Introduction to Colligative collective Properties Goal 1: Get in the way Goal 2: Create Disorder

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Introduction to Colligative (collective)
PropertiesGoal 1 Get in the wayGoal 2 Create
Disorder

• or
• When quantity not quality is the only thing that
  matters.

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An ice storm is coming. Your cars radiator has
no antifreeze in it all stores are closed, and
youve got to use whatever you have around the
house to save your cars engine.
It would be ideal to have antifreeze (ethylene
glycol) but remember its quantity not quality
that counts here.
Also remember the motto for Colligative
PropertiesGetting in the way is Good,Creating
Disorder is Better
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The next morning you wonder if what you did saved
you thousands of dollars on a new engine.
Rule one What you add has to dissolve in
water. Rule two If you add a solid that
dissolves in water, it doesnt 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. Examples 1 Sugar, salt, baking
soda, shampoo, laundry detergent, pancake
syrup. Examples 2 Rubbing alcohol, brake fluid
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Again, it seems odd that the chemical nature of
these different compounds are not a factor in
regards to how much they depress the freezing
point.
It reminds us of the ideal gas law PVnRT
The properties are not affected by what kind of
gas we are talking about (at least in situations
where they are behaving as an ideal gas).
Examples 1 Sugar, salt, baking soda, shampoo,
laundry detergent, pancake syrup.
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Freezing point depression

• How does it work?
• Get in the way.
• Create Disorder

Reason 1 As the water tries to freeze,the
other molecules get in the way. Reason 2 By
adding these other substances, youve added
disorder to the mixture. Nature tends to favor
disorder (entropy). When water tries to freeze,
it has to get organized, which will take more
energy.
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This frog is frozen, but the water in it did not
turn into ice crystals, which would have ruptured
the cells in its body. Why didnt ice crystals
form?
Glucose and glycerol in its blood and cells
prevent water from freezing.
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Dissolved sugars also play a role in helping
plants to be frost tolerant.
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Calculating the size of freezing point depression

• Since colligative properties depends on quantity
  not quality, you would expect the moles of the
  solute to be important.
• Experimentally, a kilogram of solvent (water in
  our example) is mixed with one mole of solute.
  The number of degrees below its normal freezing
  point is noted. That becomes the freezing point
  constant. It is assumed the amount of freezing
  point depression is proportional to the number of
  moles of solute. In other words, if the number of
  moles of the solute is doubled, the number of
  degrees below its normal freezing point will
  double.
• The freezing point constant is degrees Celsius of
  freezing point depression for every mole of
  solute thats dissolved in one kilogram of
  solvent. The units look like
• oC freezing pt depression
  mole solute kg solvent
• ?TKfm

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This graphic helps us see the randomness that
mixing causes. As a mixture, the liquid that
would normally freeze at a higher temperature is
prevented from freezing by the other liquid or
solid. Again the cause is the solute gets in the
way and the freezing as to reverse the randomness
(entropy) which takes more energy.
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Boiling Point Elevation
Antifreeze is often called coolant because it
not only can lower the freezing point of water,
it can elevate the boiling point of water. So,
if you dont have any antifreeze (coolant), what
could you use instead? Yes, the same items that
you picked to keep it from freezing.
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Notice that ice cream melts differently than ice.
Ice stays hard until it melts. Ice cream
gradually get softer and softer.
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Ice is a pure substance but ice cream is a
mixture. In other words, there are other
chemicals that get in the way of water freezing.
So you have get colder than 0Cto 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.
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Chocolate behaves is a similar fashion. As a
mixture, it gradually softens before it melts.
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Most anything will reduce its melting point if it
is mixed with other substances.
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Pure molten silica (SiO2) freezes into quartz at
around 2000C, but if mixed with CaO and Na2CO3,
it freezes at about 1000C. Other additives can
bring it down to 500C. Actually, at room
temperature it is still not completely frozen.
Glass is classified as a supercooled liquid. Over
many millenia, this drinking glass will slowly
melt into a pool of glass.
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Eggs are mostly water, but dissolved proteins
keep them from freezing at 0C. Chefs take
advantage of this in frozen desserts.
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OSMOSIS Another Colligative Property
How do you make red blood cells go from this
shape to this?
Get in the wayAnd quantity not quality is all
that counts.
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Inside cell
Outside cell
Inside and outside water passes through the
permeable membrane at equal rates.
Cell Membrane
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Inside cell
Outside cell (hypertonic)
The extra solute (e.g., NaCl) gets in the way of
the outside water passing through the membrane.
Note Solutes cannot pass through membrane.
Cell Membrane
Eventually an equal number will pass back and
forth, but only after the outside has a greater
share of the water molecules.
Na
Na
Cl
Na
Cl
Cl
Na
Na
Anything soluble will do the same thing (quantity
not quality) For example, any used for freezing
point depression would cause this.
Cl
Na
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OSMOTIC PRESSURE
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REVERSE OSMOSIS
Eventually the salt concentration will become too
high, so some must be drained out.
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What could you do to make lighter fluid less
likely to ignite?
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Vapor pressure
Vapor pressure is reduced by the addition of a
non-volatile solute for the same reasons as
freezing point depression and boiling point
elevation.
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Colligative Properties

• Freezing point depression
• Boiling point elevation
• Osmotic pressure
• Vapor pressure

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Colligative (collective) PropertiesGoal 1 Get
in the wayGoal 2 Create Disorder

• or
• When quantity not quality is the only thing that
  matters.

Coming up next Calculations with colligative
properties