The Solution Process

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The Solution Process

• Chapter 13
• Brown-LeMay

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I. Solution Forces

• Solution Solvent Solute
• Attractions exist between
• A. solvent and solute
• B. solute and solute
• C. solvent an solvent
• If solute by solute attraction is greater that
  solute by solvent (water) then a precipitate
  forms.

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I. Solution Forces

• Lattice Energy describes the attractive forces
  between solute molecules or ions
• Solvation solvent solute attraction is strong
  enough so the solute dissolves.
• Hydration when water is the solvent.

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II. Enthalpy of Solvation

• H (sol,total) H1 H2 H3
• H1 Energy of dispersing solute
• particles () endothermic
• H2 Energy of dispersing solvents
• particles () endothermic
• H3 Energy of solvated solute
• molecules (-) exothermic

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II. Enthalpy of Solvation

• If the energy released from solvation is greater
  than that required for dispersion (delta 1
  delta 2) then the process is exothermic
• If delta H of solvation is negative or exothermic
  it is energetically favorable or spontaneous

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• Although some delta H solvations are endothermic
  they may proceed spontaneously
• 1) the increase in disorder is the driving
  force that can overcome slightly positive
  enthalpy changes
• 2) remember disorder is measured in degrees of
  entropy delta S
• a) high entropy disorder
• b) low entropy ordered
• c) systems tend to be disordered move from
• low to high

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III. Ways to Express Conc.

• A. Dilute vs Concentration
• 1) Dilute weak few solute particles vs
• solvent particles
• 2) Concentrated strong- greater solute vs
• solvent particles
• B. Mass - mass of solute X 100
• mass of solution
• Ex A 100g NaCl solution evaporated to dryness
  weighed 5g. What was the mass Percent of the
  solution. 5g/100g X 100 5

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III. Ways to Express Conc.

• C. Parts Per Million
• ppm mass g of solute x 106
• mass g of solution
• 1) a mass of 1 would equal 10,000 ppm
• 1g/100g x 106
• 2) 1 ppm would be equal to 0.0001
• 1 xg x 106
• 100g
• 1 xg ? 100 106x 0.0001
• 106 100g

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• A 100ml sample of water is evaporated to dryness
  and as 75ug Pb2. of How many parts per million
  of lead II does it have?
• (d water is 100g per milliliter, 1ug1x106g) 75ug
  (1g/1x106ug) 7.5 x 10-5g Pb2
• 100g of water x 1 x 106 7.5 x 10-1ppm
• 7.5 x 10-5g Pb2

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D. Mole Fraction

• X moles of solute
• moles of solution
• X Pb2 sum of moles fractions in a solution
  (including the solution itself) must equal one.
• E. Molarity Moles of solute
• (M) liters of solution
• F. Molality Moles of solute
• (m) kg of solvent

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• Ex. 1 a certain beverage contains 7 ethanol
  C2H5OH by mass. Calculate the mole fraction,
  molarity, and molality. (mm ethanol 46.1 g/mol)
• 7g ethanol 7g ethanol
• 100g(C2H5OH H2O) (93g H2O7g C2H5OH)
• X eth 0.152 mol eth
  0.0286
• (0.152 mol eth 5.17 mol H20)

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• Molarity moles of solute
• (M) liters of solution
• 7g ethanol
• 100g solution(1g/ml) assume ethanol has no effect
  on density
• 7g(1mole/46.1g) 0.152 moles 1.52 M
• 100ml 0.1 liter 0.1 liter
• Molality moles of solute 0.152
• Kg of solvent 93g(1kg/1000g)
• 1.63 m

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Saturated solutions and solubility

• A. Two opposing forces
• 1. dissolving hydration of individual ions
• 2. collision of ions ions unite and increase
  crystal mass (crystallization)
• Solute solvent dissolve ? solution
• crystallize ? solution

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IV saturated solutions and solubility

• If dissolutiongtcrystallization then the crystals
  in the solvent get smaller
• If crystallizationgtdissolution crystals in the
  solvent get larger
• If crystallization dissolution the system is in
  dynamic equilibrium (saturated) no more solute
  will be dissolved

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IV saturated solutions and solubility

• The concentration of solute present at saturated
  is known as the solubility of the solute
• At higher temperatures usually more solute can be
  dissolved and solubility is higher

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b. Supersaturated Solutions

• Solute is dissolved at high temperature to
  saturation
• Solution is carefully cooled to a lower
  temperature
• At the lower temperature the concentration of
  solute is higher than at the equilibrium
  concentration at that temperature
• The introduction of a seed crystal will
  stimulate rapid crystal formation

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V. Factors Affecting Solubility

• A. Gases in Water
• 1. solute-solvent interactions
• gases have weak I.F. primarily London
  dispersion L.D. forces
• L.D. forces increase with increased molecular
  weight solubility of a gas typically decreases
  with increasing mass of a gas molecule

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Gases in Water

• If a gas molecule appears to be more soluble than
  its mass would indicate a chemical reaction may
  have taken place
• B. Polar solutes in polar solvents
• 1. interactions between polar solutes are
  typically dipole-dipole (or hydrogen bonding)
• 2. interactions between molecules of polar
  solvents are the same as the solutes

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B. Polar solutes in polar solvents

• 3. thus the energy associated with disrupting
  solute-solute and solvent-solvent interactions
  are aprox. Equal
• 4. entropic forces drive the dissolution process
• 5. polar liquids tend to dissolve in polar
  solvents
• Note liquids that mix are miscible- dont mix
  are immiscible

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c. Alcohols

• Short chained alcohols are miscible in water -
  polar - D.D. and hydrogen bonding
• Long chained alcohols tend not to be miscible
• The hydrocarbon chain is held together by C-H
  bonds that are primarily L.D. forces. Single OH
  groups at the end are not enough to make the
  liquids miscible.

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c. Alcohols

• In general like dissolves like substances with
  the same or similar intermolecular forces tend to
  be soluble in one another
• D. Effect of pressure on gases and solubility
  increasing the pressure at constant temp. results
  in more collisions of gas molecules, per unit
  time with the surface of the solvent resulting in
  greater solubility

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E. Temperature effects

• Solvated gas molecules with enough K.E. can
  escape from the surface of a liquid
• K.E. increases with temperature
• Increased temperature reduces the solubility of
  gas molecules in a solvent

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F. Temp. effects on Solid solutes and solubility

• Insolubility inability of solvent to overcome
  the solute solute attraction
• Increase temp increases K.E. of solvent and
  solute molecules
• Both separate more readily and the effect of the
  solvent is increased
• Increasing temperature increases the solubility
  of solid solutes

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Colligative Properties Vapor Pressure lowering

• Vapor pressure - The pressure of the gas that
  collects above a liquid in a closed container

• Depend upon the collective number of particles in
  a solution
• The physical bases for the behavior is the effect
  of the solute upon the vapor pressure of the water

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Colligative Properties Vapor Pressure lowering

• Raoult's Law - The vapor pressure of the solvent
  above a solution is equal to the product of the
  mole fraction of the solvent and the vapor
  pressure of the pure solvent Pa vp solution
  (cont. solute) Pao pure solvent

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Colligative Properties Vapor Pressure lowering

• Ideal Solutions solutions that obey Raoults
  law generally for dilute solutions where the
  mole fraction is closer to one, also where I.F.
  are similar among solute and solvent
• Deviations hydrogen bonding or when solute
  solvent interactions are extremely strong

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Vapor pressure lowering expression

• Given Raoults law holds true
• P Pao Xb(mole fraction solute)