Teaching AP ChemistrywithElectron Density Models

Paul D. PriceTrinity Valley SchoolFort Worth, TX 76132pricep@trinityvalleyschool.org

Teaching AP Chemistry with Electron Density Models

The Scope of AP Chemistry

The Scope of AP Chemistry

How does this all relate?

Testing Knowledge and Skills

How is an LO created?

The “Practical” Science Practices

1. Students can work with representations of chemistry.

2. Students can solve quantitative problems in chemistry.

3. Students can propose and evaluate scientific questions about chemistry.

4. Students can collect data.5. Students can analyze and evaluate data.6. Students can predict and explain the chemistry

of specific phenomena.7. Students can connect chemical concepts across

various scales and apply those concepts to other disciplines.

Testing the LO

Showing the Relationship

Knowledge + Skill = question

Consequences of “Pigeonholing”

► Inability to Determine what is being asked► Inability to Link Concepts► Memorization vs. Understanding► Misconceptions

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

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

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

What is Electron Density?

► Wavefunction ( from QM► Where are the Electrons?► ED = (r) = r)2

Isosurfaces - Picturing Electron Density

Introducing to Students - Periodic Trends

► Molecular Electron Density (MED)

• Compare to radius of Magnesium atom– 2.1 vs 1.6 Angstroms

Factors Affecting Trends

Explain why the volumes change as they do in terms of basic electrostaticsHow would Mg3+ Compare?

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.

2008 AP Q5

2008 AP Q5

► Justify why the IE of Fluorine is greater than Oxygen► What about the IE of xenon?

Xe F O

Electron Density of Molecules

► Building Molecules from Orbital Overlap

MED of H2 from Quantum Mechanics

Predicting the MED’s of Halogen Compounds

Draw what the MED’s for Cl2, I2, ClF, and IF should look like

Cl2

I2

ClF

IF

Predict Bond Lengths and

Bond Energies

Interpreting Data

Rationalize the relationship between Bond Energy and Bond Length for

•Homonuclear diatomics

•Hydrogen halides

•Heteronuclear diatomics

Mapping the asymmetry of Electron Density

► Work to pull a +1 charge away ► Positive for a proton► Negative for an electron

Molecular Electrostatic Potential

Symmetric pulls on electrons Asymmetric pulls on electrons

Red ----- Green ---- BlueNeg. Neutral Pos.

Visually Explaining Data

IF ClF

Compound E.N. Difference

ClF 1.0

IF 1.2

Testing Student Understanding

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

HF, HCl, HI

Polyatomic Molecules

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? Explainc) 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.

CS2, CO2, OCS

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.

► 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.

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

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.

• Understand how IM Forces work– Molecular dynamics

• ID with MEP Maps

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

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?

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)

Hydrohalic Acids

► Positive hydrogens but very polar (and shorter) bonds

► Poorer overlap (longer bond) + positive hydrogen = better acid

What about methane?

► Strong C-H bond► No Positive H► Poor Acid!

Organic Acids and the Inductive Effect

► Trifluoroacetic acid?

References on Electron Density

► Shusterman, Gwendolyn P.; Shusterman, Alan J. J. Chem. Educ. 1997 74 771.

► Matta, C. F.; Gillespie, R. J. J. Chem. Educ. 2002 79 1141.

► Purser, Gordon H. J. Chem. Educ. 2001 78 981.► 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

Electrostatic Maps

• Build isosurface •“Map” electrons

•Red - excess of electrons

•Blue - electron deficient

H2 and HF