04/09/2010
The Biological Medium on Earth
All living organisms require water more than
any other substance
Water molecules participate in many chemical
reactions needed to sustain life
Most cells are surrounded by water, and are
composed of 70-95% water themselves
¾ of the Earth’s surface is submerged in
water
To get the most out of these notes, you MUST
read along with pages 16-19 in the textbook
Not everything in these pages are covered in
the PowerPoint, only main points.
To understand these points, you should read
along with the textbook as it elaborates on,
and explains the main points presented in
this PowerPoint presentation.
H2O: two hydrogen atoms connected to an
oxygen atom by single covalent bonds
Oxygen is more electronegative than hydrogen
(has a stronger attraction for the electrons of a
covalent bond), so the electrons of the polar
bonds spend more time closer to the oxygen
atom
In other words, the bonds that hold together the
atoms in a water molecule are polar covalent
bonds
The water molecule is a polar molecule,
molecule meaning
the ends of the molecule have opposite charges
Intermolecular forces
The oxygen end of the molecule has a partial
negative charge, and the hydrogens have a
partial positive charge
LondonLondon-dispersion
exist between all atoms and molecules
the only intermolecular forces of attraction between noble gas
atoms and between nonpolar molecules.
formed by the temporary unequal distribution of electrons as
they randomly move about the nuclei of atoms.
Weak!
Form between small nonpolar molecules, such as methane
(CH4) and nobel gasses ie helium) That is why these materials
are gases at room temperature.
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Intermolecular forces
DipoleDipole-dipole
hold polar molecules to one another.
the partially positive side of one polar molecule attracts the
partially negative side of adjacent polar molecules. These
intermolecular forces of attraction are stronger than London
forces.
1. Cohesion
2. Moderation of Temperature
3. Insulation of Bodies of Water by Floating
Ice
4. The Solvent of Life
Atoms and molecules have kinetic energy (the
energy of motion) because they are always
moving
Heat is a measure of the total amount of
kinetic energy due to molecular motion in a
body of matter
Temperature measures the intensity of heat
due to the average kinetic energy of the
molecules
the strongest intermolecular forces of attraction. They are
just especially strong dipole–dipole forces that only form
between an electropositive H of one polar molecule and an
electronegative N, O, or F of a neighbouring polar molecule
Water molecules hold onto each other by HH-bonds. Hydrogen
bonds and an angular shape give water its many unique
properties
London dispersion forces, dipole–dipole attractions, and
hydrogen bonds are collectively
called van der Waals forces.
water molecules would be arranged like this? In liquid form, hydrogen bonds are fragile
Collectively, the hydrogen bonds hold the
substance together, a phenomenon called
cohesion
Adhesion: the clinging of one substance to
another
Surface Tension: a measure of how difficult it
is to stretch or break the surface of a liquid
Water has a greater surface tension than most
other liquids
When two objects of different temperature
come into contact, heat passes from the
warmer to the cooler object until they are the
same temperature
Molecules in the cooler object speed up (at
the expense of the kinetic energy of the
warmer object)
Calorie:
Calorie a unit of heat. The amount of heat it
takes to raise the temp. of 1g of water by 1oC
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Specific heat:
heat the amount of heat that must
be absorbed or lost for 1g of that substance
(in this case, water) to change its temperature
by 1oC
The specific heat of water is 1cal/g/oC
Compared to other substances, water has a
high specific heat capacity
Large bodies of water can absorb and store a
huge amount of heat from the sun
As a liquid evaporates, the surface of the
liquid that remains behind cools down
This evaporative cooling occurs because the
“hottest” molecules (those with the greatest
kinetic energy) are the most likely to leave as
gas
Evaporative cooling contributes to the
stability of temperature in lakes and ponds
Solution:
Solution a liquid that is a completely
homogeneous mixture of two substances
Solvent:
Solvent the dissolving agent of a solution
Solute:
Solute the substance that is dissolved
Aqueous solution:
solution water is the solvent
Water is the most versatile solvent, due to its
polarity
Hydrogen shell:
shell the sphere of water molecules
around each dissolved ion
Heat of vaporization:
vaporization the amount of heat a
liquid must absorb for 1g of it to be
converted from the liquid to gaseous state
Water has a high heat of vaporization
compared to other liquids, caused by
hydrogen bonds (which must be broken
before the molecules can make their exodus
from the liquid)
Water’s high heat of vaporization helps
moderate Earth’s climate
Water is less dense as a solid than as a liquid
So, ice floats in water
Water expands when it solidifies, again, a
result of hydrogen bonding
If ice sank, all water bodies would freeze,
killing all the life inside them
Hydrophilic:
Hydrophilic a substance that has an affinity
for water (“water-loving”)
Some substances are hydrophilic, but do not
dissolve. They remain suspended in the
aqueous liquid (this is called a “colloid
colloid”
colloid a
stable suspension of fine particles in a liquid)
Hydrophobic:
Hydrophobic a substance that repels water
due to being nonionic and nonpolar (“waterfearing”). An example is vegetable oil.
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Molecular mass:
mass the sum of the masses of all the
atoms in a molecule. For example, in sucrose:
C12H22O11, the mass of a carbon atom is 12, hydrogen
is 1, and oxygen is 16. So, sucrose has a molecular
weight of:
12(12) + 1(22) + 11(16) = 342 daltons (atom masses
given in daltons rounded to whole numbers)
Substances are often measured in units called Moles
(mol), which represents and exact number of objects
Molarity: the number of moles of solute per litre of
solution
An acid:
acid a substance increases the hydrogen ion
concentration of a solution
A base:
base a substance that reduces the hydrogen ion
concentration of a solution
pH scale:
scale the pH of a solution is defined as the
negative logarithm (base 10) of the hydrogen ion
concentration:
pH = -log[H+]
Buffers are substances that minimize changes in ion
concentration (H+ and OH-)
Acid precipitation has a pH lower or more acidic than
pH=5.6
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