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