PowerPoint Presentation Chem 111 112101 Dr' Seymour

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Wed 11/21/01 Chem 111-02,3 Dr. Seymour
Solutions and a relaxing Thanksgiving

• Short week, regular class schedule through
  Wednesday
• Capa 31 due on tonight, 32 on Monday
• Return Quiz 10 - probably 11 on Monday

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Moving from solids to solutions

• Working with the s words
  
• Solutes dissolve in solvents to form solution.
  
• Solubility, saturation and supersaturation.
• And miscible (solutions of all proportion of
  solute solvent)
• Like dissolves like social, political, religious
  and chemical
• Solubility of gases? of Liquids of solids!! p486

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Mixed drinks The -OH end attracted to water
but the organic end disrupts waters H-bonding
network
For example gasoline water dont mix. The
organic breaks up a very stable situation Water
behaving like a network, a bit like a BIG
molecule! Solubility is a balance between the
stabilizing and destabilizing forces for any
particular solute and solvent combination.
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Predictions / guidelines for solubility

• No foolproof method, empirical approach
• Evaluate possible forces dispersion, dipolar and
  H-bonding
• Requires knowledge of electronegativity and
  overall molecular structure, look for symmetry as
  a key to non-polar molecules
• Like dissolves like
• Ionic solids follow earlier solubility guidelines
• Ion-dipole (of water) interactions must be
  stronger than the coulombic cation/anion force

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Which of these vitamins is water soluble?
Vitamin E
Which vitamin is most likely to be required on a
daily basis? Which vitamin could bioaccumulate
with potential for a toxic reaction?
Vitamin B6
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Like dissolves like More mixtures, pp 486-7

• Alloy - a mixture of substances with metallic
  properties. brass and bronze are solutions
  (homogeneous mixture) , solder is heterogenous
  mixture.
• Amalgam - any solution of another metal dissolved
  in Hg
• Metals are not soluble in any common solvent, but
  they can react chemically and the new ionic
  species can dissolve
• Ni(s) H2O(l) -gt Ni(aq) H2O(l) NO - does not
  happen
• Ni(s) 2 H3O --gt Ni2(aq) H2(g) 2 H2O(l)
  YES - reaction
• Note this is a net ionic equation - some
  spectator ion has been left out of the equation.
  What might this spectator ion be?

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Dual nature molecules

• Hydrophobic - water fearing, not H2O soluble,
  non-polar
• Hydrophilic - water loving, water soluble, polar
  or ionic
• Single molecule with polar head and non-polar
  tail
• Three types of structures in water

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Uses of dual nature molecules

• Soaps, detergents and surfactants
• Wetting agents to improve physical properties
  such as flow, mixing and foaming
• Lipid bilayer in cell membranes to control flow
  of molecules in and out of the cell

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Phase equilibria and solutions

• Normal freezing point - temp at which solid and
  pure liquid coexist at 1 atm. Rate of melting
  rate of freezing
• Normal boiling point - temp at which pure liquid
  and gas coexist at 1 atm. Rate of vaporization
  rate of condensation
• How will a switch from pure liquid to a solution
  affect the equilibrium for freezing and boiling?
• Since the concentration of solvent molecules in
  solution is less, the rate of freezing is less.
  To maintain equilibrium the rate of melting must
  decrease, which requires a lower temperature.
• In a similar manner for boiling, the lower
  concentration of solvent molecules in solution
  decreases the rate of vaporization. To maintain
  equilibrium at 1 atm the rate of vaporization
  must increase by increasing the temperature.

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Quantitative Aspects of Freezing Point Depression
and Boiling Point Elevation

• ?Tf Kf cm or ?Tb Kb cm
• FP constant and BP constant are solvent dependent
• For water Kf 1.858 C kg/mol, Kb 0.512 C
  kg/mol
• cm is solute concentration in units of molality
• moles of solute particles / kg solvent
• independent of the type of solute
• Ionic solutes contribute more particles than
  molecular solutes
• NaCl has 2 moles of particle ions, CaCl2 has
  three moles ions
• Of course, moles grams solute / MM solute

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Higher solvent concentration, greater rate of
movement through membrane
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• To maintain equilibrium (increase solvent flow
  from solution) additional pressure ? - the
  osmotic pressure - must be applied to the
  solution side of the membrane.
• Or - think of it as the solvent always wanting to
  have equal solute concentrations on both sides,
  it will move to dilute the more concentrated
  solution.

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Quantitative Aspects of Osmotic Pressure

• ? MRT
• M is solute concentration in units of molarity
• moles of solute particles / L of solution
• independent of the type of solute
• Ionic solutes contribute more particles than
  molecular solutes
• NaCl has 2 moles of particle ions, CaCl2 has
  three moles ions
• Of course, moles grams solute / MM solute
• R is the gas constant, 0.08206 L atm /mol K
• T is temperature in kelvin

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Importance of Osmotic Pressure

• Cell membranes
• If fluid around a cell is low in salt
  concentration, solvent will flow into the cell
  causing it to expand or rupture
• If fluid around a cell is high in salt
  concentration solvent will flow out of the cell
  causing it to collapse
• External fluid with same salt concentration has
  no net flow of solvent - isotonic
• Water purification by reverse osmosis