Colligative PropertiesColligative Properties

Honors ChemistryHonors Chemistry

Unit 8Unit 8

Chapter 15Chapter 15

Colligative PropertiesColligative Properties

These are the effects that a These are the effects that a solutesolute has on has on a a solvent.solvent.

When water has something dissolved in it, When water has something dissolved in it, its its physicalphysical properties change. properties change.

It will no longer boil at It will no longer boil at 100100ooCC and it will no and it will no longer freeze at longer freeze at 00ooCC like pure water. like pure water.

Three Main EffectsThree Main Effects

Lowers the Lowers the vaporvapor pressure of a solvent. pressure of a solvent.– Lower vapor pressure means that Lower vapor pressure means that fewerfewer water water

molecules can escape from the liquid phase molecules can escape from the liquid phase into the gas phase at given temperature. into the gas phase at given temperature. Remember, a lower vapor pressure means a Remember, a lower vapor pressure means a higherhigher boiling point! boiling point!

2) Raises the 2) Raises the boilingboiling point of a solvent. point of a solvent.

3) 3) Lowers Lowers the freezing point of a the freezing point of a solvent.solvent.

Freezing Point DepressionFreezing Point Depression

Boiling Point ElevationBoiling Point Elevation

ApplicationsApplications

– salting icy roadssalting icy roads

– making ice creammaking ice cream

– antifreezeantifreeze

cars (-64°C to 136°C)cars (-64°C to 136°C)

fish & insectsfish & insects

Colligative PropertiesColligative Properties -These depend only on the -These depend only on the

numbernumber of dissolved particlesof dissolved particles-Not on what -Not on what kindkind of particle of particle

General RuleGeneral Rule: The : The more more solute solute particles that are present in a particles that are present in a solvent, the solvent, the greatergreater the effect. the effect.

# of Particles# of Particles– NonelectrolytesNonelectrolytes ( (covalent)covalent)

remain intact when dissolved remain intact when dissolved 1 particle1 particle

– ElectrolytesElectrolytes ( (ionicionic))dissociate into ions when dissolveddissociate into ions when dissolved2 or more particles2 or more particles

– Electrolytes have a Electrolytes have a stronger stronger affectaffect in in lowering the freezing point and elevating the lowering the freezing point and elevating the boiling point because it puts boiling point because it puts moremore particles particles into the solutioninto the solution. .

Remember the rule, the Remember the rule, the moremore particles, particles, the greater the effect!the greater the effect!

The The Dissociation FactorDissociation Factor (d.f.) for an (d.f.) for an electrolyte is the electrolyte is the number of ionsnumber of ions a a compound dissociates into. compound dissociates into.

NaCl gives NaNaCl gives Na++ ions and Cl ions and Cl-- ions, which is ions, which is 2 particles, therefore 2 particles, therefore d.f.d.f. = 2= 2

What is “What is “d.f.d.f. “ for Al(NO“ for Al(NO33))33 ? ?

Al(NO3)3 Al3+ + 3 NO3- = 4 particles

Non-electrolytesNon-electrolytesA compound that does not conduct electricity A compound that does not conduct electricity when dissolved in water. when dissolved in water.

Examples are glucose or any other sugars and Examples are glucose or any other sugars and alcohols, such as alcohols, such as ethanol (CHethanol (CH33CHCH22OH)OH)

The The d.f. d.f. = = 11 for any non-electrolyte. for any non-electrolyte.

Why? Why?

Covalent molecules do not break apart when Covalent molecules do not break apart when they become solvated by water molecules.they become solvated by water molecules.

CalculationsCalculations

tt:: change in temperaturechange in temperature ( (°°CC))

KK:: constant constant basedbased on the solvent on the solvent ( (°°CC··kgkg//molmol))

Use KUse Kbb for boiling point elevation and K for boiling point elevation and Kff

for freezing point depression. Each solvent for freezing point depression. Each solvent has it’s own unique factors!has it’s own unique factors!

mm:: molality (molality (mm) = moles solute/Kg solvent) = moles solute/Kg solvent

d.f.d.f.:: # of particles# of particles

t = m · d.f. · K

Example 1:Example 1:

If 48 moles of ethylene glycol, CIf 48 moles of ethylene glycol, C22HH44(OH)(OH)22, is , is

dissolved in 5.0kg of water. What is the boiling dissolved in 5.0kg of water. What is the boiling point of the solution?point of the solution?∆∆Tbp = m Tbp = m d.f. d.f. Kb Kb

d.f. = 1 ( it is a non-eletrolyte)d.f. = 1 ( it is a non-eletrolyte)

Kb = 0.52Kb = 0.52ooC/m (a constant for water)C/m (a constant for water)

Molality = m = moles solute/kg solventMolality = m = moles solute/kg solvent

∆∆Tbp = Tbp = (48 moles)(48 moles) (1) (0.52 (1) (0.52ooC/molal)C/molal)

(5.0 Kg)(5.0 Kg)

∆∆Tbp = 5.0 Tbp = 5.0 ooCC New BP = 100 + 5 = 105 oC

Example 2:Example 2:

55.5 grams of CaCl55.5 grams of CaCl22 are dissolved in 2.0kg of are dissolved in 2.0kg of

water. What is the freezing point of this water. What is the freezing point of this solution?solution?∆∆Tfp = m Tfp = m d.f. d.f. Kf Kf

d.f. = 3 (CaCld.f. = 3 (CaCl22 produces 3 particles) produces 3 particles)

CaClCaCl22 Ca Ca2+2+ + 2 Cl + 2 Cl--

Kf = 1.86Kf = 1.86ooC/m (a constant for water)C/m (a constant for water)

∆∆Tfp = (0.25molal) (3) (1.86Tfp = (0.25molal) (3) (1.86ooC/molal)C/molal)

Must calculate molality

Molality = mass/molar mass

Kg Solvent

= (55.5 gram/111g/mole) 2.0 kg

= 0.25m

= 1.4 oC

New FP = -1.4 oC