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Solution Formation. Solutions. . . . the components of a mixture are uniformly intermingled (the mixture is homogeneous ). Solution Components. Solute Solvent. Steps in Solution Formation. Step 1 - Expanding the solute (endothermic, ∆H 1 )

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solutions
Solutions

. . . the components of a mixture are uniformly intermingled (the mixture is homogeneous).

solution components
Solution Components

Solute

Solvent

steps in solution formation
Steps in Solution Formation

Step 1 - Expanding the solute (endothermic, ∆H1)

Step 2 - Expanding the solvent (endothermic, ∆H2)

solute solute solvent solvent forces
Solute-solute/solvent-solvent forces
  • Ion-ion
  • H-bonding
  • Dipole-dipole
  • London dispersion
steps in solution formation1
Steps in Solution Formation

Step 3 - Allowing the solute and solvent to interact to form a solution (exothermic, ∆H3)

solvent solute forces
Solvent-solute forces
  • Ion-dipole
  • Dipole-dipole
  • London dispersion

*(H-bonds = strong dipole)

water dissolving an ionic salt
Water dissolving an ionic salt
  • Water – polar solvent
  • Ionic salt – positive (cations) and negative (anions) ions
aqueous solutions of ionic compounds
Aqueous Solutions Of Ionic Compounds
  • The forces causing an ionic solid to dissolve in water are ion-dipole forces, the attraction of water dipoles for cations and anions.
  • The attractions of water dipoles for ions pulls the ions out of the crystalline lattice and into aqueous solution.
  • The extent to which an ionic solid dissolves in water is determined largely by the competition between:
    • inter-ionic attractions that hold ions in a crystal and
    • ion-dipole attractions that pull ions into solution.
dissolving like dissolves like
Dissolving –”like dissolves like”
  • Polar solvents will dissolve polar solutes
  • Nonpolar solvents will dissolve nonpolar solutes
enthalpy of solution
Enthalpy Of Solution

Solution formation can be considered to take place in three steps:

  • Move the molecules of solvent apart to make room for the solute molecules. DH1 > 0 (endothermic)
  • Separate the molecules of solute to the distances found between them in the solution. DH2 > 0 (endothermic)
  • Allow the separated solute and solvent molecules to mix randomly. DH3 < 0 (exothermic)

DHsoln = DH1 + DH2 + DH3

visualizing enthalpy of solution
Visualizing Enthalpy of Solution

For dissolving to occur, the magnitudes of DH1 + DH2 and of DH3 must be roughly comparable.

intermolecular forces in solution formation
Intermolecular Forces In Solution Formation
  • If all intermolecular forces are of comparable strength, this type of solution is called an ideal solution and DHsoln = 0.
  • If the intermolecular forces between solute and solvent molecules are stronger than other intermolecular forces, DHsoln < 0.
  • If the intermolecular forces between solute and solvent molecules are weaker than other intermolecular forces, DHsoln > 0
  • If the intermolecular forces between solute and solvent molecules are much weaker than other intermolecular forces, the solute does not dissolve in the solvent.
    • Not enough energy is released by solute-solvent interactions to separate solute particles or solvent particles.
slide18

“like dissolves like”

Two substances with similar intermolecular forces are likely to be soluble in each other.

  • non-polar molecules are soluble in non-polar solvents
      • CCl4 in C6H6
  • polar molecules are soluble in polar solvents
      • C2H5OH in H2O
  • ionic compounds are more soluble in polar solvents
      • NaCl in H2O or NH3 (l)

12.2

practice
Practice
  • Rank the following compounds according to increasing solubility in water.

I. CH3–CH2–CH2–CH3

II. CH3–CH2–O–CH2–CH3

III. CH3–CH2–OH

IV. CH3–OH