Chemical Compounds – WATER / pH / Buffers
Background Information:
- Life as we know it, is based on the chemistry of
organic molecules. To be “organic” a molecule
must have a CARBON and hydrogen in it.
- For example; Vinegar (CH3CH2OH) is organic,
whereas water (H20) and (CO2) are not, they are
inorganic.
- The organic molecules in living things are easily
grouped into four types. An understanding of
these four biochemicals is the purpose of the next
unit.
1. Proteins
2. Carbohydrates
3. Lipids
4. Nucleic Acids
- Why is water so incredibly important? What
happens to the quality of life if the chemical
conditions of living things change?
- In the formation of chemical compounds,
organic or inorganic, atoms must be bonded
together to form stable structures. In order to do
this, the electron configurations around the
various nuclei in the compound must also be
stable. The stability can be achieved through
some degree of sharing electrons between the
atoms.
- If the atoms share the electrons reasonably
equally, their association is termed covalent bond.
- The alternative is a very unequal sharing. This
is called Ionic bonding. Where it seems as though
one atom actually gives away its electrons.
Neutral atoms become positive ions when they lose
electrons.
- In between these two extremes is what is termed
polar covalent. Polar covalent molecules have
dipoles ( regions with slight positive and negative
natures ). The water molecule is an example of
this kind of molecule.
Water
Structure of water:
In a covalent bond atoms share electrons to
become stable. In water, each hydrogen shares a
pair of electrons with oxygen. The oxygen atom is
much larger and tends to attract the shared
electrons more. Therefore, oxygen has a partial
negative charge while each hydrogen has a partial
positive charge.
- Any molecule that ends with a different charge is
called a polar molecule. Hence it would be a
dipole.
- One end being the negative dipole and the other
being the positive dipole.
- This means that water molecules are loosely
attracted to one another. The negative charge on
the oxygen of one molecule is attracted to the
positively charged hydrogen on another water
molecule to produce a weak bond called a
Hydrogen Bond.
Therefore, water tends to clump together.
Although the bond is weak, the vast number of
these bonds gives water its unique properties.
Roles of Water
1) Water acts as a solvent (The “Universal
Solvent”) and is able to dissolve many
chemical substances especially other polar
molecules such as salts.
- Since blood is mainly water, the ability of water
to dissolve and transport substances greatly aids
in bringing about necessary chemical reactions in
the body.
2) Moderates Climate (both internal and external
climates) due to its high Specific Heat Capacity.
- Water can absorb a great deal of heat while only
rising in temperature a slight amount due to the
H-bonding. It also releases heat slowly.
Example: Oceans keep surrounding land masses
cool in summer and warmer in winter.
Example: Body systems are mainly water,
therefore, bodies tend to stay at relatively constant
temperatures despite external conditions.
3) Liquid water is more dense than ice. (very rare
for compounds).
- Therefore, ice forms on top of water, insulating
lower levels. This allows living things to function
under the ice surface. Hydrogen bonding allows
molecules to come closer together.
If ice formed at the bottom, all lakes and oceans
would freeze solid in the winter.
4) Water is transparent.
- Allows light to penetrate well into water bodies
to the organisms below. This allows photosynthesis to take place in aquatic environments
5) Water molecules are cohesive.
They stick to each other and to other surfaces.
This keeps surfaces moist and lubricated.
- Diffusion of gases occurs much more efficiently
across moist surfaces.
Acids, Bases and Buffers
Acids : are molecules that dissociate to release
hydrogen ions (H+). HCl (hydrochloric acid) is a
strong acid because it dissociates to free up many
many H+ ions.
- Systems with HCl in them have a lot of free H+
(Hydrogen ions) floating around.
Bases : are molecules that release Hydroxide ions
(OH-). Molecules like NaOH are bases. Bases
have a neutralizing effect on acids because the H+
ions will combine with OH- ions from a base and
form water and a salt.
At pH 7, the concentration of hydrogen ions = the
concentration of hydroxide ions. [ H+ ] = [ OH- ]
- At pH 6, there are 10 times more H+ than at pH 7
- pH 3 has 100 times more H+ than pH 5
- The pH scale is just a comparison between the [
H+ ] and [ OH- ].
- pH under 7 is acidic.
- pH = 7 is neutral.
- pH over 7 is basic.
Most enzymes (organic catalysts), which control
the chemical reactions in your body, can only
operate at certain pH levels.
Example: Blood must be at pH 7.4 or else we
become ill. The blood cannot transport O2 + CO2
efficiently if blood pH is off.
Buffers : are chemicals or a combination of
chemicals that are able to keep pH levels constant.
They absorb excess H+ or OH- ions from
substances.
Example: Bicarbonate (HCO3-) absorbs excess
acid (H+ ions)
Without a Buffer
With a Buffer
Note: The buffer keeps pH constant despite
adding more and more acid.
Example of Buffering System
# 2 #1
H2CO3
Carbonic
Acid


H+
+
Hydrogen
Ion
HCO3Bicarbonate
Ion
- If H+ is added, the reaction will run in the
direction of arrow #2
- If OH- is getting too high, the reaction will run
in the direction of arrow #1. This will release
H+ ions to join to the OH- ions to form water.