Nuclear Reactions, e = mc^2
||Chemistry: Nuclear Chemistry|
||High School Chemistry I (grade 10 or 11)|
two 45 minute class periods
This will be one of the last lessons in the unit on Atomic Structure and Nuclear Chemistry. By this time students should be familiar with basic atomic structure, and the concept of isotopes, and have an understanding of radioactivity and the various processes of radioactive decay. Specifically, students should be able to identify the major subatomic particles (proton, neutron, electron) in the modern atomic model and explain how they interact, as well as the three main types of radioactive decay (alpha, beta, and gamma).
of New Content
Students will be presented with the concepts of nuclear binding energy, nuclear fusion and nuclear fission, and the concept that in these reactions matter is transformed into energy, as related in the equation, e = mc2. Nuclear fusion will be explored further as the means by which elements heavier than hydrogen are created in the cores of stars.
Students will gain an understanding of the fundamental ideas behind fusion and fission, the “iron limit” (which states that fusion reactions are no longer energetically favorable in producing nuclei heaver than that of iron), the concept of nuclear binding energy. They should also gain a general understanding that mass and energy may be interconverted in certain processes, as represented in Einstein’s famous equation. Furthermore, students should gain an appreciation for the awe-inspiring fact that all of the elements on earth and inside their bodies (except hydrogen) were fused in the cores of stars.
||Computer, Projector, MS Powerpoint, a comprehensive list of the atomic masses of the naturally occurring isotopes of all elements.|
Use student feedback to improve the lesson, or make it more or less difficult. At all points in the lesson where students are asked to break into groups and discuss important questions, ask them to record their thoughts/hypothesis and turn them in at the end of class. Have the students fill out the following worksheet during the course of the lesson:
Nuclear Reactions, e = mc2
1. Describe as best you can what is occurring in the atomic bomb explosions in Orson Welles’s movie, “Dr. Strangelove”? What is the name of the process that provides the power behind these explosions? What occurs during this type of reaction?
2. Describe what you see occurring in the photograph of the Sun. What is the name of the process that powers the Sun and other stars? What occurs during this type of reaction?
3. What do the two processes described above have in common? Where do you think this huge amount of energy comes from?
4. In order for a reaction to release energy, what needs to happen (what characteristics do the products have in relation to the reactants)? Keeping this in mind, what do you think is changing energy in “nuclear” reactions?
5. The process that occurs in the Sun fuses two atoms of 2H (deuterium) into one atom of helium (4He). Write the reaction, and verify that the Law of Conservation of Mass applies in this reaction, given the isotope masses discussed in class. What are your findings? Are you surprised by these findings?
6. Another process that occurs in the Sun is the fusion of 3 4He nuclei to form one 12C atom. Again, write the reaction and please verify that the Law of Conservation of Mass applies in this reaction, given the isotope masses discussed in class. What are your findings? Are you surprised?
7. Where do you think this “missing mass” has gone? Do you think it might have any relationship to the huge amount of energy released in these types of reactions?
8. Try to verify the Law of Conservation of Mass for the fission reactions in the atomic explosions in “Dr. Strangelove.” The reaction involved is the splitting of 235U to produce 90Rb, 143Cs, and two neutrons. As you did with the two fusion reactions above, write this reaction, and verify that the Law of Conservation of Mass applies, given the isotope masses discussed in class.
9. Write the definition of Nuclear Binding Energy.
10. Write Einstein’s famous equation, and explain in a few words what it means.
11. Use your answers to questions 9 and 10 to try and explain where the large amounts of energy released by the Sun (nuclear fusion) and atomic bombs (nuclear fission) comes from. Why does the Law of Conservation of Mass not seem to hold for these reactions? What role does the loss of mass play in the release of energy in these reactions?
12. Calculate the change in mass for the reverse reactions of those mentioned in questions 5, 6, and 8. Is mass being produced or consumed in these reactions? Why do you think these reactions do not actually occur?
13. After the class discussion, explain what happens to the “missing mass” in the fusion and fission reactions we talked about today. Was this what you had originally hypothesized?
14. Explain what the “Iron limit” means, and why the fission of 4He not favorable, while the fission of 235U is favorable.
15. Given the “Iron limit”, how is it possible for stars to create elements heavier than Iron?
Discuss the “Iron limit” and ask students to think about this question: if all elements are created by fusion reactions in stars, given the “Iron limit”, how can elements heavier than Iron be produced? Ask them to propose hypotheses, and guide them to realize that, while unfavorable, these reactions can and do occur. Ask them to think about what conditions these unfavorable fusion reactions might occur. Finally, explain to them that in the extreme energy released by a supernova explosion at the end of a star’s life, there is adequate energy for fusion processes to occur that produce elements heavier than Iron.
|References||for the ending sequence of "Dr. Strangelove": http://youtube.com/watch?v=wxrWz9XVvls|