an unusually large amount of heat to raise the temperature of water
specific heat for water is 1 calorie per gram per degree C (definition of the calorie)
C = 1.00 cal/ g ·C or q = C m _T
heat energy (cal) = specific heat x mass (g) x temperature change (·C)
the relatively large value of specific heat is due to H-bonding
it takes a lot of energy to get individual water molecules to move around (i.e., get hot)
it also takes an unusually large amount of heat to melt ice or to boil water
melting ice at 0·C requires 80 cal/g (heat of fusion)
boiling water at 100·C requires 540 cal/g (heat of vaporization)
thus hot water (or steam) is excellent for storing heat
and cold water (or ice) is excellent for removing heat
Water Sources and Purification
only a tiny fraction of all water on earth is fresh water available for use (see Fig. 5.12)
fresh water recycles through the water cycle, i.e., evaporation, clouds, rain
rainwater typically dissolves minerals as it passes through soil and rocks
purification processes include: filtration to remove large particles and many bacteria
settling, usually with alum and lime, to remove dirt and clay
chlorination to kill bacteria (ozone sometimes used)
fluoride to prevent tooth decay
water with excessive mineral content is called hard water
problems are caused by Ca +2 and Mg +2 , which form deposits with soaps
ion exchange replaces Ca +2 and Mg +2 with Na + , which is always soluble
distillation - boiling and recondensing a liquid - a purification method for any liquid
distilling water is energy-intensive (high specific heat and heat of vaporization)
desalination - removal of salts from salt water
accomplished by distillation or osmosis through a semipermeable membrane