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Colligative Properties - PowerPoint PPT Presentation

Colligative Properties. Properties that depend only on the # solute particles in soln not the nature of the solute particles. Three Colligative properties affect solutions: . Vapor Pressure Depression: .

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Properties that depend only on the # solute particles in soln not the nature of the solute particles

Three Colligative properties affect solutions:

VP is when a pure liquid is placed into a closed container, some of the solvent evaporates into the empty space above the liquid and has a measurable pressure

Pure Solvent

101.3

4.3

0

Temperature (°C)

BUT when a solute is added to a solvent, it takes up space within the solvent

Allowing less of the solvent to evaporate into the space above the mixture

This process decreases (lowers) the vapor pressure of the soln

Vapor Pressure Depression within the solvent is when the vapor pressure of a pure liquid is lowered by the addition of a solute

Pure Solvent

Solvent w/ solute

Vapor Pressure (kPa) within the solvent

for Water

for Solution

101.3

4.3

0

Temperature (°C)

Boiling Point Elevation: within the solvent

The BP of a pure liquid is increased with the addition of a solute to a pure liquid

WHY?

Because decreased vapor pressure of a pure liquid changes the BP

Freezing Point Depression: within the solvent

Is decreased(lower) with the addition of a solute to a pure liquid

Ex: Salt melting snow; the soln of salt and snow has a lower freezing point than the snow does alone

Ex: Ethylene Glycol (antifreeze) combines with water to depress the freezing point and increase the boiling point allowing vehicles to run in extreme temperatures

Vapor Pressure (kPa) within the solvent

for Water

for Solution

101.3

4.3

Boiling Point Elevation

0

Freezing Point Depression

Temperature (°C)

If Boiling Point Elevation and Freezing Point Depressions are directly proportional to the # of solute particles

Why is there a difference between freezing points of 1 molal soln of sugar (FP 1.86°C) and 1molal soln of salt (FP -3.72)?

C6H12O(s) H2O C6H12O(aq)

NaCl(s)

NaCl(s)

H2O Na+(aq) + Cl-(aq)

NaCl is an ionic substance which when dissolved in water it disassociates and releases ions into soln; therefore the more ions in soln the higher the BP rises and the lower the FP decreases

ΔTf = kfm

Kf :Freezing Point constant (each solvent has there own constant)

Ex: kf water = 1.86°C/molal

ΔTb = change in freezing temp

m: molality

ΔTb = kbm

Kb :Boiling Point constant (each solvent has there own constant)

Ex: kf water= 0.52°C/molal

ΔTb = change in freezing temp

m: molality

Dissociation vs. Ionization calculations

Two different processes that occur when a solute

dissolves in a solvent

Dissociation:

Happens when an ionic solute (ex: NaCl, KOH, K2SO4, or CaCl2) dissolves in a polar solvent (water)

Ionic solute in a polar solvent: calculations

(+) ions of the solute are attracted to the (-) poles of the molecule…and (-) ions are attracted to the (+) pole of the molecule thru molecule-ion attractions

These attractions are so strong…they pull the ions out of the crystal lattice…forming a soln of mobile charged ions

a) These ions are now considered to be “hydrated”

b) This soln can now conduct electricity

Ionization: the crystal lattice…forming a soln of

the process by which polar molecules interact to form ions from molecular substs not containing ions to begin with

CH3COOH(l) + H2O(l) CH3COO-(aq) + H3O+(aq)

acetate ion + hydronium ion

ethanoic acid + water

Polar molecules are poor conductors of heat and electricity

Ions will now conduct small amounts of electricity

1) When 2 polar solvents are mixed, the opposite poles of each molecule are attracted to the other molecules pole (dipole-dipole forces)

CH3COOH(l) + H2O(l) CH3COO-(aq) + H3O+(aq)

acetate ion + hydronium ion

2) These forces are so strong that an ion is transferred…forming a soln of oppositely charged ions, allowing the soln to conduct electricity.

ethanoic acid + water