Lab 4

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Lab 4 Diffusion and Osmosis
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The process in which molecules move from a region
of higher concentration to one of lower
concentration is called diffusion
Diffusion occurs in gases, liquids, and solids,
and is due to the intrinsic kinetic energy of
molecules
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This intrinsic kinetic energy is called thermal
motion and is the equivalent of heat
All molecules that exist above absolute zero (0
degrees K) possess some degree of thermal motion
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How fast do molecules in air move when they are
close to room temperature?
Air is composed of about 80 nitrogen. We will
assume that nitrogen behaves like an ideal gas
and determine the rms speed of nitrogen at 293K
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The average kinetic energy per particle for
nitrogen is
The mass of a nitrogen molecule is
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Now we can calculate the rms speed of nitrogen
That is about 1143mph!
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We have just seen that a gas molecule has a
translational rms speed of hundreds of meters per
second at room temperature
At such a speed, a molecule could travel across
an ordinary room in just a fraction of a second
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Yet it often takes several seconds, and sometimes
minutes, for the fragrance of a perfume to reach
the other side of a room
Why does it take so long?
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When a perfume molecule diffuses through the air,
it makes millions of collisions each second with
air molecules. Although the perfume molecule does
move very fast between collisions, it wanders
only slowly away from the source because of the
zigzag path resulting from the collisions
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These collisions between molecules, whether in a
gas or a liquid, are completely random and
independent
Diffusion of a population of molecules may be
directional, howeverWhy?
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The second law of thermodynamics states that
every energy transfer or transformation increases
the entropy of the universethat is, the more
random a collection of matter is, the greater its
entropy
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In any system, there is a tendency to go from
order to disorder this is entropy
More Order Higher Energy
Equilibrium Lowest Energy
Disorder Lower Energy
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Once the system reaches a dynamic equilibrium,
work must be done to move all of the molecules
back on one side of the membrane
We can view the highest-ordered system in terms
of its chemical potential µ
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Chemical potential is a measure of the free
energy available to do the work of moving a mole
of molecules from one location to another and, in
some cases, through a barrier such as a cell
membrane
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In a solution, the greater the concentration of
solutes, the higher the chemical potential (free
energy per mole) of that substance
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Molecules tend to spontaneously move from areas
of higher chemical potential (higher
concentration) to areas of lower chemical
potential (lower concentration), thus increasing
the entropy of the universe
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The driving force behind the net movement of a
substance down its concentration gradient is the
difference in chemical potential between two
areas. The difference in chemical potential can
be described by the following equation
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?µ is the difference between the chemical
potential on the outside and inside of a
membrane. R is the ideal gas constant, T is the
temperature in Kelvin, and X is the concentration
of the substance
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The movement of X across a membrane in one
direction or another is known as unidirectional
flux
The algebraic sum of the two unidirectional
fluxes (out and in) is the net flux, or the net
transport rate
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Cell membrane
Unidirectional flux in
Minus flux out
Equals net flux
Higher solutes higher chemical potential
Lower solutes lower chemical potential
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The random thermal movement of molecules in a
liquid can also cause a phenomenon known as
Brownian Motion
Particles suspended in water suffer impacts from
the thermal motion of the fluid medium
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As a result of these impacts, the suspended
particles have the same average translational
kinetic energy as the fluid molecules
But unlike the fluid molecules the particles have
a much larger mass, and a comparatively small
average speed
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Osmosis
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The diffusion of water across a selectively
permeable membrane is a special case of passive
transport known as osmosis
Osmosis can be described in terms of differences
in water potential or osmotic pressure
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Water potential is effectively the same as
chemical potentialThe water potential is a
measure of the chemical potential, or free energy
per mole, of water molecules
We can therefore predict what direction water
will move
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Water potential is affected by the amount of
other substances (solutes) dissolved in the water
The addition of solutes to water lowers its water
potential there will be less free water
because water molecules bind (hydrate) the solute
molecules
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Water potential is abbreviated by the Greek
letter psi
Water potential is the combined effects of solute
concentration and pressure
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These combined effects are incorporated into this
equation
This is the solute potential, which is
proportional to the solute concentration
This is the pressure potential, which is the
physical pressure on a solution
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We can also describe osmosis in terms of osmotic
pressure p
Osmotic pressure is the pressure required to
prevent osmosis (movement of water), and is found
to obey a law similar in form to the ideal gas
law
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V is the volume of solution in liters, n is the
number of moles of solute, R is the gas-law
constant, and T is the temperature on the Kelvin
scale
moles per liter molarity of the solution
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The two solutions above are separated by a
semipermeable membrane, and water moves from
right to left as if the solutions were driven to
attain equal concentrations
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Eventually, the pressure difference resulting
from the uneven heights of the liquid in the two
arms becomes so large that the net flow of water
ceases
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If two solutions of identical osmotic pressure
are separated by a semipermeable membrane, no
osmosis will occur
The amounts of water moving in both directions
will be equal there will be no net movement of
water
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Example
The average osmotic pressure of blood is 7.7 atm
at 25 C. What concentration of glucose will be
isotonic with blood?
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Tonicity?
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How living cells react to changes in the solute
concentrations of their environments
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Web Resources
Chemistry Comes Alive!
http//jchemed.chem.wisc.edu/jcesoft/cca/cca2/CCAM
AIN.HTM
Osmotic Pressure Calculation at Hyperphysics
http//hyperphysics.phy-astr.gsu.edu/hbase/kinetic
/ospcal.html