6.4.P3
Prerequisites:
One of the most important energy transforming processes in biology is that of chemical reactions. The storage of energy in chemical structures and the extraction of that energy that results from transforming bound atoms from one structure to another is found in the most basic processes of life such as photosynthesis to respiration.
In this problem we'll explore how to think about this storage of energy conceptually and, through the use of a mechanical metaphor, we'll see how it relates to our macroscopically developed energy concepts: kinetic energy, potential energy, and work. These are related by the work-energy theorem, which says that the change in the kinetic plus potential energy of a system is equal to the work done on the system by outside forces.
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In a molecule, the atoms are bound together by interactions whose effect is described by a potential energy. (See Chemical bonding) As a model of this we consider "four atoms bound together" -- a magnet and three steel spheres, shown in the figure at the right. The magnet exerts an attractive force on each of the steel spheres that falls off rapidly as a function of distance (i.e., the attraction is stronger when the sphere is closer).
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A. Just looking at the system consisting of the strong magnet and the steel spheres labeled 1, 2, and 3, I would conjecture that "sphere 1 is the most strongly bound, sphere 2 is less strongly bound, and sphere 3 is weakly bound." Discuss to what extent this is a good description of the situation, how you might test it if you had the apparatus in front of you, and why it might be a misleading description (especially for spheres 1 and 2).
B. If an external metal sphere (labeled "0" collides with this molecule, it is strongly attracted. Watch the video of what happens below.
If we think of this as a collision between the two mechanical objects, it is a super-elastic collision*, that is, the kinetic energy of the two objects after the collision is greater than the kinetic energy of the two objects before the collision. Where did the energy come from? Tell the story of the collision, walking us through what happens describing carefully the forces and energies involved so that we understand the source of the kinetic energy of ball 3 at the end of the video.
C. To what extent do you think that this is a decent analogy for an exothermic chemical reaction -- one in which some energy is made available to do other things as a result of the reaction? Explain why you think so.
* Or, in chemistry language, it is an exothermic reaction.
Joe Redish 1/20/12
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