discussion: colligative properties

Discussion: Colligative Properties

Students will be able to:Explain how dissolved solutes affect the boiling

and freezing points of solutions

Remember

Packet # 1 is due today

Lab next time!

The Affect of Adding Non-volatile Solutes

What happens to the boiling point and freezing point as solutes are added?

Will the Freezing Point Ever Go Up?

NO! Adding Solutes Will Always Make the Freezing Point Go Down

Will the Boiling Point Ever Go Down?

Sometimes. If the solute is volatile the boiling point goes down.

An example would be adding alcohol to water. The mixture will boil at a lower temperature than the pure water.

The boiling point of alcohol is only around 70oC so adding it makes the boiling point go down

Ice Will Not Melt if the Outside Temp is Lower than Zero

Since the Fp = 0 and the outside temp is -5, this ice won’t melt.

If the Outside Temperature is -5oC Salt and Ice Mixtures Can Melt

If you can add salt the mixture will not freeze until it is -10oC

Why do people add salt to roads?

Sentence #1:

Sentence #2:

Sentence #3:

Colligative Properties

What are two properties of solutions that depend on the number of dissolved particles?

Boiling Point (normally goes up) Freezing Point

Ethylene Glycol

This is the molecule that is commonly called antifreeze. C2H6O2

62 grams/mol There are two reasons it is used for

antifreeze. It has

a high boiling point and it is very soluble in water

Antifreeze

Why is antifreeze added to cars? The mixture of water and ethylene

glycol has a higher boiling point than pure water.

The mixture of water and ethylene glycol has a lower freezing point than pure water.

Molarity and molality

Molarity is the ratio of moles solute to liters of solution

Molality is the ratio of moles solute to kilograms of solvent

Notice you divide by the solvent not the solution

Calculate Molality (on discussion)

30.0 grams of C2H6O2 (the solute) change to moles

Mixed with 30.0 grams of water (the solvent)change to Kg

What Temp Will it Freeze?

What are the two factors that influence how much it drops? a. the molality of the solution b. the freezing point constant of water (1.86)

Identify The Symbols

How to Calculate the Boiling Point

ΔTb = m x Kb ΔTb = m x 0.512 ΔTb = 16.13 x 0.512 8.25 Recall the temp of boiling goes up. Boiling

is normally 100oC. So the temp of boiling is……

108.25

70 grams of water (the solvent) mixed with 30 grams of

antifreeze (C2H6O2) the solute.