Forms of Energy and Their Interconversion

Modern atomic theory allows us to consider other forms of energy—solar, electrical, nuclear, and chemical—as examples of potential and kinetic energy on the atomic and molecular scales. No matter what the details of the situation, when energy is transferred from one object to another, it appears as work and/or as heat. In this section, we examine this idea in terms of the loss or gain of energy that takes place during a chemical or physical change. 

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Each of these forms of energy plays a role in chemistry, and each is described at the appropriate point in this textbook. Here we concentrate on the nature of potential and kinetic energy and their interconversion. The nnderstanding we gain here is essential background for understanding these other forms of energy. 

Fig. 3. Chair and boat forms of cyclohexane and possible transition states for their interconversion energies (kcal/mole) relative to the chair form in parentheses...

Nineteenth-century chemists did not pay much attention to the developments in thermodynamics, while experiments done by chemists—such as Gay-Lussac s experiments on the expansion of a gas into vacuum—were taken up and discussed by the physicists for their thermodynamic implications. The interconversion of heat into other forms of energy was a matter of great interest, but mostly to the physicists. Among the chemists, the concept of heat as an indestructible caloric, a view supported by Lavoisier, largely prevailed [1]. As we noted in Chapter 2, the work of the Russian chemist Germain Hess on heats of reaction was an exception. 

The energy barrier between the chair and boat form of cyclohexane is about 35 k J/mole and this is not large enough to prevent their rapid interconversion at room temperature. This is why it is not possible to isolate each conformation. 

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