Chapter 10 Properties of Solutions

1
Chapter 10Properties of Solutions

• 17.1 Solution Composition
• 17.2 The Thermodynamics of Solution Formation
  (skip)
• 17.3 Factors Affecting Solubility
• 17.4 The Vapor Pressures of Solutions
• 17.5 Boiling-Point Elevation and Freezing-Point
  Depression
• 17.6 Osmotic Pressure
• 17.7 Colligative Properties of Electrolyte
  Solutions
• 17.8 Colloids

2
Colligative Properties of Solutions

• For Colligative properties, the difference
  between a pure solvent and dilute solution
  depends only on the number of solute particles
  present and not on their chemical identity.
• Examples
• Vapor Pressure Depression
• Boiling Point Elevation
• Melting Point Depression
• Osmotic Pressure

3

• Lowering of Vapor Pressure
• Vapor Pressure of a solvent above a dilute
  solution is always less than the vapor pressure
  above the pure solvent.
• Elevation of Boiling Point
• The boiling point of a solution of a non-volatile
  solute in a volatile solvent always exceeds the
  boiling point of a pure solvent

• Boiling
• liquid in equilibrium with its vapor at the
  external pressure.
• Boiling Point
• Vapor press external pressure
• Normal boiling point
• Vap press. 1 atm

4
Elevation of Boiling Point Vapor Pressure
Depression
Phase diagrams for pure water (red lines) and
for an aqueous solution containing a nonvolatile
solution (blue lines).
5
Solution Composition
The solute and solvent can be any combination of
solid (s), liquid (l), and gaseous (g) phases.
Dissolution
Two (or more) substances mix at the level of
individual atoms, molecules, or ions.
Solution
A homogeneous mixture (mixed at level of atoms
molecules or ions
Solvent
The major component
Solute
The minor component
6
Solution Composition
Mass Fraction, Mole Fraction, Molality and
Molarity
Mass percentage (weight percentage)
mass percentage of the component
Mole fraction
The amount of a given component (in moles)
divided by the total amount (in moles)
X1 n1/(n1 n2) for a two component system
X2 n2/(n1 n2) 1 X1 or X1X21
7
Molality
msolute moles solute per kilogram solvent
moles per kg or (mol kg-1)
Molarity (biochemists pay attention)
csolute moles solute per volume solution
moles per liter of solution (mol L-1)
8
Factors Affecting Solubility

• Molecular Interactions
• Review chapter 4
• Polar molecules, water soluble, hydrophilic
  (water loving)
• E.g., Vitamins B and C water-soluble
• Non-polar molecules, soluble in non-polar
  molecules, hydrophobic (water fearing)
• E.g., Vitamins A, D, K and E fat-soluble

9
Factors Affecting Solubility of Gases

• Structure Effects
• Pressure Effects

10
Henrys Law (for dilute solutions)

• The mole fraction of volatile solute is
  proportional to the vapor pressure of the solute.
• P kH X
• kH Henrys Law constant, X mole fraction.
• Increasing the partial pressure of a gas over a
  liquid increases the amount of gas disolved in
  the liquid.
• kH depends on temperature.

11
When the partial pressure of nitrogen over a
sample of water at 19.4C is 9.20 atm, the
concentration of nitrogen in the water is 5.76 x
10-3 mol L-1. Compute Henrys law constant for
nitrogen in water at this temperature.
12
When the partial pressure of nitrogen over a
sample of water at 19.4C is 9.20 atm, then the
concentration of nitrogen in the water is 5.76 x
10-3 mol L-1. Compute Henrys law constant for
nitrogen in water at this temperature.
13
When the partial pressure of nitrogen over a
sample of water at 19.4C is 9.20 atm, then the
concentration of nitrogen in the water is 5.76 x
10-3 mol L-1. Compute Henrys law constant for
nitrogen in water at this temperature.
14
Factors Affecting Solubility

• Structure Effects
• Pressure Effects
• Temperature Effects for Aqueous Solutions

The solubility of somesolids as a function of
temperature.
The aqueous solubilities of most solids increase
with increasing temperature, some decrease with
temp.
Endothermic heat is absorbed by the system
(think evaporation of water, or melting of
ice) Exothermic heat is evolved by the system
(think fire, or freezing of water).
15
Factors Affecting Solubility

• Structure Effects
• Pressure Effects
• Temperature Effects for Aqueous Solutions

The solubility of some gases in water as a
function of temperature at a constant pressure of
1 atm.
16
(No Transcript)
17
(No Transcript)
18
(No Transcript)
19
(No Transcript)
20
The Person Behind the Science
Francois-Marie Raoult (1830-1901)

• Highlights
• 1886 Raoult's law , the partial pressure of a
  solvent vapor in equilibrium with a solution is
  proportional to the ratio of the number of
  solvent molecules to non-volatile solute
  molecules.
• allows molecular weights to be determined, and
  provides the explanation for freezing point
  depression and boiling point elevation.
• Moments in a Life
• Raoult was a prominent member of the group which
  created physical chemistry, including Arrhenius,
  Nernst, van t'Hoff, Planck.

For ideal solutions
Psoln XsolventPsolvent
21
Raoults Law, non-volatile solute

• Consider a non-volatile solute (component 2)
  dissolved in a volatile solvent (component 1).
• X1 the mole fraction of solvent

Raoults Law
P1X1 P1 P1 the vapor pressure of pure
component 1
22
Raoults Law, volatile solute

• Volatile solute (component 1)
• Volatile solvent (component 2)

P1 X1 P1
P2 X2 P2
Ptot P1 P2
23
Vapor pressure for a solution of two volatile
liquids.
Positive deviation solute-solvent attractions lt
solvent-solvent attractions
For non-ideal Solutions
Negative deviation solute-solvent attractions gt
solvent-solvent attractions
24
(No Transcript)
25
Osmotic Pressure

• Fourth Colligative Property
• Important for transport of molecules across cell
  membranes, called semipermeable membranes
• Osmotic Pressure ?
• ? M RT
• ?V n RT

PV nRT
Molarity (M) moles/L or n/V
26
The normal flow of solvent into the solution
(osmosis) can be prevented by applying an
external pressure to the solution.
Osmotic Pressure

• Osmotic Pressure useful for
• Determining the Molar Mass of protein and other
  macromolecules
• small concentrations cause large osmotic
  pressures
• Can prevent transfer of all solute particles
• Dialysis at the wall of most plant and animal
  cells

27
Dialysis Representation of the functioning of an
artificial kidney

• A cellophane (polymeric) tube acts as the
  semi-permeable membrane
• Purifies blood by washing impurities (solutes)
  into the dialyzing solution.

28
A dilute aqueous solution of a non-dissociating
compound contains 1.19 g of the compound per
liter of solution and has an osmotic pressure of
0.0288 atm at a temperature of 37C. Compute the
molar mass of the compound.
29
A dilute aqueous solution of a non-dissociating
compound contains 1.19 g of the compound per
liter of solution and has an osmotic pressure of
0.0288 atm at a temperature of 37C. Compute the
molar mass of the compound
30
The Person Behind the Science
J.H. vant Hoff (1852-1901)

• Highlights
• Discovery of the laws of chemical dynamics and
  osmotic pressure in solutions
• Mathematical laws that closely resemble the laws
  describing the behavior of gases.
• his work led to Arrhenius's theory of
  electrolytic dissociation or ionization
• Studies in molecular structure laid the
  foundation of stereochemistry.
• Moments in a Life
• 1901 awarded first Noble Prize in Chemistry

vant Hoff Factor (i)

• ?T - i m K

31
Colligative Properties of Electrolyte Solutions
Elevation of Boiling Point ?Tb m Kb Where m
molality (Molality is moles of solute per
kilogram of solvent) The Effect of
Dissociation ?Tb i m Kb i the number of
particles released into the solution per formula
unit of solute e.g., NaCl dissociates into i
2 e.g., Na2SO4 dissociates into i 3
(2 Na 1 SO4-2) e.g., acetic acid (a weak acid
and weak electrolyte) does not dissociate i 1

• also
• Depression of Freezing Point
• ?Tf - m Kf
• ?Tf - i m Kf

32
(No Transcript)
33
(No Transcript)
34
Elevation of Boiling Point The Effect of
Dissociation ?Tb i m Kb
35
Colloids Colloidal Dispersions

• Colloids are large particles dispersed in
  solution
• 1nm to 1000 nm in size
• E.g., Globular proteins 500 nm
• Examples
• Opal (water in solid SiO2)
• Aerosols (liquids in Gas)
• Smoke (solids in Air)
• Milk (fat droplets solids in water)
• Mayonnaise (water droplets in oil)
• Paint (solid pigments in liquid)
• Biological fluids (proteins fats in water)
• Characteristics
• Large particle size colloids translucent,
  cloudy, milky)
• Small particle size colloids can be clear

36
Colloidal Dispersions

• Tyndall Effect
• Light Scattering

37
(No Transcript)