Docstoc

Solutions

Document Sample
Solutions Powered By Docstoc
					Solutions

     Homogeneous Mixtures
Key Definitions
   Solute
       Dissolved particles in a
        solution
       The lesser component
   Solvent
       The dissolving medium in a
        solution
       The greater component
   Example: Saline Solution
    for contact lenses
       Solute = sodium chloride
       Solvent = water
More definitions
   Miscible
       Liquids that will
        dissolve in each
        other
            Ex. Oil and gasoline
             for 2 stroke engines
   Immiscible
       2 liquids that are
        insoluble in each
        other
            Ex. Oil and water
    How much solute is dissolved?
   Saturated solution
       Contains the maximum
        amount of dissolved
        solute
            If you add more solute, it
             will fall to the bottom
   Unsaturated solution
       Contains less than the
        maximum amount of
        dissolved solute
            If you add more solute, it
             will dissolve
Relative Humidity
   A measure of how much water vapor is
    in the air, compared to maximum
    amount of water the air can hold at that
    temperature
       100% humidity: rain is possible!

       People feel most comfortable indoors when
        the relative humidity is between 40-60%.
Supersaturated Solutions
   Sometimes solutions can be prepared
    that have MORE dissolved solute
    than a saturated solution
       Unstable!
       Often used in candy making processes


   Video clip:
Solubility Diagrams
                         Solubility
                          information is
                          often
                          presented
                          graphically
Interpreting Solubility
Diagrams
   Most substance become more soluble
    as temperature increases.
       Gases typically become LESS soluble as
        temperature increases.
   Note the units on the y axis: often
    given as grams of solute/100 g solvent.
To speed up the solution process:
   Increase the surface area of the solute
       Which would dissolve faster: a sugar cube
        or powdered sugar?
   Stir the solution
   Increase the temperature of the
    solution
Rule of thumb for solubility
   “Like dissolves like”
       Solutes tend to dissolve in solvents with
        similar properties.
       Nonpolar substances typically dissolve in
        nonpolar solvents.
       Ionic solids tend to dissolve in polar
        solvents.
Solution Concentrations
   Ratio of solute to solvent
   Concentrated: high ratio of solute to solvent
   Dilute: low solute/solvent ratio
   Several different ways to calculate
       Most common:
            Molarity symbol: M
            Percent solutions: m/m, m/v, v/v
            Parts per million
            Molality
Molarity
   Definition:



   Since mol = g/gfm,
Percent Solutions
   Easy and reliable to calculate
       Mass/mass, mass/volume, volume/volume




   Frequently used in biology & medicine
Molality
   Used to calculate changes in physical
    properties of solutions




   Note subtle differences from molarity
    formula!
   “molal” solutions
Vapor Pressure
   Particles at the surface of a liquid may
    evaporate or vaporize, if they have
    enough kinetic energy.
   If this happens in a sealed container,
    the particles of escaped liquid (now in
    the gas phase) will collide with the
    container walls and exert a “vapor
    pressure” above the liquid
     Vapor Pressure Diagrams

   Vapor pressure
    data is often
    presented
    graphically
Boiling Point
   Boiling
       when bubbles of gas form anywhere in the liquid
        and then rise to the surface
   Boiling point
       the temperature at which the vapor pressure of
        the liquid is just equal to the external pressure
   Normal boiling point
       the boiling point of a liquid at a pressure of 101.3
        kPa (1 atm)
Colligative Properties
   Addition of a solute changes the
    properties of the system:
       The solution will have different properties
        than the pure solvent
       Adding solute changes the freezing point,
        boiling point, vapor pressure
       These changes depend only on the number
        of particles added (NOT the identity of the
        particles)
Phase Diagrams
Phase Diagrams
   Closed system
   Triple Point
       Where solid, liquid, and gas can coexist
   Critical Point
       Above this point, gas and liquid are
        indistinguishable
       “supercritical fluids”
Colligative Properties
   Freezing Point Depression
       The solution will freeze at a lower
        temperature than the pure solvent
            Why we put salt on sidewalks during winter
            Antifreeze in cars
   Boiling Point Elevation
       The solution will boil at a higher
        temperature than the pure solvent
   Cool simulation
van’t Hoff Factor (i)
   Some substances dissociate into ions,
    resulting in more solute particles
    dissolving
   van’t Hoff Factor = the theoretical
    (maximum) number of particles formed
    when a substance dissociates
       The real degree of ionization is typically
        slightly less than predicted by the van’t
        Hoff factor
Predicting van’t Hoff factors
   For all covalently bonded substances,
       i=1
   For ionic substances
       I = number of ions present in formula
            Ex.
                      NaCl       i=2
                      Al(NO3)3   i=4
                      MgCl2      i=3
Calculating Colligative
Properties
   Tbp = kbpim where kbp is a constant for
    each solvent
       Calculate the new boiling point by adding Tbp to
        the boiling point of the pure solvent
   Tfp = kfpim where kfp is a constant for
    each solvent
       Calculate the new boiling point by subtracting Tbp
        from the freezing point of the pure solvent

				
DOCUMENT INFO