Lecture17222

955 views

Published on

a supplemental resource for students

Published in: Education, Technology
0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total views
955
On SlideShare
0
From Embeds
0
Number of Embeds
31
Actions
Shares
0
Downloads
24
Comments
0
Likes
0
Embeds 0
No embeds

No notes for slide

Lecture17222

  1. 1. The Properties of Mixtures: the Solution Process Lecture 17
  2. 2. Similia similibus solvuntur
  3. 3. Macroscopic rule “like dissolves like” is based on microscopic interactions. How do enthalpy and entropy change in solute-solvent interaction?
  4. 4. Three events in the process of solution: <ul><li>Solute particles separate from each other (some energy must be absorbed); </li></ul><ul><li>Some solvent particles separate to make room for the solute particles; </li></ul><ul><li>Solute and solvent particles mix together (some energy must be released). </li></ul><ul><li>There must be change in enthalpy! </li></ul>
  5. 5. Solution: separating particles
  6. 6. Solute particles separate from each other: <ul><li>Solute (aggregated) + heat  solute (separated) </li></ul><ul><li>To overcome intermolecular attractions, energy is needed. </li></ul><ul><li>So the process is endothermic. </li></ul><ul><li>∆ H solute > 0 </li></ul>
  7. 7. Solvent particles separate from each other: <ul><li>Solvent (aggregated) + heat  solvent (separated) </li></ul><ul><li>To overcome intermolecular attractions, energy is needed. </li></ul><ul><li>So the process is endothermic. </li></ul><ul><li>∆ H solvent > 0 </li></ul>
  8. 8. Solute and solvent particles mix: <ul><li>Solute (separated) + solvent (separated)  solution + heat </li></ul><ul><li>The particles attract each other, energy is released. </li></ul><ul><li>So the process is exothermic. </li></ul><ul><li>∆ H mix < 0 </li></ul>
  9. 9. The three events in solution
  10. 10. Heat of solution (∆H soln ) is the total enthalpy change that occurs when a solution forms from solute and solvent. May be both exothermic and endothermic.
  11. 11. Thermochemical solution cycle: ∆ H soln = ∆H solute + ∆H solvent + ∆H mix Resembles Hess’s law and Born-Haber cycle.
  12. 12. Enthalpy components of the heat of solution
  13. 13. Solution implies solvation. Solvation is a process of surrounding a solute particle with solvent particles. Hydration is a process of surrounding a solute particle with water molecules.
  14. 14. Heat of hydration: ∆ H soln = ∆H solute + (∆H solvent + ∆H mix ) ∆ H hydr = ∆H solvent + ∆H mix ∆ H soln = ∆H solute + ∆H hydr
  15. 15. Heat of hydration <ul><li>NaCl (g)  Na + (g) + Cl - (g) </li></ul><ul><li>Na + (g) + 6H 2 O (l)  [Na(H 2 O) 6 ] + (aq) </li></ul><ul><li>Cl - (g) + 6H 2 O (l)  [Cl(H 2 O) 6 ] - (aq) </li></ul><ul><li>------------------------------------------- </li></ul><ul><li>NaCl (s) + 6H 2 O (l)  [Na(H 2 O) 6 ] + (aq) +[Cl(H 2 O) 6 ] - (aq) </li></ul><ul><li>M + (g) [or X - (g) ] + H 2 O  M + (aq) [or X - (aq) ] </li></ul><ul><li>∆ H hydr of the ion < 0, always </li></ul>
  16. 16. Charge density of an ion is the ratio of the ion’s charge to its volume. In general, the higher the charge density is, the more negative  H hydr is.
  17. 17. Coulomb’s law <ul><li>A 2+ ion attracts H 2 O molecules more strongly than a 1+ ion of similar size; </li></ul><ul><li>A small 1+ ion attracts H 2 O molecules more strongly than a large 1+ ion. </li></ul>
  18. 18. Charge densities and heats of hydration <ul><li>decrease down a group of ions (Li + —Na + —K + —Rb + —Cs + —Fr + ) - 1A </li></ul><ul><li>(F - —Cl - —Br - —I - ) - 7A group </li></ul><ul><li>increase across a period of ions (Na + —Mg 2+ —Al 3+ ) - 3rd period </li></ul>
  19. 19. The heat of solution for ionic compounds in water:  H soln =  H lattice +  H hydration of the ions  H lattice is always positive  H hydration is always negative
  20. 20. Dissolving ionic compounds in water
  21. 21. Hot (CaCl 2 ) and cold (NH 4 NO 3 ) packs
  22. 22. The heat of solution  H soln is only one of two factors determining whether a solute dissolves in a solvent. The other factor is entropy S.
  23. 25. Entropy is directly related to the number of ways that a system can distribute its energy. It is closely related to the freedom of motion of the particles and the number of ways they can be arranged.
  24. 26. <ul><li>Ludwig Eduard Boltzmann (1844–1906), Austrian scientist </li></ul>
  25. 27. Freedom of particle motion and entropy <ul><li>S liquid > S solid ; ∆S melting > 0 </li></ul><ul><li>S gas > S liquid ; ∆S vaporization > 0 </li></ul><ul><li>S solid > S gas ; ∆S sublimation > 0 </li></ul>
  26. 28. Solid state: minimum entropy
  27. 29. A solution usually has higher entropy than the pure solute and pure solvent: S soln > (S solute + S solvent ) ∆ S soln > 0
  28. 30. Systems tend toward a state of lower enthalpy and higher entropy.
  29. 31. Entropy is higher when mixed
  30. 32. THE END

×