Business
• Buy your lab manual ASAP ($3.00 cash),
– Friday, 1 - 2, BI261,
• Quiz next Thursday covering reading
materials through that day, i.e. Proteins,
• Lectures…updated by morning of the
lecture.
Water is common to all life on Earth.
H2O Polarity
• polar molecule,
• each water
molecule can form
hydrogen bonds to
as many as four
other molecules.
H2O Bonding
• Forms a rough
tetrahedron,
• hydrogens at two
corners,
• unpaired electrons
at two corners.
Polar Molecules
…when electrons are shared unevenly in a
covalent bond, the bond is polar,
– substances with polar bonds are hydrophilic.
Effects of Water’s Polarity
• The polarity of water molecules results in
hydrogen bonds,
• Organisms depend on the hydrogen bonding
of of water molecules,
– cohesion,
– adhesion,
• Water moderates temperatures on Earth,
• Water is the solvent of life.
Cohesion
• water molecules stick together because of
hydrogen bonding,
– hydrogen bonds in liquid water last only a few
trillionths of a second,
– collectively, the hydrogen bonds hold the water
together.
Cohesion
Evaporation from leaves ‘pulls’ water up from the roots.
An unbroken ‘transpiration’ stream is required for water transport.
Adhesion
…water also sticks to other substances.
Adhesion of the water to the vessel walls also helps to
resist the downward flow of gravity.
Surface Tension
…a measure of how difficult it is to stretch or
break the surface of a liquid,
– water has a high surface tension due to the
hydrogen bonding of surface molecules.
Water - Air Interface
• water molecules in
solution have attractions
in all directions,
• at the surface, nothing
counterbalances the
downward pull.
Water Moderates Temperatures
…water is effective as a heat bank because it
can absorb or release a relatively large
amount of heat with only a small change in
its own temperature.
Specific Heat
…the amount of heat that must be absorbed or
lost for 1 g of a substance to change its
temperature 1o C,
...calorie: the amount of heat it takes to raise
o
1 g of H2O by 1 C.
Specific Heat Examples
Substance
Alcohol
Gold
Granite
Iron
olive oil
water
Specific Heat*
0.58
0.03
0.19
0.10
0.47
1.00
* cal/g oC
Hydrogen Bonds Again
…water’s high specific heat is due to the
breakdown and formation of hydrogen
breaking
bonds,
…heat energy is absorbed or dissipated by the
numerous hydrogen bonds.
forming
Remember: heat is a measure of total quantity of kinetic
energy due to molecular motion in a body of matter
Moderates Temperatures
…absorbs heat from the sun during the day,
– prevents drastic high temperatures,
…slowly releases heat at night,
– prevents drastic cold temperatures,
…moderates large bodies of water and nearby land
masses,
…also moderates temperatures within organisms.
Heat of Vaporization
…is the amount of heat a liquid must absorb
for 1 g of it to be converted from the liquid
to the gaseous state,
– 580 calories of heat are required to convert 1 g
of water to steam.
Heat of Vaporization Examples
Substance
HV*
Methanol
Ethanol
Acetone
Benzene
Butane
water
1100
854
523
394
381
2,260
* cal/g oC
Climate
…water’s high heat of vaporization helps
moderate Earth’s climate,
– the extreme heat absorbed by tropical seas is
consumed during the evaporation of surface
water,
– as moist tropical air circulates pole-ward, heat
is released as it condenses to form rain.
Evaporative Cooling
…occurs because the ‘hottest’ molecules are
most likely to leave as gas,
• bodies of water,
• animals,
• plants.
Sweat
Ice Floats
…water is less dense as a solid than it is as a
liquid,
Oceans and Lakes Don’t Freeze Solid
…if ice sank, as does
most solids, bodies of
water would freeze,
…floating ice insulates
the water, maintaining
temperatures near 0o
C.
Water is the Solvent of Life
…a liquid that is a complete mixture of two or
more substances is called a solution,
– the dissolving agent is the solvent,
– the dissolved substance is the solute,
…aqueous solution: is one in which water is
the solvent.
Water is Not a Universal Solvent
…or else nothing would hold it,
– biological systems use hydrophobic molecules
to ‘contain’ water.
Mole
…the standard reference unit for the amount
of each pure chemical substance is the
molecular weight of the substance taken in
grams,
– H2O: H is 1.008 Dalton, O is 16 Dalton,
• molecular weight of H2O is 18.016,
...one mole of H2O is equal to 18.016 grams.
Avagadro’s Number
…the number of atoms or molecules
in that quantity of a substance that,
expressed in grams, is numerically
equivalent to the atomic weight:
6.02 X 1023,
…or one mole = 6.02 X 1023 atoms or
molecules.
Molarity
…the number of moles of solute per liter of
solution,
– Sugar, C12H22O16 (MW = 342),
– 1 mole of sugar = 342 grams,
– 342 grams in a liter of water = 1 M, a 1 molar
solution.
Self Ionization of Water
+
H is a Proton
…in pure water, only one in 554 million
molecules is auto-ionized,
+
-7
– [H ] (hydrogen ions) in pure water is 10 M
(at 25o C),
– [OH-] (hydroxide ions) is the same.
Acids
…acids: increase [H+] in aqueous solutions,
– when HCl (hydrocloric acid) is added to water,
HCl ----> H+ +
Cl-
hydrogen ion
Bases
+
…bases: reduces [H ] in aqueous solutions,
– when NaOH (hydrocloric acid) is added to
water,
NaOH ----> Na+ +
hydroxide ion
combines with
H+ to form
water.
OH-
OH- + H+ ----> H2O
removes H+ .
Bases
+
…bases: reduces [H ] in aqueous solutions,
– when NH3 (ammonia) is added to water,
NH3 + H+
Ammonium ion takes H+.
NH4+
Strong/Weak Acids
…strong acids: dissociate completely when
mixed in water,
HCl ----> H+ + Cl-
…weak acids: the binding and dissociation
of the H+ ion is readily reversible,
H2CO3
HCO3-
+
H+
Strong/Weak Bases
…strong bases: dissociate completely when
mixed in water,
NaOH ----> Na+ + OH-
…weak bases: the binding and dissociation
of the OH- ion is readily reversible,
NH3 + H+
NH4+
+
pH is the measure of [H ]
…acid and base refer to compounds and ions,
…acidic and basic refer to solutions.
+
-
H and OH ions
…in any aqueous solution the concentration of
+
-14
the H and OH ions is constant at 10 M,
+
-
[H ] [OH ] = 10
+
-7
-
-14
M
-7
…if [H ] is 10 , then [OH ] is 10 ,
...if [H+] is 10-5, then [OH-] is 10-9.
pH
…to conveniently express the variation in the
+
concentrations of H and OH ions, scientists
use the pH scale,
– pH (potential of Hydrogen): is the negative
+
logarithm of [H ] ,
pH = -log10 [H+]
+
pH = -log10 [H ]
pH = -log10 [10-7] = -(-7) = 7
pH = -log10 [10-4] = -(-4) = 4
-2
pH = -log10 [10 ] = -(-2) = 2
+
pH and [H ] are Inversely Proportional
higher pH
fewer H+
lower pH
more H+
pH = -log10 [10-12] = -(-12) = 12
pH = -log10 [10-7] = -(-7) = 7
pH = -log10 [10-4] = -(-4) = 4
pH = -log10 [10-2] = -(-2) = 2
10 Fold Steps
…each pH unit represents 10 fold difference
in H+ and OH- ions,
…a solution of pH 3 is not twice as acidic as a
solution of pH 6,
…it is 103, or 1000 times more acidic,
• Small changes in pH represent large
changes in H+ and OH- concentrations.
Buffers
…a solution characterized by the ability to
resist changes in pH when limited amounts
of acid or base are added to it,
Buffers Minimize pH Changes
To Do
• Read through pp. 48-55, as assigned in the
syllabus,
• Note, for Tuesday, read pp. 58-60.