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Solution Formation

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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Solution Formation

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  1. Solution Formation

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

  3. Solution Components Solute Solvent

  4. Steps in Solution Formation Step 1 - Expanding the solute (endothermic, ∆H1) Step 2 - Expanding the solvent (endothermic, ∆H2)

  5. Solute-solute/solvent-solvent forces • Ion-ion • H-bonding • Dipole-dipole • London dispersion

  6. Steps in Solution Formation Step 3 - Allowing the solute and solvent to interact to form a solution (exothermic, ∆H3)

  7. Solvent-solute forces • Ion-dipole • Dipole-dipole • London dispersion *(H-bonds = strong dipole)

  8. Steps in Solution Formation

  9. Water dissolving an ionic salt • Water – polar solvent • Ionic salt – positive (cations) and negative (anions) ions

  10. 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.

  11. Oxygens attracted to cations

  12. Hydrogens attracted to anions

  13. Dissolving –”like dissolves like” • Polar solvents will dissolve polar solutes • Nonpolar solvents will dissolve nonpolar solutes

  14. 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

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

  16. 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.

  17. “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

  18. 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

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