Cl2O4 in the Stratosphere  Week 3: Follow-Up Discussion

1. Ab Initio Results
   
   
2. Questions
   
   

Ab Initio Results

The reported sum of electronic and thermal enthalpies for the molecules are given below.

These are calculated values of the enthalpies of these molecules. They are not enthalpies of formation because the calculations did not assume that elements in their standard states have enthalpies equal to zero. When you calculate the enthalpy for the reaction, you should include the enthalpy calculated for dioxygen.

Assignments 3: Spontaneity of the Reaction of ClOClO₃ With Ozone

Deadline: 5:00 PM, Friday April 20

Answer the questions below.

1. We are assuming that the entropy change for this reaction is small. Is this a good assumption? Why or why not?

2. If we assume that the entropy change is small, why can we use the enthalpy change to predict if the reaction will be spontaneous?

3. Because of the conservation of spin, in a thermal reaction the net spin of the products of a reaction must be the same as the net spin of the reactants. ClOClO₃, O₃, and O₂ClOClO₃ have all their electrons paired. Would the other product be singlet or triplet oxygen molecules?

4. The results reported from the MP2 calculations were the sum of the electronic and thermal enthalpies for each compound in the reaction. These represent calculated values of the enthalpies of the compounds. Using the state of O₂ from your answer to problem 3, calculate the enthalpy change for the reaction. Do the calculations suggest that the reaction between ClOClO₃ and ozone would have to be spontaneous? The enthalpies calculated by Gaussian are not enthalpies of formation because they are not defined so that elements in their standard state have enthalpies = 0. Use the value of the enthalpy calculated for O₂ .

5. In the chapter in your textbook on electronic spectroscopy, you should find a discussion of selection rules for electronic transitions. The ground state of molecular oxygen is the triplet state and the singlet state is an excited state. The lifetime of singlet oxygen is 74 min.¹⁶ Is this long lifetime consistent with dipole selection rules for electronic transitions? Why or why not?

6. In the chapter in your textbook on electronic spectroscopy, you should find a discussion of phosphorescence. Thinking about phosphorescence, how do you think it is possible for the conversion of the singlet state to the triplet state to occur?

7. Assume the triplet state of O₂ is accessible through collisional deactivation. Given this assumption and the results from the MP2 calculations, would you expect the reaction between ClOClO₃ and ozone to be spontaneous? (Hint: Think about the enthalpy change again, this time with the triplet state as a product.)

8. Here are the first two steps of the proposed mechanism. If we predict that the second step of this mechanism will be thermodynamically spontaneous, is it certain that this mechanism will be significant? Discuss why or why not. You might think about two concepts - quantum yield and chemical kinetics.

Send your answers to Dr. Lever, who will forward them to the entire group.

Partial support for this work was provided by the National Science Foundation's Division of Undergraduate Education through grant DUE #9751605 and by CAChe Scientific through a Higher Education Program grant.

The PCOL community that partial support for this work was provided by the National Science Foundation's Division of Undergraduate Education through grant DUE #9950809. Additional support was provided by the Camille and Henry Dreyfus Foundation. PCOL faculty also acknowledge the National Science Teachers Association which awarded the PCOL Faculty Consortium the 1998 Gustav Ohaus Award for Innovation in College Science Teaching.

This site created by David Whisnant (whisnantdm@wofford.edu).
This page was last updated on April 16, 2007
llever@uscupstate.edu