Unit 9 Lecture Day 4-Colligative Properties

8/22/2019 Unit 9 Lecture Day 4-Colligative Properties

1/41

COLLIGATIVEPROPERTIES

Unit 9Chapter 12


2/41

Definition:

The properties of a solution that are dependant

only on the number of solute particles in

solution.

Vapor pressure lowering

Boiling point elevation

Freezing point depression

Osmotic pressure

COLLIGATIVE PROPERTIES


3/41

The equilibrium vapor pressure of a solvent with anonvolatile solute is shown in (a).

The equilibrium vapor pressure of a pure solvent is shownin (b).

VAPOR PRESSURELOWERING


4/41

Which equilibrium has a higher vapor

pressure?



5/41

The presence of solute particles results in

fewer solvent molecules sitting at the surface

to escape.



6/41

According to RaoultsLaw, the vapor pressure

of a solution with a non-volatile solute is equal

to the vapor pressure of the pure solvent at

that temperature multiplied by its mole fraction.

Psolv = Psolv Xsolv


The vapor pressure of the solvent is

proportional to the relative number of solventmolecules at the surface the mole fraction


7/41

When the solute-solvent attractions are stronger thanthe original solvent-solvent attractions, the solutionvapor pressure will be lower. This is typically the casewhen the solute is nonvolatile.



8/41

Definition:

When the vapor pressure of a solvent

containing a solute is compared to the vapor

pressure of the pure solvent, the solutions

vapor pressure is found to be lower.Psolv Psolv= P


Vapor pressure of

the pure solvent

Vapor pressure of

the solvent insolution

Amount by which

the solvents vaporpressure changed


9/41

Eugenol, C10H12O2, is the chief constituent of oil of clove. It is a

pale yellow liquid that dissolves in ethanol, C2H5OH; it has aboiling point of 255C (thus, it has a relatively low vapor

pressure at room temperature). What is the vapor pressure

lowering at 20.0C of ethanol containing 8.56 g of eugenol in

50.0 g of ethanol? The vapor pressure of ethanol at 20.0C is

44.6 mmHg.


Psolv = Psolv

Xsolv

Psolv Psolv= P

Solute

Solvent

P

Psolv = 44.6 mm Hg Psolv Psolv= P

Psolv Psolv Xsolv =

P


10/41






44.6 mmHg.


Psolv = Psolv

Xsolv

Psolv Psolv= P

Solute

Psolv = 44.6 mm Hg

Psolv Psolv Xsolv =

P(44.6)(44.6) Xsolv =

P

Solvent

8.56gEugenol 1mol

164.22g 0.0521mol

50.0gEthanol 1mol

46..07g 1.086mol


11/41






44.6 mmHg.


Psolv = Psolv

Xsolv

Psolv Psolv= P

Solute

Psolv = 44.6 mm Hg

Psolv Psolv Xsolv =

P(44.6)(44.6) Xsolv =

P

Solvent

solv 1.086mol

1.086mol 0.0521molsolv 0.954


12/41






44.6 mmHg.


Psolv = Psolv

Xsolv

Psolv Psolv= P

Solute

Psolv = 44.6 mm Hg

Psolv Psolv Xsolv =

P(44.6)(44.6)(0.954) = P

Solvent

2.05 mm Hg = P


13/41

Remember that for a gaseous solution

(homogeneous mixture), the total pressure of

the solution is the sum of the partial pressures.

(Whos law is that?)

Therefore

Ptotal = Psolvent + Psolute



14/41

This means that Raoults law can also beapplied to the vapor pressure of a solution witha volatile solute. The total vapor pressure ofthe mixture would be equal to the vaporpressure created by the solvent added to the

vapor pressure created by the volatile solute.

Ptotal = Psolv Xsolv + Psolute

Xsolute



15/41

PRESSURE LOWERING

AFFECT? In the diagram notice

that the vapor pressurelowers when a solventis mixed with anonvolatile solute.

The Psolv increasesas the temperatureincreases; similarly, thePsolv increases as thetemperature increases.

This means that thelowering of the vaporpressure leads to ahigher boiling point ofthe solution.

Vapor Pressure

(mm Hg)

Temperature (C)

Solvent + Solute

Solvent

P


16/41

Definition:

The boiling point of a solution is greater than

the boiling point of the pure solvent because

the solution (which has a lower vapor

pressure) will need to be heated to a highertemperature in order for the vapor pressure to

become equal to the external pressure (i.e.,

the boiling point).

BOILING POINT ELEVATION


17/41

The boiling point elevation, Tb, is the

difference between the higher boiling point of

the solution and the boiling point of the pure

solvent.

Tb = Tb Tb


Boiling point

elevation

Boiling point

of solution

Boiling point of

pure solvent


18/41

The boiling point elevation, Tb

, is directly

proportional to the molality of the solute.

Tb = kb msolute


Boiling point

elevation

molal boiling point

elevation constant

molality of

the solute


19/41

MOLAL BOILING POINT

ELEVATION CONSTANT

Each solvent has a

different boiling

point constant.

Solvent

Tb ofpure

solvent

(C)

kb

(C/m)

Water 100.00 +0.5121

Benzene 80.10 +2.53

Camphor 207.4 +5.611

Chloroform 61.70 +3.63


20/41

A solution was made up of eugenol, C10H12O2, in diethyl ether. If

the solution was 0.575 m eugenol in ether, what was the boilingpoint of the solution? The boiling point of pure ether is 34.6C

and the boiling-point-elevation constant is 2.02C/m.


Tb = kb msoluteTb = Tb Tb

Solute

Solve

ntTb

Tb = 34.6Ckb = 2.02C/m

m = 0.575 m

Tb = ??

kb msolute= Tb = TbTb(2.02C/m)(0.575 m) = Tb

(34.6C)35.8C = Tb


21/41

antifreeze, cocoa, gasoline and ethanol,

salted pasta water (just kidding)



22/41

Addition of a nonvolatile solute to the volatile

solvent increases the attractions (and lowers

the vapor pressure). This means that the

particles are closer together so a lower

temperature allows them to freeze.

FREEZING POINTDEPRESSION


23/41

The freezing point depression, Tf

, is the

difference between the higher freezing point of

the pure solvent and the freezing point of the

solution.

Tf= Tf Tf


Freezing point

depression

Freezing point

of pure solvent

Freezing point

of solution


24/41

The freezing point depression, Tf

, is directly

proportional to the molality of the solute.

Tf= kf msolute


Freezing point

depression

molal freezing

point depression

constant

molality of

the solute

MOLAL FREEZING POINT


25/41

MOLAL FREEZING POINT

DEPRESSION CONSTANT

Each solvent has a

different freezing

point constant.

Solvent

Tf ofpure

solvent

(C)

kb

(C/m)

Water 0.0 +1.86

Benzene 5.50 +5.12

Camphor 179.95 +39.7


26/41

A solution was made up of 0.575 m eugenol, C10H12O2, in diethyl

ether. What was the freezing point of the solution? The freezingpoint of pure ether is -116.3C and the freezing-point-depression

constant is

1.79C/m.


Tf= kf msoluteTf= Tf Tf

SoluteSolve

ntTf

Tf =116.3Ckf= 1.79C/m

m = 0.575 m

Tf= ??

kf msolute= Tf= TfTf(1.79C/m)(0.575 m) = (116.3C)

Tf117.3C = Tb


27/41

antifreeze (HEAT), salt on icy roads,

making ice cream, Crystal Lake freezes

before the pond in Veterans Acres



28/41

COLLIGATIVE PROPERTYANIMATION
http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/propOfSoln/colligative.htmlhttp://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/propOfSoln/colligative.htmlhttp://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/propOfSoln/colligative.htmlhttp://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/propOfSoln/colligative.html


29/41

Osmosis is the diffusion of small molecules

through a semi-permeable membrane.

Usually, osmosis is seen in the net movement

of the solvent from the pure solvent (low solute

concentration) to solution (high soluteconcentration).

OSMOTIC PRESSURE


30/41

The membrane is

termed semi-

permeable because

small molecules such

as water or small ions(Na+ or K+) may pass

in either direction

through the

membrane.

OSMOTIC PRESSURE


31/41

The solute reduces the mole fraction of the

solvent in a solution. The lower the mole

fraction of solvent, the greater the net flow

of solvent molecules into the solution (and

greater osmotic pressure).

OSMOSIS


32/41

OSMOTIC PRESSURE

Osmotic pressure is the pressure necessary to just

stop osmosis. This is done by pressing on the

solution side to increase the movement of solvent

particles from the solution back into the puresolvent.


33/41

Osmotic pressure is proportional to the

molar concentration of the solute.

OSMOTIC PRESSURE

MRT PV nRTP

n

VRT

Notice how this equation is

based on the ideal gas

law:

Osmotic

pressure

Molarity of

the solute


34/41

Dextran, a polymer of glucose units, is produced by bacteria

growing in sucrose solutions. Solutions of dextran in water havebeen used as a blood plasma substitute. What is the osmotic

pressure (in mmHg) at 21C of a solution containing 1.50 g of

dextran dissolved in 100.0 mL of aqueous solution, if the average

molecular weight of the dextran is 4.0 x 104 amu?

OSMOTIC PRESSURE

SoluteSolvent =

H2O

MRT

MRT

1.50g 1mol

4.0x104 g 3.75x105 mol

M 3.75x10

5 mol

0.1000L 3.75x104 M

3.75x104 M 0.0821 atmLmolL 294K 0.00905atm 6.9mmHg


35/41

reverse osmosis, vinegar and eggs, water

transportation in plants, blood pressure

(normal osmotic pressure when compared

to pure water), dialysis of cell walls,

desalinating ocean water

OSMOTIC PRESSURE


36/41

When a nonelectrolyte solute dissolves in a

solvent, it dissolves without separating into

ions.

Like methanol dissolving in waterCH3OH(l) CH3OH(aq)

Nonelectrolyte Solutes


37/41

When an electrolyte solute dissolves in a

solvent, it does separate into ions.

Like sodium sulfate dissolving in water

Na2SO4(s) 2 Na+(aq) + 1 SO42(aq)

Electrolyte Solutes


38/41

Since colligative properties depend on the

number of solute particles, not the identity

of the solute, an electrolyte influences those

properties more than a nonelectrolyte.

A 1.0 M sodium sulfate solution creates how many

particles?

Na2SO4(s) 2 Na+(aq) + 1 SO4

2(aq)

Electrolyte Solutes

2 + 1 = 3 particles


39/41

A strong electrolyte, such as Na2SO4, will

dissolve 100 %, creating one particle for

each dissociated ion.

These particles are called the vant Hoff

factor, i.

What is i for potassium phosphate?

Electrolyte Solutes

K3PO4(s) 3 K+(aq) + 1 PO4

3(aq)

4


40/41

A weak electrolyte, such as Ag3PO4, will

NOT dissolve 100 %. This means that

fewer particles will be able to influence the

colligative properties.

What is i for silver phosphate?

Electrolyte Solutes

Ag3PO4(s) 3 Ag+(aq) + 1 PO4

3(aq)


41/41

How does this change the colligative

properties?

Electrolyte Solutes

Vapor Pressure

LoweringBoiling Point Elevation

Freezing Point

DepressionOsmotic Pressure

P1 1P1o

moleSolvent

moleSolvent (i)moleSolute

P1o

Tb kb m i Tb Tb

o

Tf kf m i Tf

o Tf

iMRT

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Unit 9 Lecture Day 4-Colligative Properties

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