Chapter 3
Water and Life
Let’s Review Bonding
Covalent
Ionic
Hydrogen
Van der Waals
Interactions
All important to life
Between nonmetals
Form cell’s
molecules
Strongest bond
Between metal
and non-metal
Quick reactions/
responses
H bonds to other
electronegative
atoms
Fleeting
attractions
between close
atoms and
molecules
Break polar
molecules
Depend on
relative orientation
of molecules
Weaker bond
Even weaker
especially in water
Weakest bond
All bonds affect molecule’s SHAPE
 affect molecule’s FUNCTION
Similar shapes become mimics
morphine, heroin, opiates mimic endorphin
(euphoria, relieve pain)
Chemical Reactions
• Reactants  Products
ex: 6CO2 + 6H2O  C6H12O6 + O2
• Some reactions are reversible:
ex: 3H2 + N2
2NH3
• Chemical equilibrium: point at which forward and
reverse reactions offset one another exactly
• Reactions still occurring, but no net change
concentrations of reactants/products
(dynamic equilibrium)
WATER AND SOLUTIONS
• universal solvent in organisms
• one of few liquid compounds found naturally on earth, most solid
• expands in solid form
• covers > 75% of earth
• most abundant compound in living organisms
(human body ~ 70%)
• most important compound in organisms
- cells surrounded by it
- filled with it
- cellular events occur in it)
- carries or dissolves other substances
Properties of Water
1.
Polar molecule (polarity)
Will carry or dissolve other
substances in it which are vital for
life.
- Hydrophilic: substances that
dissolve rapidly in water
*Generally polar molecules
or ions (unlike charges
highly attracted to each
other)
H2O can form up to
4 bonds
- Hydrophobic: substance
that are insoluble in water
*Generally non-polar
molecules (water
molecules are more
attracted to each other than
the non polar molecules)
• Water is the solvent of life
- Polarity makes H2O a good solvent
- polar H2O molecules surround +& –ions
- solvents dissolve solutes creating solutions
2. Cohesion/Adhesion
• Cohesion: attraction of same substances of same kind
H bonding between like molecules (water to water)
• Adhesion: attraction of water to other materials
- oxygen end: negative charge
- hydrogen end: positive charge.
The hydrogens of one water molecule are
attracted to the oxygen from other water
molecules. This attractive force is what gives
water its cohesive and adhesive properties.
Examples of cohesion and adhesion:
• Transpiration: movement of water up plants
– H2O clings to each other by cohesion; cling to xylem tubes
by adhesion
• Adhesion of water to vessels walls counters
downward pull of gravity.
• Water is a sticky molecule due to adhesion
Ex: ice- floats, less dense than water
H bonds form a crystal
3.
Surface Tension: cohesion of water molecules at the
surface of a body of water
- Each molecule in the water drop is attracted to
the other water molecules in the drop.
- surface molecules attractive forces greater
- All the water molecules on the surface of the
bead are creating surface tension. (like a large
group of people tightly holding hands)
4. Capillary Action
adhesion of water
molecules to other
substances which allow it
to be drawn up the surface
of the other subst.
Ex:
meniscus
plants pull water
into themselves
water “climbing”
up a straw
5. Moderation of Temperature
•
Acts as a heat bank: absords or releases a relatively
large amount of heat with only slight change in its
own temperature.
•
This property keeps temperature fluctuations to a
minimum in order support life on land and in
water.
How does this work?
Heat: total amount of KE in system
Temperature: measure intensity of heat due to average
KE of molecules
• Whenever two objects of different temp. are brought
together, heat passes from the warmer to the cooler
object until two are same temp.
• Calorie: amount of heat it takes to change 1g H2O by
1*C
(1 cal/g *C)
• Kilocalorie: 1000 calories (1 kcal/g *C)
Specific Heat: amount of heat that must be
absorbed or lost for 1g H2O to change its temp
by 1*C
• H2O has a high specific heat
- H2O resists changes in temperature
- takes a lot to heat it up
- takes a lot to cool it down
- due to H bonding (heat needed to disrupt
H bonds)
Heat of Vaporization: quantity of heat a liquid must
absorb for 1g to be converted to a gas
- water has a high heat of vaporization
(high amount of E needed to vaporize H2O i.e. break H bonds)
- importance: temperature moderation
Evaporative Cooling
–
Molecules with greatest KE leave as gas
–
Surface of remaining liquid cools down
–
Stable temperatures in lakes & ponds
–
Cools plants
–
Prevents terrestrial organisms from overheating
H2O moderates temperatures on Earth
• Large bodies of water absorb and store more heat
 warmer coastal areas
• Create stable marine/land environment
• Humans ~65% H2O  stable temp, resist temp.
change
Insulation by ice
less dense, floating ice insulates liquid H2O below
–
–
–
10% less dense then H2O
Life exists under frozen surface (ponds, lakes,
oceans)
Ice - solid habitat (polar bears)
Solution
•
•
•
•
Homogeneous mixture
Solvent: dissolving agent
Solute: dissolved substance
Aqueous solution: water is solvent
• Hydration shell: sphere of
H2O molecules around
each dissolved ion
Water is the solvent of life.
• Rule of solubility: “like dissolves like”
Hydrophilic
Hydrophobic
Affinity for H2O
Repel H2O
Polar, ions
Nonpolar
Cellulose, sugar, salt
Oils, lipids
Blood
Cell membrane
IONS AND LIVING CELLS
Salt is a very important polar molecule.
- When mixed with water, the ionic bonds are broken and the
Na and Cl ions separate (dissociation)
- The charged ends of each ion are attracted to the polar ends of
water
- This is very important because ions such as Na, Cl, K, and Ca
are involved in many reactions inside the cell
Ionization
• Process of water (non-ionic) being converted into ions
• H atom shifts from one molecule to another
Result:
H+ + H2O  H3O+ (hydronium ion)
H2O – H+  OH- (hydroxide ion)
reversible process
pH: number of H ions in a solution
Acid: any compound that releases H ions into water
- H3O+ (hydronium ion is formed)
ex: hydrochloric acid in water
HCl

H+
+
Cl-
**** Most reactive ion due to no
electrons- attacks bonds in
many molecules
Base: compound that releases OH- ions into water
Ex: NaOH   Na+ +
OH-
• Neutralization reaction: production of
H2O from mixture of strong acid and base
neutral
H
=
OH
acidic
H
>
OH
basic, alkaline
H
<
OH
The pH scale
Acid: below ph 7 (more H+)
Base: above pH 7 (more OH-)
IMPORTANCE OF PH:
• Most organisms pH ~ 7
• most reactions in organisms can
only occur with enzymes
• enzymes very pH specific
***failed kidney function is most
often reason for inability to
maintain normal blood pH
Calculating pH
[H+][OH-] = 10-14
If [H+] = 10-6 M, then [OH-] = 10-8
pH = -log [H+]
If [H+] = 10-2
• -log 10-2 = -(-2) = 2
• Therefore, pH = 2
If [OH-] = 10-10
• [H+] = 10-4
• -log 10-4 = -(-4) = 4
• Therefore, pH = 4
Buffer: substance which resists changes in pH when small
quantities of an acid or an alkali are added to it
pH controlled by buffers
reservoir of H+
donate H+ when [H+] falls
absorb H+ when [H+] rises
blood pH 7.4 controlled by
carbonic acid/bicarbonate buffer system
Carbonic acid
Bicarb ion
Ocean acidification threatens coral reef ecosystems
CO2 mixes with seawater and creates carbonic acid
lowering pH.