edsu 531, lessonwaudio[1]
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Freezing Point Depression
Chemistry Standard 6e: “Students know the relationship between...a
solute in a solution and the solution’s depressed freezing point...”
What is Freezing Point Depression?
▪Homemade ice cream...
(And why does it matter?)
▪Why mix salt into ice?
▪Freezing pt of ice = 0oC
▪But the freezing pt of ice cream is lower!▸-16oC▸We need a way to make ice colder...
What is Freezing Point Depression?
▪Ice Cream Activity:▸Mix salt and ice in big plastic bag▸Mix: ½ c. cream, ½ c. milk, 1/4 c. sugar, and 1/4 tsp
vanilla in little bag▸Shake!
Let’s get started!
Freezing Point Depression
▪Adding a solute (salt) to a solution (ice & water) decreases it’s freezing point
▪WHY?▸Ice in equilibrium at freezing point
How do we make an ice solution colder?
Solid
Liquid
= water
Freezing Point Depression
▪Solute disrupts equilibrium▸Solute wants to be equally distributed in solution
–More molecules to water (= melting)–Melting requires the absorbtion of heat (making surrounding
colder)
Solid
Liquid
= water= salt
Freezing Point Depression
▪Adding salt to the ice facilitated melting▸Notice your ice mixture has a lot of water▸This happened because equilibrium was disrupted
▪Melting of ice required heat▸Takes heat away from ice cream mixture▸Makes ice cream colder
What’s happening to your ice cream?
Freezing Point Depression
▪Salting icy roads▸Melts ice▸Can stay as liquid until -20oC
▪Anti-freeze▸Keeps engine coolant from freezing
Other Applications...
Freezing Point Depression
▪Calculating freezing point depression▸ΔTF = KF m i
–ΔTF is freezing point depression
–Kf is f.pt depression constant
–m = molality– i = van’t Hoff factor (# particles/mole)
▪Practice problems
▪Applications in chemistry
▪Introduce boiling point elevation
What’s next?
Freezing Point Depression
▸Adding solute to solution decreases freezing point
▸Instead of 0oC, water stays liquid until -20oC
▸The extra heat required to keep water liquid will be absorbed from surroundings
–Keeps surroundings colder
▸Enjoy your ice cream!
Wrap Up