Teaching AP Chemistry with Electron Density Models

1. Paul D. Price Trinity Valley School Fort Worth, TX 76132 pricep@trinityvalleyschool.org Teaching AP ChemistrywithElectron Density Models

2. Teaching AP Chemistry with Electron Density Models

3. The Scope of AP Chemistry

4. The Scope of AP Chemistry

5. Testing Knowledge and Skills How does this all relate?

6. How is an LO created?

7. The “Practical” Science Practices Students can work with representations of chemistry. Students can solve quantitative problems in chemistry. Students can propose and evaluate scientific questions about chemistry. Students can collect data. Students can analyze and evaluate data. Students can predict and explain the chemistry of specific phenomena. Students can connect chemical concepts across various scales and apply those concepts to other disciplines.

8. Testing the LO

9. Showing the Relationship Knowledge + Skill = question

10. Consequences of “Pigeonholing” • Inability to Determine what is being asked • Inability to Link Concepts • Memorization vs. Understanding • Misconceptions

11. Students Hate Essays • Predict the relative atomic sizes of oxygen and fluorine • Why does methane not behave as an ideal gas at low temperature and high pressure? • Which is a stronger acid - Water or Methane? • Fluorine is larger because it has more electrons • Methane is polar • Methane - it has more hydrogens

12. Misconceptions in Atomic and Molecular Structure The Real Problem • Students never “see” what is occurring on an atomic and molecular level • Inexperience with waves • Orbitals vs Lewis Structures • Polarity - E.N. in bonds and molecules • Interpreting IM Forces • Molecular Nature of Acids and Bases

13. Consistent Molecular Visualization is a Solution • Spiral with Big Ideas • Lots of Visual and Verbal Explanation • Interpretation of Data • Electron Density as an Example • College Board Curriculum Module • Sci. Prac 1,2,5,6,7 • BI 1,2,3,6

14. What is Electron Density? • Wavefunction ( from QM • Where are the Electrons? • ED = (r) = r)2

15. Isosurfaces - Picturing Electron Density

16. Introducing to Students - Periodic Trends • Molecular Electron Density (MED) • Compare to radius of Magnesium atom • 2.1 vs 1.6 Angstroms

17. Factors Affecting Trends Explain why the volumes change as they do in terms of basic electrostatics How would Mg3+ Compare?

18. Isoelectronic Species of Be • Write down the electron configurations of each species. • Identify which MED corresponds to Be, B+, and Li-. Justify your choices. • Would the species represented by the MED’s above be paramagnetic or diamagnetic? Explain. • Which of the above MED’s would have the lowest ionization energy? Explain.

19. 2008 AP Q5

20. 2008 AP Q5 • Justify why the IE of Fluorine is greater than Oxygen • What about the IE of xenon? Xe F O

21. Electron Density of Molecules • Building Molecules from Orbital Overlap MED of H2 from Quantum Mechanics

22. Predicting the MED’s of Halogen Compounds Draw what the MED’s for Cl2, I2, ClF, and IF should look like Predict Bond Lengths and Bond Energies I2 Cl2 IF ClF

23. Interpreting Data • Rationalize the relationship between Bond Energy and Bond Length for • Homonuclear diatomics • Hydrogen halides • Heteronuclear diatomics

24. Mapping the asymmetry of Electron Density • Work to pull a +1 charge away • Positive for a proton • Negative for an electron

25. Molecular Electrostatic Potential Symmetric pulls on electrons Asymmetric pulls on electrons Red ----- Green ---- Blue Neg. Neutral Pos.

26. Visually Explaining Data IF ClF

27. Testing Student Understanding HF, HCl, HI a) Which of the molecules has the greatest polarity? Justify your answer b) Determine which structure belongs to each molecule. Again, justify your answer c) Finally, using your answers from parts a) and b) in addition to your discussion about bond strength in homonuclear molecules, rationalize the trends in bond energy among the hydrogen halides

28. Polyatomic Molecules CS2, CO2, OCS a) Identify which MEP corresponds to each Lewis diagram. Justify your answer. b) Do the MEP’s indicate that any of the bonds in each molecule are polar? Explain c) If your answer to b) was yes, which of the bonds does the MEP say is most polar? Do the electronegativities support this of the atoms support this? d) CO2 and CS2 are nonpolar molecules. How do the MEP’s illustrate this? e) Is OCS polar or nonpolar? Explain.

29. Applications - Organic Chem. • Shown above are the MEP’s of ethane, ethene, and ethyne, members of three important classes of organic molecules. • Draw the Lewis diagrams for ethane, ethane, and ethyne.

30. Identify, which MEP corresponds to each structure. Justify your choice. • Identify the hybridization of each carbon atom in the three molecules. • Based on the MEP’s are the molecules polar or nonpolar? Explain • Based on the Lewis diagrams and the MEP’s, account for the negative areas of electrostatic potential in the first two structures.

31. Isomerism and IM Forces • Several isomers exist with the formula C2H2F2. These isomers fall into one of two categories: structural isomers and geometric isomers. • Explain the difference between a structural isomer and a geometric isomer. • Draw Lewis diagrams for all possible isomers (both structural and geometric) for C2H2F2

32. Shown above are the MEP’s for all possible isomers of C2H2F2. • Identify which MEP belongs to each Lewis diagram. Justify your choice. • Which of the isomers is polar? Explain • Based on the MEP’s, if more than one isomer is polar, which has the greatest dipole moment? Justify your answer. • Which of the molecules above would have the lowest boiling point? Explain.

33. Understand how IM Forces work • Molecular dynamics • ID with MEP Maps

34. d) The normal boiling point of Cl2 (l)(238 K) is higher than the normal boiling point of HCl(l) (188 K). Account for the difference in normal boiling points based on the types of intermolecular forces in the substances. You must discuss both of the substances in your answer. Polarity vs. Polarizability

35. Misconceptions in acids and bases • Bronsted-Lowry theory and protic acids • HA(aq) + H2O(l) --> H3O+(aq) + A-(aq) HCl(aq) + H2O(l) --> H3O+(aq) + Cl-(aq) CH4(aq) + H2O(l) --> H3O+(aq) + CH3-(aq) • Does the ability to write an eqn. make something an acid?

36. What are the Molecular Properties of a Good Protic Acid? • Have a proton • The proton must be positive • The H-X bond must be weak • Poor overlap (bond length - hydrohalic) • Removal of electron density (inductive effect - oxoacids and organic acids)

37. Hydrohalic Acids • Positive hydrogens but very polar (and shorter) bonds • Poorer overlap (longer bond) + positive hydrogen = better acid

38. What about methane? • Strong C-H bond • No Positive H • Poor Acid!

39. Organic Acids and the Inductive Effect • Trifluoroacetic acid?

40. References on Electron Density • Shusterman, Gwendolyn P.; Shusterman, Alan J. J. Chem. Educ.199774 771. • Matta, C. F.; Gillespie, R. J. J. Chem. Educ.200279 1141. • Purser, Gordon H. J. Chem. Educ.200178 981. • Spartan and Odyssey, Wavefunction Inc., Irvine, CA. http://www.wavefun.com • Price, Paul D. Using Electron Density to build Molecular Comprehension: Applications to Periodic Properties and Molecular Polarity, College Board Curriculum Module Questions and Thank You! pricep@trinityvalleyschool.org

41. Electrostatic Maps H2 and HF • Build isosurface • “Map” electrons • Red - excess of electrons • Blue - electron deficient