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Tutor profile: Jeff K.

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Jeff K.
Graduate Student at Boston College
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Questions

Subject: Physical Chemistry

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Question:

Marcus theory predicts that increasing the exergonicity of a reaction will speed up the rate of outer-sphere electron transfer processes until a point is reached at which the rate begins to slow down again. What is the name for this point? Explain why this behavior would be observed.

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Jeff K.
Answer:

The point at which the rate of outer-sphere electron transfer processes slow down again is known as the "inverted region." Marcus theory describes the electron donor and acceptor species as harmonic oscillators. If both energy wells possess the same ground state energy, then an electron in the donor species will need to be excited to a minimum energy at which point the two energy wells cross for electron transfer to occur. If the acceptor species is further stabilized (i.e. would be drawn lower on a plot of E vs. x), the point at which the two energy wells cross will become lower until it is effectively barrierless. However, if the acceptor species is stabilized beyond this point, while the overall reaction may appear energetically downhill, the point at which the two wells cross now beings to climb the opposite wall of the energy well. This results in a slower electron transfer as fewer electrons in the donor species will have the energy necessary to cross the barrier based on the Botlzmann distribution. For faster transfer, more energy must be again applied to the system. (NOTE: This problem is greatly aided by diagramming the plot of E vs. x for all three regions described.)

Subject: Inorganic Chemistry

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Question:

What is the point group for Wilkinson's catalyst?

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Jeff K.
Answer:

Point group: Cnv. Wilkinson's catalyst possesses one C2 rotational axis, but no perpendicular rotational axis. The catalyst also has a plane of symmetry coincident with the primary rotational axis.

Subject: Basic Chemistry

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Question:

Rank the following molecules in order from highest to lowest boiling point at standard atmosphere and pressure: (a) pentane, (b) isopentane, (c) pentanol Explain your reasoning.

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Jeff K.
Answer:

(c) >> (a) > (b) Pentanol has the highest boiling point as it is the only molecule capable of hydrogen-bonding interactions. Pentane has a higher boiling point than isopentane as a result of being longer and flatter, allowing for a greater number of weak-field (i.e. Van der Waals) interactions between molecules. More interactions means that it takes more energy to force the molecules into the gas state.

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