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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/2013/08/21/how-the-alaska-wood-frog-survives-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 PROBLEMSThe 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?