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Solutions and Solubility

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					Solutions and Solubility
     Solutions and Solubility
•   Water is a polar compound and for a
    substance to dissolve in water its molecules
    or ions must compete with the attraction of
    the polar molecules of water and the water
    must “win”.
     Solutions and Solubility
•   In each of the three states of matter there are
    spaces between the molecules or atoms.
•   When the molecules or atoms of one
    substance can fit themselves between the
    molecules or atoms of another substance,
    that substance becomes dispersed
    throughout the other substance.
•   The resultant system is called a solution of
    the two substances.
     Solutions and Solubility
•   The definition of a solution is: a
    homogeneous molecular dispersion of two or
    more substances.
•   To be a solution, the individual particles must
    be of molecular or atomic size and there can
    be no chemical reaction.
•   For example, a mixture of sawdust and iron
    filings is not a solution.
  Solutions of Gases in Gases
• A mixture of two
  different gases is
  actually a solution of
  a gas within a gas.
    Solutions of Gases in Gases
•   Gas solutions differ from other solutions in
    that they can mix homogeneously no matter
    what the proportion of each gas may be.
•   a mixture becomes homogeneous in a very
    short time for three reasons
    –   high molecular kinetic energy
    –   weak bonds between molecules
    –   large distances between molecules
    Solutions of Gases in Gases
•   Hydrogen gas mixed with oxygen gas
    –   Both gases have strong bonds within their
        molecules but weak bonds between their
        molecules
    –   The oxygen does not settle to the bottom of the
        lighter hydrogen and the homogeneous dispersion
        remains indefinitely.
    Solutions of Gases in Gases
•   Hydrogen gas mixed with oxygen gas
    –   Another property of solutions exhibited here is that
        the various components can be recovered
        unchanged and the hydrogen is easy to separate
        due to its lighter molecular weight.
             Solubility Limits
•   Gases are said to be completely miscible
    because they will form solutions with each
    other regardless of the proportions in which
    they are present.
            Solubility Limits
•   Two or more substances that do not mix
    at all to form solutions are said to be
    immiscible.
             Solubility Limits
•   Partially miscible are substances that will mix
    with each other but only in certain
    proportions to form solutions.
             Solubility Limits
•   The solubility of a substance refers to the
    amount of a substance that will dissolve in a
    given amount of a second substance.
    Solutions of Gases in Liquids
                  (water)
•   The amount of a gas that will dissolve in a
    given amount of water depends upon three
    things
        Solutions of Gases in Liquids
                      (water)
•   the nature of the gas
    –    hydrogen, oxygen, and nitrogen are slightly
         soluble in water because the polar water
         molecules have little attraction to the non polar
         molecules of these gases
        Solutions of Gases in Liquids
                      (water)
•   the temperature of the solution
    –    gases become less soluble as the temperature
         of the water increases
     Solutions of Gases in Liquids
                   (water)
•   the pressure of the gas above the solution
    – the greater the concentration of gas molecules
      above the surface of the water, the greater the
      solubility of the gas
      Saturations of Solutions
• When a solution is formed by dissolving a
  solid in a liquid, the solid is referred to as the
  solute and the liquid is the solvent.
      Saturations of Solutions
• Solid covalent iodine (I2) will slightly dissolve
  in water at about 0.03 g in 100 ml of water at
  a given temperature (20 g I2 per 100 ml of
  alcohol).
• It is solid due to van der Waal forces, which
  are overcome by the polar water molecules
  and their kinetic energy propels them into the
  water (diffusion).
      Saturations of Solutions
•   Adding more solid iodine will result in
    crystals setting at the bottom of the
    container.
•   The solute is then in equilibrium with the
    solvent and the molecules of iodine are
    moving in and out of the solution at equal
    rates.
       Saturations of Solutions
•   Ionic solids dissolve in water through the
    breaking of the ionic bonds.
•   If one of the ions is a polyatomic ion, it does
    not get broken into smaller parts.
    – (NH4)2SO4 will dissolve as NH4+ SO4=
        Saturations of Solutions
•   saturated solution
    –   equilibrium exists between the solute and the
        solvent and increasing the amount of the solute
        has no effect on the saturation
        Saturations of Solutions
•   unsaturated solution
    –   equilibrium concentration has not yet been
        reached because there is insufficient solute to
        attain that level
        Saturations of Solutions
•   supersaturated solution
    –   equilibrium is exceeded since the solvent is
        holding excess solute and a supersaturated
        solution cannot exist if any excess solute is
        present in a solid form
    –   formed by slowly evaporating the solvent from a
        solution or lowering the temperature of an
        unsaturated solution
    –   common with gases in liquids such as
        carbonated beverages
      Saturations of Solutions
•   The saturated solution is regarded as the
    standard of reference since a
    supersaturated and an unsaturated solution
    both try to reach this level.
•   If a crystal of solute is added to a
    supersaturated solution, it will add to the
    crystal as it tries to be just saturated.
•   The unsaturated solution will cause
    molecules or ions of the solute to pass into
    the solution as it approached equilibrium.
        Factors That Affect the Rate of
                     Solution
•   the nature of the solute and solvent
    –   the rate varies greatly for different solutes in
        different solvents and no general statement can
        be made
        Factors That Affect the Rate of
                     Solution
•   the nature of the solute and solvent
    –   some ionic solids dissolve in water and some do
        not, it depends upon the strength of the ionic
        bonds
        Factors That Affect the Rate of
                     Solution
•   the nature of the solute and solvent
    –   ionic solids will not dissolve in non-polar liquids
        since the ionic bonds cannot be separated by
        the solvent where the molecules are held
        together only by weak van der Waals forces
        (carbon tetrachloride, benzene, pentane)
        Factors That Affect the Rate of
                     Solution
•   the nature of the solute and solvent
    –   covalent solids are soluble in non-polar liquids
        since both are held together by weak van der
        Waals forces, but the higher the molecular
        weight of the solid, the less soluble it is in the
        same non-polar solvent (waxes and greases in
        carbon tetrachloride and benzene).
        Factors That Affect the Rate of
                     Solution
•   the nature of the solute and solvent
    –   covalent solids are insoluble in polar solvents
        Factors That Affect the Rate of
                     Solution
•   the particle size of the solid
    –   greater areas of contact between the solute and
        solvent will accelerate the rate of solution
        Factors That Affect the Rate of
                     Solution
•   temperature of the solution
    –   increasing the temperature will accelerate the
        rate of solution since both the solute and solvent
        have increased their thermal agitation
        Factors That Affect the Rate of
                     Solution
•   degree to which the mixture is agitated
    –   crystals at the bottom of the container are
        brought into better contact with the solvent,
        undissolved crystals are brought into contact
        with fresh solvent, diffusion is speeded up
      Solubility and Temperature
•   Solubility-temperature curves are the best way
    to show the relationship between temperature
    and solubility.
     Solubility and Temperature
• Solubility-
  temperature curves
  – set up a graph
    having solubility
    increasing from
    bottom to top and
    temperature
    increasing from left
    to right
        Solubility and Temperature
•   Solubility-temperature curves
    –   solubility is expressed in grams of solid solute
        per 100 g. of solvent and temperature in
        degrees centigrade
        Solubility and Temperature
•   Solubility-
    temperature curves
    –   At 30°C
        approximately 12g of
        KClO3 will dissolve in
        100g of water. If the
        temperature is
        increased to 80°C,
        approximately 42g of
        the substance will
        dissolve in 100g (or
        100mL) of water.
What mass of solute will dissolve in 100mL of water at the following
temperatures? Also determine which of the three substances is most
soluble in water at 15°C.


• 1. KNO3at 70°C
    – 130g / 100mL
• 2. NaCl at 100°C
    – 40g / 100mL
• 3. NH4Cl at 90°C
    – 70g / 100mL
• 4. NaCl is the most
  soluble at 15°C
        Solubility and Temperature
•   Solubility-temperature curves
    –   any point on the curve represents a saturated
        solution while below the curve is unsaturated
        and above is supersaturated
        Solubility and Temperature
•   Solubility-temperature curves
    –   the solubility curve indicates the temperature
        required to bring about crystallization of the
        solute
        Solubility and Temperature
•   Solubility-
    temperature
    curves
    –    the solubility curve
         can indicate
         temperatures to
         use to separate
         contaminates from
         a salt in a
         purification process
          Solutions and Their
            Concentrations
• Methods of expressing concentrations
  – Weight-weight (number of grams of solute per
    100 grams of solution)
  – Weight-volume (number of grams of solute
    per 100 ml of solvent)
  – Volume-volume (volume of each liquid that is
    dissolved in the total volume of the liquid)
     Solutions and Their
       Concentrations
– Molarity (mole-volume)
  • Number of gram molecular weights (gram moles)
    of solute in 1 l. of solution
  • Works well because all chemical reactions occur
    according to a fixed ratio of the gram molecular
    weights of the reactants (stoichiometry) and
    because volumes are easily measured
  • A 1 molar solution is 1 gram molecular weight of a
    solute dissolved in 1 l. of solution. NaOH has 40 g.
    dissolved in 1 l. of solution in a 1 molar solutuion.
        Solutions and Their
          Concentrations
• Preparation of solutions
  – Mixing a solute into a solvent will usually
    increase the volume of the resulting solution
    above the starting volume of the solvent.
    Sometimes it has no effect while other times it
    may actually reduce the volume.
  – A solute is dissolved in a small amount of
    solvent and then more solvent is added until a
    volume of 1 l. is reached.
        Solutions and Their
          Concentrations
• Calculating Molarities
  – Convert any values to moles and liters
  – Use ratio and proportion (moles to liters
    equals moles to liters)
        Solutions and Their
          Concentrations
• Calculating molality
  – Molality of a solution equals the number of
    moles per kg. of a solvent

				
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posted:8/5/2011
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