COLLIGATIVE PROPERTIES OF SOLUTIONS14.4: Pgs. 498 - 504

COLLIGATIVE PROPERTIES Physical properties of solutions differ from

the solvent they were made from Differences are due to the number of solute

particles in solution (not the type of solute) These differences in physical properties are

called colligative properties Colligative properties include:

vapor pressure freezing point boiling point Osmotic pressure

1. VAPOR PRESSURE

Review: What is vapor pressure?

Volatile vs. Nonvolatile substances

1. VAPOR PRESSURE

Ions and water molecules interact so that less are able to leave the surface, creating a lower vapor pressure.

1. VAPOR PRESSURE

Solutions that contain nonvolatile solutes will have a LOWER vapor pressure than the pure solventGlucose and sodium

chloride are both nonvolatile solutes

Example – salt water solution

1. VAPOR PRESSURE

The decrease in vapor pressure (and all colligative properties) is proportional to the number of solute particles in the solution NaCl fully ionizes into 2 ions, while the glucose

molecules do not break apart when dissolving Each NaCl will have a GREATER effect on vapor

pressure because it dissociates into more particles than glucose

2. BOILING-POINT

Since adding a solute to water decreases the vapor pressure, what will happen to the boiling point? It will increase! Called boiling-point elevation

CALCULATIONS WITH BOILING POINT

Kb = (called the boiling-point-elevation constant), depends only on the solvent

m = molal concentration of the solution

i = number or parts the solute breaks into

Δ T = Kb m i

3. FREEZING-POINT DEPRESSION

Presence of solute in water disrupts the freezing pattern

Need to withdraw MORE energy from a solution for it to solidify Called freezing-point

depression

CALCULATIONS WITH FREEZING POINT

Kf = (called the freezing-point-elevation constant), depends only on the solvent

m = molal concentration of the solution

i = number or parts the solute breaks into

Δ T = Kf m i

Phase diagram showing the effect of nonvolatile solute on freezing point and boiling point.

Purple = solution

Blue = pure solvent

TABLE OF KB AND KF

A PROBLEM TO CONSIDER….

Table 12.3 gives Kb and Kf for water as 0.512 oC/m and 1.86 oC/m, respectively. Therefore,

An aqueous solution is 0.0222 m in glucose. What is the boiling point and freezing point for this solution?

The boiling point of the solution is 100.0114oC and the freezing point is –0.0413oC.

4. OSMOTIC PRESSURE Solute concentration effects osmosis rateOsmosis: when a solvent moves from an

area of high concentration to an area of lower concentration through a semipermeable membrane that allows only the solvent to pass.

AN EXPERIMENT WITH OSMOSIS:Osmotic Pressure = amount of additional pressure caused by water molecules what moved into the concentrated solution

LET’S MAKE ICE CREAM! Part 1 – Table Group:

Mass large Ziplock bag Fill it 1/3 full with ice

cubes and mass again. Determine mass of ice

cubes used Put thermometer in ice,

and record temp when it stabilizes

Mass 100g of salt in a paper cup and add to Ziplock bag, stirring occasionally

Record lowest temp reached

Part 2 – Individual Groups: Measure ½ cup milk, 1-2

Tbsp sugar and ¼ tsp vanilla and put into small Ziplock bag Seal and squeeze out air

Place sealed small bag in large bag (both lab groups go in one large bag) Seal and squeeze air out

of bag Shake/mix mixture until a

thick ice cream appearance Takes about 5 – 8 minutes.

EAT!