Colligative Properties
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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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Colligative Properties

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Colligative properties

Colligative Properties

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

Three Colligative properties affect solutions:


Colligative properties

Vapor Pressure Depression:

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


Colligative properties

Vapor Pressure (kPa)

101.3

4.3

0

Temperature (°C)


Colligative properties

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


Colligative properties

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

Pure Solvent

Solvent w/ solute


Colligative properties

Vapor Pressure (kPa)

for Water

for Solution

101.3

4.3

0

Temperature (°C)


Colligative properties

Boiling Point Elevation:

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


Colligative properties

Freezing Point Depression:

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


Colligative properties

Vapor Pressure (kPa)

for Water

for Solution

101.3

4.3

Boiling Point Elevation

0

Freezing Point Depression

Temperature (°C)


Colligative properties

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


Colligative properties

Freezing Point Depression and Boiling Point Elevation calculations

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


Colligative properties

Dissociation vs. Ionization

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)


Colligative properties

Ionic solute in a polar solvent:

(+) 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


Colligative properties

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


Colligative properties

Ionization:

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


Colligative properties

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


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