6.4.P10

The figure at the right refers to a highly simplified model of a chemical reaction that can be described as   A + BC → (ABC)  → AB + C.   where A, B, and C might represent atoms or clusters of bound atoms. In the middle of the reaction, when everything that is reacting is close together, we model the reaction complex, (ABC),  as a combination of three parts, shown as red, green, and blue blobs (surrounded perhaps by a shared electron cloud). We treat this in a simple "toy model", assuming that each part retains its identity in the cluster and can be pulled away from the remaining pair or vibrate against them. (Note: In general, the actual reaction complex may not look like this simple combination. But the "toy model" can help you follow the energy balance taking place in the reaction.)

Two potential energy curves are shown: one in blue that shows what happens to the potential energy as the blue part is pulled away from the red-green pair, one in red that shows what happens to the potential energy as the red part is pulled away from the blue-green pair. The reaction complex has a number of excited states, some that can be described as either the blue or the red cluster vibrating against the remaining pair (modeled as fixed). Three vibrational states of each of these types are shown. The states where the blue part is vibrating are labeled E₁, E₂, and E₃. The states where the red part is vibrating are labeled E_a, E_b, and E_c.

A. If you model the small oscillations of the red and blue parts of the molecule around their stable points, which would have a larger spring constant?

a. The blue part.
b. The red part.
c. They will each have the same spring constant.
d. You can’t tell from the information given.

B. Which part takes less energy to break off from the molecule, blue or red?

a. The blue part.
b. The red part.
c. They will each take the same energy to remove.
d. You can’t tell from the information given.

C. Suppose the chemical reaction Blue + (Red-Green) →Red + (Blue-Green). Would this require energy input to happen or would it give off energy?

a. It would require energy.
b. It would give off energy.

c. It would happen but would neither require input of energy or give it off.

d. You can't tell from the information in the graphs

Joe Redish 5/11/12