Colligative Properties of Solutions: A Comprehensive Guide, Slides of Chemistry

Download Colligative Properties of Solutions: A Comprehensive Guide and more Slides Chemistry in PDF only on Docsity!

Colligative Properties

of Solutions

Yanovska Anna Olexandrivna

PhD physical chemistry

Lecturer of General chemistry

department

In our life we can see decreasing of freezing point by adding salt on the road. Due to this road is free of snow and ice.

If compounds completely

dissociates on ions

they are called

ELECTROLYTES

HCl, MgCl

2

, and NaCl

are strong

electrolytes.

Degree of dissociation 

for such electrolytes is

1 or nearly 100%.

Aqueous Solutions

Electrolytes in the Body  Carry messages to and from the brain as electrical signals  Maintain cellular function with the correct concentrations electrolytes Make your own 50 - 70 g sugar One liter of warm water Pinch of salt 200ml of sugar free fruit squash Mix, cool and drink

Constant of dissociation for weak electrolytes

Kd

Colligative properties  Properties of ideal solutions don’t depend on nature of the solute they only depend on its concentration.  So properties that depend only on the number of solute particles and not on their identity named – colligative properties:  Vapor pressure lowering  Boiling point elevation  Freezing Point depression

Vapour pressure depression

Vapour pressure depression  The particles of solute are surrounded by and attracted to particles of solvent.  Now the solvent particles have less kinetic energy and tend less to escape into the space above the liquid.  So the vapor pressure is less.

Isotonic, hypotonic and hypertonic solutions The solution around the cells has the same concentrations like inside of them (first picture), too high concentration in which case the water flows out of the cell and they shrivel up (second picture), or too low concentration in which case water flows into the cells and they lyse or rupture (third picture.)

 Example 1.2 L of solution contains 20.5 mg of

sucrose (C

12

H

22

O

11

). Calculate the osmotic

pressure of the solution at 22ºС.

 Solution

Answer: Posmotic= 12 3 Pа.

M ( C H O ) 342 g / mol

12 22 11

    ( ) ( ) ( ) ( ) ( ) 12 22 11 12 22 11 12 22 11 M C H O V solution m C H O V solution n C H O CM mol L g mol L g

5. 0 10 / 342 / 1. 2
6. (^5105) 2                C RT 5.0 10 mol/L 8.314J/mol K 295 K 5 Posmotic M 0.123 kPa 123 Pa.

Freezing Point Depression and Boiling Point Elevation

Boiling Point Elevation

 ∆T

b

=k

b

Cm (for water k

b

o

C/m)

 Freezing Point Depression

 ∆T

f

=k

f

Cm (for water k

f

o

C/m)

 Note : C m is the molality of the particles , so if the

solute is ionic, multiply by the i - Vant Hoff

coefficient of particles it dissociates to ().

 i=1+  (  - 1)

Boiling Point Elevation and Freezing Point Depression ∆T = K•Cm•i

i = van’t Hoff factor = number of particles

produced per molecule/formula unit. For

covalent compounds, i = 1. For ionic

compounds, i = the number of ions present

(both + and - )

Compound Theoretical Value of i

glycol 1

NaCl 2

CaCl

2

Ca

3

(PO

4

2

How many particles do each of the following give upon solvation?

 NaCl

 CaCl

2

 Glucose

M(solute)g/mol m(H O)g m(solute)g 1000 ΔT K K kg/mol 2 b    b   Which is more effective for lowering the freezing point of water? NaCl, CaCl 2 or Glucose

M(solute)g/mol m(H O)g

m(solute)g 1000

ΔT K K kg/mol

2 f

 f  

Freezing Point Depression ΔTf  Kf  Сm  Kf  Сm

f

ΔT  i  Kf  Сm

f

ΔT