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Midtermexamsolutions2 - Chemistry 124 Second Examination...

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Chemistry 124 Second Examination Answers October 20, 2008 The exam budgets 50 minutes, but you may have 60 minutes to finish it. Good answers can fit in the space provided. Question values correspond to alloted time. Don't waste too much time on cheap questions. Read each question carefully to see what it asks for (bold face is used to help highlight questions). Make sure you are answering the question, not just saying something vaguely relevant to its topic. 1. (4 min) Label the symmetric atomic orbital pairs that would give the following four profiles for overlap integral versus C-C distance. 2. (4 min) Explain whether Correlation Energy should raise or lower a molecular energy calculated by the SCF procedure. “Correlation Energy” is the name for the error that remains after a complete SCF molecular orbital calculation. SCF necessarily overestimates the energy, because the electrons are kept in fixed clouds and are unable to avoid one another by correlating their motion. Thus correction for correlation energy always lowers the energy calculated by SCF. 3. (2 min) Name two accurate QUANTITATIVE tools that would have been available to you, if you were a chemistry student in Yale’s Sheffield Scientific School in the class of 1901. Burettes for titration Analytical balances for precise weighing 4. (4 min) Assuming, perhaps unrealistically, that the atom-atom distances and overlap integrals are identical, explain , in terms of electron energies, which of the following bonds should be stronger. (A diagram would help.) the 1-electron bond of H 2 + the 3-electron bond of He 2 + In either case there is perfect energy match, so if we assume identical overlap, the shifts of the electronic energy levels will be identical. In the two-electron case, one electron falls the same as in the one-electron case, but the fall of the second electron is more than offset by the rise of the third , because of the different normalization constants (<1/sqr(2) for the bonding orbital; >1/sqr(2) for the antibonding). Thus the 3-electron bond should be weaker. [Note that it is not trivial to predict the effect of change in electron repulsion on bonding, or the influence of small changes in bond distance and nuclear repulsion.] sp-sp s-s p-p σ p-p π
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