Download 08.Colligative properties Solution

Lecture 8. Colligative
properties of solutions
Prepared by PhD Halina Falfushynska
A space-filling model of the water
molecule.
GENERAL PROPERTIES OF SOLUTIONS
1. A solution is a homogeneous mixture of two or
more components.
2. It has variable composition.
3. The dissolved solute is molecular or ionic in
size.
4. A solution may be either colored or colorless
nut is generally transparent.
5. The solute remains uniformly distributed
throughout the solution and will not settle out
through time.
6. The solute can be separated from the solvent
by physical methods.
Polar water molecules interact with the
positive and negative ions of a salt,
assisting with the dissolving process.
Electrical Conductivity of Ionic Solutions
Electrical
Conductivity
Comparison of a Concentrated and
Dilute Solution
Comparison of an Unsaturated and
Saturated Solution
Molarity (Concentration of Solutions)= M
M=
Moles of Solute
=
Liters of Solution
Moles
L
solute = material dissolved into the solvent
In air (gas), Nitrogen is the solvent and oxygen,
carbon dioxide, etc. are the solutes.
In sea water (liquid), Water is the solvent, and salt,
magnesium chloride, etc. are the solutes.
In brass , Copper is the solvent (90%), and Zinc is the solute(10%)
MOLALITY
• Molality = moles of solute per kg of solvent
• m = nsolute / kg solvent
• If the concentration of a solution is given in terms of
molality, it is referred to as a molal solution.
Q. Calculate the molality of a solution consisting of
25 g of KCl in 250.0 mL of pure water at 20oC?
First calculate the mass in kilograms of solvent using the density of solvent:
250.0 mL of H2O (1 g/ 1 mL) = 250.0 g of H2O (1 kg / 1000 g) = 0.2500 kg of H2O
Next calculate the moles of solute using the molar mass:
25 g KCl (1 mol / 54.5 g) = 0.46 moles of solute
Lastly calculate the molality:
m = n / kg = 0.46 mol / 0.2500 kg = 1.8 m (molal) solution
HEAT EFFECT ON THE GAS DILUTION IN
WATER
PRESSURE AFFECTS GAS SOLUBILITY
HENRY’S LAW
m = kP
m – mass of soluble gas;
k – Henry’s constant;
P – partial gas pressure.
СО2 pressure in bottle is 4 atm.
Decreases of pressure of saturated vapor
under solution
Colligative: particles are particles
• Colligative comes from colligate – to tie together
• Colligative properties depend on
amount of solute but do not depend on
its chemical identity
• Solute particles exert their effect merely by being
rather than doing
• The effect is the same for all solutes
COLLIGATIVE PROPERTIES FOR
NONVOLATILE SOLUTES:
•
•
•
•
Vapour pressure is always lower
Boiling point is always higher
Freezing point is always lower
Osmotic pressure drives solvent from
lower concentration to higher
concentration
NON-VOLATILE SOLUTES AND
RAOULT’S LAW
• Vapor pressure of solvent in solution
containing non-volatile solute is always
lower than vapor pressure of pure
solvent at same T
–At equilibrium rate of vaporization =
rate of condensation
–Solute particles occupy volume
reducing rate of evaporation the number
of solvent molecules at the surface
–The rate of evaporation decreases and
so the vapor pressure above the solution
must decrease to recover the
equilibrium
Molecular view of Raoult’s law:
Boiling point elevation
• In solution vapor pressure is reduced
compared to pure solvent
• Liquid boils when vapor pressure =
atmospheric pressure
• Must increase T to make vapor pressure
= atmospheric
Colligative Properties – BP Elevation
• The addition of a
nonvolatile solute
causes solutions to
have higher boiling
points than the pure
solvent.
 Higher temperature
– Vapor pressure
decreases with addition
of non-volatile solute.
is
needed in order for vapor
pressure to equal 1 atm.
MOLECULAR VIEW OF RAOULT’S LAW:
FREEZING POINT DEPRESSION
• Depends on the solute
only being in the liquid
phase
– Fewer water
molecules at surface:
rate of freezing drops
– Ice turns into liquid
– Lower temperature to regain balance
– Depression of freezing point
Colligative Properties - Freezing Pt Depression
• The addition of a
nonvolatile solute
causes solutions to have
lower freezing points
than the pure solvent.
• Solid-liquid equilibrium
line rises ~ vertically
from the triple point,
which is lower than that
of pure solvent.
 Freezing
point of the
solution is lower than
that of the pure
solvent.
RAOULT’S LAW
• Vapor pressure above solution is vapor
pressure of solvent times mole fraction
of solvent in solution
Psoln  Psolv X solv
• Vapour pressure lowering follows:
Psoln  Psolv X solute
MAGNITUDE OF ELEVATION
• Depends on the number of particles
present
• Concentration is measured in molality
(independent of T)
b
b
• Kb is the molal boiling point elevation
constant
T  K m
Boiling point elevation
(ebullioscopy)
• The boiling point of a pure solvent is increased
by the addition of a non-volatile solute, and the
elevation can be measured by ebullioscopy.
• Here i is the van't Hoff factor as above, Kb is
the ebullioscopic constant of the solvent (equal to
0.512°C kg/mol for water), and m is themolality of
the solution
MAGNITUDE OF DEPRESSION
• Analagous to boiling point, the freezing
point depression is proportional to the molal
concentration of solute particles
T f  K f m
• For solutes which are not completely
dissociated, the van’t Hoff factor is applied
to modify m:
T f  K f m  i
Freezing point depression
(cryoscopy)
• The freezing point of a pure solvent is lowered
by the addition of a solute which is insoluble in
the solid solvent, and the measurement of this
difference is called cryoscopy.
• Here Kf is the cryoscopic constant, equal to
1.86°C kg/mol for the freezing point of water.
Again i is the van't Hoff factor and m the
molality.
OSMOSIS: MOLECULAR
DISCRIMINATION
• A semi-permeable membrane discriminates
on the basis of molecular type
– Solvent molecules pass through
– Large molecules or ions are blocked
• Solvent molecules will pass from a place of
lower solute concentration to higher
concentration to achieve equilibrium
OSMOTIC PRESSURE
• Solvent passes into more conc solution
increasing its volume
• The passage of the solvent can be
prevented by application of a pressure
• The pressure to prevent transport is the
osmotic pressure
CALCULATING OSMOTIC
PRESSURE
• The ideal gas law states
• But n/V = M and so
PV  nRT
  MRT
• Where M is the molar concentration of
particles and Π is the osmotic pressure
Determining molar mass
• A solution contains 20.0 mg insulin in 5.00 ml
develops an osmotic pressure of 12.5 mm Hg
at 300 K

M
12.5mmHg 1
RT
760mmHg
4
M
 6.68 10 M
L  atm
0.0821
300 K
mol  K
VOLATILE SOLUTE: TWO
LIQUIDS
• Total pressure is the sum of the
pressures of the two components
Ptotal  PA  PB
Ptotal  P X A  P X B

A

B
p = gh
OSMOMETER
Colligative Properties - Osmosis
• Osmosis plays an important role
in living systems:
– Membranes of red blood cells are
semipermeable.
• Placing a red blood cell in a
hypertonic solution (solute
concentration outside the cell is
greater than inside the cell)
causes water to flow out of the
cell in a process called
CRENATION.
Colligative Properties
• Placing a red blood cell in a hypotonic
solution (solute concentration outside the
cell is less than that inside the cell) causes
water to flow into the cell.
– The cell ruptures in a process called
HEMOLYSIS.