Title: Importance%20of%20Water
1Lecture 3 Importance of Water 9/2/05
2Last time talked about Water as a Polar Covalent
Molecule No NET CHARGE on the Water
Molecule just ASYMMETRY WITHIN the water Molecule
RESULT is force of attraction Between Polar
Molecules sticky
HYDROGEN BONDING
3- Water is not the ONLY Polar Molecule
?
the molecule is LOPSIDED NO NET CHARGE JUST
ASYMMETRY
?
?
Typical Polar arrangements
R-O-H R-N-H R-CO R-S-H
A hydrogen bond
most abundant polar molecule is WATER
4Hydrogen Bonding
S
N
S
N
S
N
5S
N
Hydrogen bonding
Electronegative and Electropositive poles
6Van der Waals Interactions
Another Type of FORCE OF ATTRACTION
Very Weak attraction between nonpolar molecules
Transient Asymmetry of electrons Due to
probability
7Effect of Van der Waals force on Nonpolar
Molecules
Gas
Methane CH4
Very weak, fleeting but cumulative
8Helium atoms have almost NO attraction for one
another - Helium is a GAS until cool to 4 K
(-269oC) Brrrr!
9- The polarity of water molecules
- Allows them to stick together
- Contributes to the various properties water
exhibits
Takes energy to break hydrogen bonds
Extremely Important for life on this planet!
10O
Importance of Water
H
H
- 70 of cell material is water
- - Interactions cellular molecules
- influences/dictates their structure
- often is part of metabolic reactions (made or
broken apart) or acts as a catalyst
Polar Covalent Molecule
Unequal sharing
11Properties of water due to Polarity
- Cohesion/surface tension
- Temperature moderation
- High specific heat
- Evaporative cooling
- Ice floats
- Solvent Ability
- Hydrophilicity dissolves polar things ions
- Hydrophobicity herds away nonpolar things
- Ionization ability (pH)
12- Emergent properties of water contribute to
Earths fitness for life
1. Cohesion - water molecules stick to one another
13Liquid
142. Temperature Moderation - water has a high
specific heat (energy to raise 1g of substance
1oC)
- heat is absorbed when Hydrogen bonds break -
heat is released when Hydrogen bonds form keeps
temperature of earth from fluctuating
wildly 75 of earth surface is water a heat
reservoir
15Some consequences Water hydrogen bonding
- Evaporative cooling
- Is due to waters high heat of vaporization
- Allows water to cool a surface
- heat capacities in change of state
(solid-liquid-gas) (heat of vaporization, heat
of fusion) high boiling point of water
16Gas Steam
Liquid
17Water is WEIRD Solid Solid Water ICE Is less
dense than Water
Insulates bodies of water, melts from surface
18A weird consequence
193. Water acts as a Great Solvent for Other
Polar Molecules Polar and Ionic things readily
dissolve in Water
Polar and Ionic Things are Hydrated by water
molecules
20- Water hydrates polar molecules
O-H
Sugar
Polar groups Are hydrogen Bonded to
Surrounding Water molecules
O-H
21Hydrophilic
water loving
Water and Oil Dont Mix
Hydrophobic
Nonpolar molecules
water fearing
22Nonpolar cannot form hydrogen bonds with H2O
23NONpolar covalent molecules do
not dissolve in water
Because disrupt hydrogen bonding
24Time
Water HERDS NONpolar things away to minimize
contact maximize of hydrogen bonds between
water molecules
Hydrophobicity
water fearing
25Hydrophobicity and Hydrophilicity affect the 3-D
Structure of Macromolecules
P
NP
P
P
P
P
NP
NP
NP
NP
P
NP
NP
NP
P
P
NP
NP
P
Properties of Water drive shapes and
structures of biological macromolecules
P
P
P
P
P
P
NP
NP
NP
NP
P
NP
NP
P
NP
NP
NP
P
NP
P
P
P
P
26Basis for Cell Membranes
Hydrophilic like water (polar things
ions)
OUTSIDE
WATER
POLAR POLAR POLAR POLAR POLAR
POLAR POLAR
NONPOLAR NONPOLAR NONPOLAR NONPOLAR
NONPOLAR
NONPOLAR NONPOLAR NONPOLAR NONPOLAR
NONPOLAR
POLAR POLAR POLAR POLAR POLAR
POLAR POLAR
WATER
INSIDE
Hydrophobic shy away from water (nonpolar
things)
274. Last Property that Makes Water so Weird and
Versatile
Water can IONIZE transiently
Important participant in biochemical rxns
O
H3O OH- Hydronium Hydroxide
ion ion
H
H
Water molecule no charge
H ion
28Dissociation is a rare event
Occurs constantly but at any instant only Tiny
fraction water molecules dissociate to ions At
any instant in time MOST REMAIN as MOLECULAR
H2O But always some ionized
29Definitions
- A mole
- Represents an exact number of molecules of a
substance in a given mass - 6.02x1023 molecules per mole
- Water 18g/mole, hydrogen H2 2g/mole
- CO2 44g/mole
Molarity Is the number of moles of solute per
liter of aqueous solution
30- H2O H OH-
- 1x10-7M 1x10-7M
- Define dissociation constant for water
Kw 1x10-7 x 1x10-7 1x10-14
Kw will always equal 1x10-14
(so if H 1x10-4M, what is OH-?)
1x10-10M
31So concentration of H in pure water
1
1
H
1 ten millionth of a Mole
10,000,000
107
1
1 x 10-7
H
(an awkward number)
107
So math shorthand
log H -7
negative log H 7
p negative log of
pH 7
32Add something else to water which contributes
extra H
More H around, maybe as common as 1 thousandth
M
1
1
1 x 10-3 so, pH 3
1,000
103
-OH Conc ?
Conversely,
Add something else to water which depletes H
Fewer H around, maybe as common as 1 billionth
M
1
1
1 x 10-9 so, pH 9
109
1,000,000,000
-OH Conc ?
33Low pH H abundant -OH depleted
Acidic
Pure water
High pH H depleted -OH abundant
Basic
34pH scale tells you HOW MUCH H is around
- pH important for
- mechanism of harvesting energy in metabolism
- affecting macromolecular functions
- affecting macromolecular structures
- e.g., solubility of some molecules changes with pH
35- Summary
- Water - polar covalent
- Hydrogen bonds
- High cohesion, liquid at room temp
- High specific heat modulates temperature
- Solid less dense than liquid coldest floats
- 3. Other polar molecules dissolve
- hydrated by water molecules
- Hydrophobic hydrophilic interactions
- 4. Water transiently ionizes - pH
36Next time (Wed 9/7) Chemistry of Carbon
Compounds Functional Groups Buffers