WOOOAAAHHHHH
The wood frog is a remarkable
creature because it can
survive being frozen.
Scientists believe that a
substance (glycerin and
glucose) in the cells of this
frog acts as a natural
antifreeze, which prevents the
cells from freezing. You will
discover how a solute can
change the freezing point of a
solution.
http://newswatch.nationalgeographic.com/2
013/08/21/how-the-alaska-wood-frogsurvives-being-frozen/
WHAT IS A COLLIGATIVE PROPERTY?
A property that depends ONLY upon the number of solute
particles, and not upon their identity.
So what does that mean?
“I don’t care who you are or what you look like! I don’t
care what you are made of! I just need a lot of
ANYBODY to get this done!”
THEREFORE,
The amount of solute particles is the only thing that affects colligative
properties, and colligative properties apply to all types of substances–
no matter what they are made of.
Examples of Colligative Properties:
1) Freezing-Point Depression (If you freeze now… that’s it. It’s over!
Don’t you dare freeze on me!)
2) Boiling-Point Elevation (No! Don’t boil yet! Hold on! Wait!)
3) Vapor Pressure Lowering ( Do not vaporize as much as you are!
You are not as volatile as you think!)
ACTIVITY
An ice storm is coming. Your car’s
radiator has no antifreeze in it; all
stores are closed, and you’ve got to
use whatever you have around the
house to keep the water from freezing
in the car’s engine and splitting the
engine block.
Rule one: What you add has to dissolve in
water.
Rule two: If you add a solid or liquid that
dissolves in water, it doesn’t matter what it is,
just the amount.
Rule three: Just like antifreeze, your goal is to
replace about ½ of the water with a solid or
liquid that is miscible with water.
The items on the left is what you
had. So you mixed all of these into
a gallon of water and poured it into
your radiator.
Note: This is where it doesn't matter
what substance you use. It's the
count (the moles) that matters, not
its chemical makeup.
It was a long cold night and the next
morning you wonder if what you did
saved you thousands of dollars on a
new engine….
But your car looks like
this…
So after a few days, the
ice thaws and you check
your engine. It worked! It
didn’t freeze! Of course,
you now replace your
mixture with actual
antifreeze from the store.
Hmmm… How does antifreeze lower
the freezing point of the water?
1) FREEZING POINT DEPRESSION
• The difference in temperature between
freezing point of the solution and the freezing
point of the pure solvent.
Freezing point of Solution is a LOWER temperature than freezing
point of pure solvent.
WHY??
Reason 1: As the water tries to freeze, the other
molecules get in the way.
Reason 2: By adding these other substances, you’ve
added disorder to the mixture. Nature tends to favor
disorder (entropy). When water tries to freeze, it has
to get organized, which will be more difficult because
it’s mixed with all of these other molecules.
ORDER AND ENTROPY
Pure substances are more
orderly. When mixed, they
lose that order and become
more random (more entropy).
Again, nature favors entropy.
You know entropy from experience. You may clean and organize your room,
but it doesn't seem to take long for everything to become disorganized and
cluttered. It takes energy to organize and reduce clutter. It seems easy to
make it disorganized
OTHER FREEZING POINT DEPRESSIONS
Eggs are mostly
water, but
dissolved proteins
keep them from
freezing at 0°C.
Chefs take
advantage of this
in frozen desserts.
Notice that ice cream melts differently
than ice. Ice stays hard until it
melts. Ice cream gradually get softer
and softer.
Ice is a pure substance but ice cream is a
mixture. In other words, there are
other chemicals that get in the way of
ice freezing. So you have to get
colder than 0°C to get it to freeze.
About 30% of the water in ice cream
never freezes because of the high
level of dissolved solids like sugar,
fats, and proteins.
2) BOILING POINT ELEVATION
Antifreeze is also called a coolant
because it will raise the
temperature at which a liquid
will begin to boil.
Why is this a good thing?
2) BOILING POINT ELEVATION
• The difference in temperature between the
boiling point of solution and the boiling point
of the pure solvent.
Boiling point of solution is HIGHER than boiling point of pure solvent.
WHY??
By adding non-volatile substances, it interferes
with the particles and lowers the vapor
pressure.
Therefore, MORE energy such as heat
must be added to push those particles
to boil.
3) VAPOR PRESSURE LOWERING
In a solution, solute particles reduce the number of
free solvent particles able to escape the liquid.
The more solute particles you add to
the solution, the lower the vapor
pressure will be.
REMEMBER
Colligative Properties DO
NOT depend on the actual
particle.
It only depends on the AMOUNT of
particles.
PROBLEM
In college, I stored sodas in a
small refrigerator. One day I
noticed that the Diet Coke
had exploded from freezing
but the regular Coke did not
freeze and therefore did not
explode. I thought why did
the Diet Coke freeze and
not regular Coke?
SOLUTION
Regular coke has about 39 grams of sugar
dissolved in it. Diet Coke has only about 0.1
gram of Aspartame sweetener in it. The 39
grams of sugar interfered with the water trying
to freeze. So the freezing point of regular Coke
was lower than that of Diet Coke. Diet Coke's
freezing point is basically the same as water
since very little is dissolved in Diet Coke.
CALCULATIONS INVOLVING COLLIGATIVE
PROPERTIES
Review:
We learned that to calculate the concentration of a
solution, we can use molarity.
Molarity = M = mol/L
MOLALITY
Molality is a different way to calculate the concentration of a
solution.
It is represented by a little m.
CALCULATE MOLALITY
What is the molality of a
solution that is made by
adding 29.3g NaCl to 1000
g of water?
CALCULATING GRAMS FROM MOLALITY
Relating Concepts
How are freezing-point depression and boiling-point
elevation related to molality?
Freezing Point Depression
Boiling Point Elevation
What is the normal freezing point
of water before adding a solute?
What is the normal boiling point
of water before adding a solute?
PRACTICE PROBLEMS
Anything that can be is used
to depress the freezing
point of water can also
be used to cause ice to
melt.
Here is your ice covered
Porsche after an ice
storm.
What substances could you
use to de-ice it?
PRACTICE PROBLEMS
The ingredients say 39 grams of
sugar is used in 1 can of coke.
The molar mass of fructose (sugar)
is 180.16 g/mol.
1 can =12 oz = 355 mL.= about
355 g of solution
Kf= 1.86 C·kg/mol for water
Question: .So how many degrees Celsius
does the sugar reduce the freezing point
of water in the can?
What is the new boiling point?
PRACTICE PROBLEMS
How many grams of sodium bromide must be dissolved
in 400.0g of water to produce a 0.500 molal solution?