CHAPTER 12  PHYSICAL PROPERTIES OF SOLUTIONS

12.1 Types of Solutions

Saturated Solutions and Solubility

The opposite of the solution process (dissolution) is
        called crystallization.

There is an dynamic equilibrium between the solution

    and solute + solvent; example: a liquid at its boiling
    point
  is where the gas and liquid phases are in
    dynamic
equilibrium).

Unsaturated solution is one where the solvent can
easily accept more solute, i.e., sugar in water.

A saturated solution is one where no more
    solute can be dissolved in the solvent.

Definition:  amount of solute required to form a
    saturated solution for a given amount of solvent is
    called the solubility of that solute in the solvent in
    that experiment at that temperature.

Supersaturated solutions are also possible, in which
    more solute is
 dissolved than normally would be
    at
equilibrium (think about supercooling).
 

12.2 A Molecular View of the Solution Process

Factors Affecting Solubility:

Solute-Solvent Interactions:
Polar liquids tend to dissolve in polar solvents;
    non-polar liquids dissolve in non-polar solvents.
               
LIKE DISSOLVES LIKE!!!

   Miscible liquids: mix in any proportions.
            Ex.:  water and alcohol;
                    water and ethylene glycol (antifreeze).

   Immiscible liquids: do not mix at all.
             Ex.:  water and olive oil;

                     water and gasoline.

Intermolecular forces are important: water and
    ethanol are miscible because the broken hydrogen 
    bonds in both pure liquids are re-established
    (albeit in a different way) in the mixture (solution).
 
 The number of carbon atoms in a chain can affect
    solubility:
    the more C atoms in a compound, the less soluble
          that compound is in water.

The number of -OH groups within a molecule increases
    its solubility in water;  sugars have many -OH groups,
    and are very soluble in water.


         (REMEMBER::::
LIKE DISSOLVES LIKE!)

  The more polar bonds in the molecule, the better it
    dissolves in a polar solvent.
 The less polar the molecule, the less it dissolves in a
    polar solvent and the better it dissolves in a
    non-polar solvent (ex.:  benzene dissolves in CCl4,
    but not in H
2O.

 Network solids (e.g., SiO2) do not dissolve because
    the strong intermolecular forces in the solid are
    not re-established in any kind of solution.
 

12.3 Concentration Units


1)  PERCENT BY MASS:  % = pph; ppt; ppm; parts per trillion

2)  Molarity M:  CALCULATE M OF 23.5 g NaOH DISSOLVED
            IN ENOUGH WATER TO MAKE 450 mL OF SOLUTION.

3)  molality m:  CALCULATE m OF 23.5 g NaOH DISSOLVED
            IN 450 mL OF WATER.
              NOTICE THE DIFFERENCE BETWEEN THESE LAST TWO!!!


Examples:

1 -  CALCULATE M OF 25.5 g BENZENE (MW = 78.0 g/mol)
        DISSOLVED IN ENOUGH CARBON TETRACHLORIDE
        TO MAKE 550. mL OF SOLUTION.
        M = 25.5g (1 mol/78.0g) / 0.550 L = 0.594 M

2 -  CALCULATE m OF 25.5 g BENZENE (MW = 78.0 g/mol)
        DISSOLVED IN 550. mL OF CARBON TETRACHLORIDE
        (DENSITY OF CCl4   =  1.60 g/mL).
 m = 25.5g (1 mol/78.0g)/550. mL(1.60 g/mL)(1kg/1000g)
        m  = 0.32602mol/ 0.880 kg = 0.370 m
 

12.4  Effect of Temperature on Solubility

For solids in liquids.....we note that sugar dissolves
    more easily in warm water than in cold; the sugar is
    more soluble in hot  tea than in ice tea.
 Therefore, as temperature increases, the solubility of
    solids
  in solvents generally increases.

 For gases in liquids....experience tells us that
     carbonated beverages go flat as they get warm.
 Gases are less soluble at higher temperatures.

  Thermal pollution: if lakes get too warm, CO2 and O2
    become less soluble and are not available for
    plants or animals.


12.5 Effect of Pressure on the Solubility
        of Gases


Solubility of a gas in a liquid is a function of the

    pressure of the gas.
      The higher the pressure of a gas ABOVE the solution,
    the more molecules of gas
are close to the surface
    of the solvent and the
greater the chance of a gas
    molecule
striking the surface and entering the
    solution.

Therefore, the higher the pressure of the gas, the
    greater the
solubility of the gas in the liquid.

The lower the pressure of the gas ABOVE the solution,
    fewer molecules of gas
are close to the solvent,
    and the lower the solubility.

            Henry's law:             kPgas = Cgas

where Pgas is the partial pressure of gas above the
    solution, in atm.
    Cgas is the molar concentration of dissolved gas and
    k = Henry's law constant for a particular gas and a
          particular solvent at some T.

Calculate the solubility of CO2 in water at atmospheric
    conditions if the solubility at 25oC and 1 atm is
    0.034 mol/L.  The partial pressure of CO2 in air is
    0.00030 atm.             kPgas = Cgas

 Find k:   k = C/P = 0.034 mol/L /1atm = 0.034 mol/L-atm

  For atmosphere: C = kP = 0.034mol/L-atm(0.00030atm)
                               C = 1.0 x 10-5 mol/L
 

Carbonated beverages are bottled under Pgas > 1 atm.
    As the bottle is opened, Pgas decreases and the
    solubility of CO2 decreases.  Therefore, bubbles of
    CO2 escape from solution.

IMPORTANT EXCEPTIONS:  IF THE DISSOLVED GAS
REACTS
  WITH WATER, HIGHER SOLUBILITIES RESULT.
EX:  AMMONIA(g) + WATER GIVES THE AMMONIUM ION
AND HYDROXIDE ION.  CARBON DIOXIDE REACTS
WITH WATER TO MAKE CARBONIC ACID.
 
 

12.6 Colligative Properties of Nonelectrolyte
        Solutions
          (or collective properties)

These properties depend on the quantity (numbers)
    of individual solute particles (molecules OR ions):

            Vapor Pressure Lowering
            Boiling-Point Elevation
            Freezing-Point Depression
            Osmotic Pressure


Vapor Pressure Lowering:

  1)  SOLUTE = non-volatile:  solutes reduce the ability
    of the surface solvent molecules to escape the liquid.
    Therefore, vapor pressure of solvent is lowered and
    the amount of vapor pressure lowering depends on
    the amount of solute.  EX:  sugar dissolved in water.

            Raoult's Law:       P1 = X1 P1o

    P1 is the vapor pressure of the solution, P1o is the
    vapor pressure of the pure solvent,  and X1 is the
    mole fraction of the solvent in the solution.

           But:   P1 = (1 - X2)P1= P1o - P1oX2
                         P1 - P1o = -P1oX2
                        P1o - P1 = DP = X2P1o

 NOTE: the decrease in vapor pressure, DP, is directly
  proportional to the concentration (mole fraction =X2)
 of
the solute present.                   LOWERING!!!

 
  2) BOTH COMPONENTS ARE VOLATILE:  THE VAPOR

    PRESSURE OF THE SOLUTION IS THE SUM OF THE
    INDIVIDUAL PARTIAL PRESSURES OF THE
    COMPONENTS.

         PA=XA PAo          PA AND PB ARE PARTIAL PRESSURES
         PB=XB PBo        PAo AND PBo ARE VAPOR PRESSURES 
                                    OF THE PURE SUBSTANCES.

      AND  PTOTAL= P+ PB       IS THE VAPOR PRESSURE OF
                THE SOLUTION

      REMEMBER:  DALTON'S LAW OF PARTIAL PRESSURE

                                PTOTAL= P+ P+ P3 +.....


RAL 123109

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