LECTURE № 5 Colligative Properties of Solutions

in water
compounds in water
a heterogeneous mixture
a homogeneous mixture

given amount of solvent is represented by the …
Volume of

the solution

Mass of the solute

Mass of the solution

Concentration of the solute

of equivalent-moles of solute per 1 liter of solution
the gram

of a solute per 1 milliliter of solution

the number of moles of solute per 1 kilogram of solvent

the number of moles of solute per 1 liter of solution

equivalent-moles of solute per 1 liter of solution
the gram of

a solute per 1 milliliter of solution

the number of moles of solute per 1 kilogram of solvent

the number of moles of solute per 1 liter of solution

and do not conduct electricity when placed in water.
An electrolyte

is a substance that, when dissolved in water form ions, as results a solution can conduct electricity:

NaCl => Na+ + Cl-
cation anion

in polar solvents dissociate (break apart) in solution to make

ions
may be strong (100% dissociation) or weak (less than 100%)
Strong Electrolyte – all or almost all of compound dissociates; example: strong acids (H2SO4, HNO3, HClO4, HCl, HBr, HI)
Weak Electrolyte – small amount of compound dissociates; example – weak acids (HF, CH3COOH, H3PO4)

Nonelectrolyte – solution that does not conduct electricity
solute is dispersed but does not dissociate
Example: sugar (dissolves but does not dissociate), organic acids (contain carboxyl groups)

only on the concentration of solute particles (ions or molecules)

in the solution and not the type.

In other words, it doesn’t matter if it is salt, sugar, gasoline, or tennis balls – it will behave the same way!

Freezing Point Depression
Salt on the road
Anti-freeze in your radiator
And Boiling

Point Elevation (2nd Raoult’s Law)
Anti-freeze in your radiator

3) Osmotic Pressure (Van’t Hoff’s Law)
Membrane diffusion
The Great Sugar Fountain

Colligative PropertIES:

thermodynamic equilibrium with its condensed state.
When ↑t , ↑p0

H2O:
00C – 4,6 mm Hg
200C – 17,4 mm Hg
1000C – 760 mm Hg

P0

liquid,
the presence of dissolved substances in the liquid or

solid.

According to Raoult's law, the vapor pressure of a pure liquid or solid is lowered by the addition of a solute.

become less volatile (=decreased vapor pressure).

Solute-solvent interactions contribute to this

effect.

P0

P0

pressure is proportional to its concentration. This was discovered by

French chemist Francois Raoult (1830-1907).
1st Raoult’s law states that for non-electrolyte solutions, the partial vapor pressure of a solvent over a solution (P1) is equal to the vapor pressure of the pure solvent (P0) multiplied by the mole fraction of the solute (X2). This law is mathematically expressed as:

the number of dissolving solute moles

the number of solvent mole

the mole fraction

atmospheric pressure (p0= pаtm) is called boiling point

The temperature at

which vapor pressure of solvent in its liquid and solid phase become equal is called freezing point

at 00C water is freezing,
and at 1000C water is boiling

the boiling point of a solution made of a liquid

solvent with a nonvolatile solute is greater than the boiling point of the pure solvent. The boiling point of a liquid or is defined as the temperature at which the vapor pressure of that liquid equals the atmospheric pressure. For a solution, the vapor pressure of the solvent is lower at any given temperature. This pattern applies to the freezing point of the solution. Therefore, a higher temperature is required to boil the solution and a low temperature – to freeze the solution than the pure solvent.

for aqueous solutions.
This means that, for example, 1 mole

of sugar (non-electrolyte) in 1 kilogram of water will increase the boiling point from 100C to 100,52C. And Cm is the molal concentration of solute.
It is also important to note that Δtboil is a positive quantity and should be added to the boiling point of pure solvent (water), which is 100C.

The change in boiling point is proportional to the molality of the solution:

-1,86C/m for aqueous solutions.
Again, for example, 1 mole of

sugar (non-electrolyte) in 1 kilogram of water will decrease the freezing point from 0C to -1,86C.
And since we have already established that the freezing point of the solution is less than that of the pure solvent, then Δtfreez is a negative quantity and should be subtracted from the freezing point of pure solvent (water), which is 0C.

The change in freezing point can be found similarly:

a pig's bladder filled with a concentrated solution of alcohol

in water expanded when it was immersed in water. The bladder acted as a semipermeable membrane, which allowed water molecules to enter the solution, but kept alcohol molecules from moving in the other direction. Movement of one component of a solution through a membrane to dilute the solution is called osmosis, and the pressure this produces is called the osmotic pressure ().
Osmotic pressure can be demonstrated with the apparatus shown in the figure below. A semipermeable membrane is tied across the open end of a thistle tube. The tube is then partially filled with a solution of sugar or alcohol in water and immersed in a beaker of water. Water will flow into the tube until the pressure on the column of water due to the force of gravity balances the osmotic pressure driving water through the membrane.

solvent diffuses through a membrane but solutes do not.
Osmosis

is defined as the flow of water/solvent molecules through a semipermeable membrane from a region of low to high solute concentration, until equilib-
rium is established.

hydrostatic pressure equalizes the solvent flow in both directions.

Because the

liquid level for the solution is higher, there is greater hydrostatic pressure on the solution than on the pure solvent .

Osmotic Pressure:
The excess hydrostatic pressure on the solution compared to the pure solvent.

in the right arm of a U-tube, and the left

arm is filled to the same height with pure water; a semipermeable membrane separates the two arms. Because the flow of pure solvent through the membrane from left to right (from pure water to the solution) is greater than the flow of solvent in the reverse direction, the level of liquid in the right tube rises. (b) At equilibrium, the pressure differential, equal to the osmotic pressure of the solution (Πsoln), equalizes the flow rate of solvent in both directions. (c) Applying an external pressure equal to the osmotic pressure of the original glucose solution to the liquid in the right arm reverses the flow of solvent and restores the original situation.

to stop osmosis. The osmotic pressure of the solution is

given by:

where CM is the molarity of solution, R the gas constant (0.0821 L . atm / K . mol), and T the absolute temperature (in Kelvins).
The osmotic pressure is expressed in atm.

Van’t Hoff’s equation

Mendeleev Clapeyron equation

following?

QUIZ ME
A) freezing­point depression

B) vapor pressure

C) boiling­point evaluation
D) osmotic

pressure

to the solution
Hypertonic (higher solute concentration) to the solution
Isotonic solutions

are equal in their solute concentrations. We say that they are isotonic to each other.

is the Van’t Hoff factor, named after Jacobus Henricus Vant’

Hoff (1852-1911), who won the very first Nobel Prize in chemistry in 1901 for his work on colligative properties of solution.
The i factor gives the number of particles per formula unit of the solute.

<<

i

Na+

Cl-

by next ratio:
For Electrolyte Solution:

are dissolved in 1 kg of distilled water, the one

with the lowest boiling point will be:
NaI
B) AlCl3
C) Mg(NO3)2
D) CH3COOH
E) C6H12O6

QUIZ ME

are properties that depend only on the number of solute

particles in solution and not on the nature of the solute particles.
2. The four colligative properties of a solution are vapor pressure, osmotic pressure, boiling point and freezing point.
3. The change in vapor pressure where the solute is less volatile than the solvent is regulated by Raoult’s law, which states that the vapor pressure of a solvent over a solution is equal to the mole fraction of the solvent times the vapor pressure of pure solvent.
4. The osmotic pressure of a solution is the pressure required to stop osmosis.
5. The freezing point of the solution is always less than the freezing point of the pure solvent.

the formation of ion pairs. The Van’t Hoff factor (i)

provides a measure of the extent of dissociation of electrolytes in solution.
8. Solutions can be classified as hypertonic, hypotonic or isotonic depending on the concentration of solute inside and outside the cell.
9. A hypertonic solution has a higher concentration of particle than the cell causing the cell to shrink.
10. A hypotonic solution has a lower concentration of particles than the cell causing the cell to swell.
11. An isotonic solution has the same osmotic strength on both sides of the semipermeable membrane.
12. Osmosis is the spontaneous net movement of solvent molecules through a semi-permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides.