Entropy, Free Energy, and Equilibrium

At the end of this chapter, you will be able to answer several questions regarding entropy changes [ H Page 806]. 

About the three laws of thermodynamics and how to use thermodynamic quantities to determine whether or not a proeess is expected to be spontaneous. 

An understanding of thermodynamics enables us to predict whether or not a reaction will occur when reactants are combined. This is important in the synthesis of new compounds in the laboratory, the manufacturing of chemicals on an industrial scale, and the nnderstanding of natural proeesses such as cell function. A process that does occur under a specifie set of conditions is called a spontaneous process. One that does not occur under a specific set of eonditions is called nonspontaneous. Table 18.1 lists examples of familiar spontaneous processes and their nonspon-taneous counterparts. These examples illustrate what we know intuitively Under a given set of conditions, a process that occurs spontaneously in one direction does not also oecur spontaneously in the opposite direction. 

Processes that result in a decrease in the energy of a system often are spontaneous. For example, the combustion of methane is exothermic  

the energy of the system is lowered because heat is given off during the eourse of the reaction. Likewise, in the aeid-base neutralization reaction. 

The production of quicklime (CaO) from limestone (CaC03) in a rotary kiln. The models show structures of CaC03 and CaO and carbon dioxide molecules. 

Thermodynamics is an extensive and far-reaching scientific discipline that deals with the interconversion of heat and other forms of energy. Thermodynamics enables us to use information gained from experiments on a system to draw conclusions about other aspects of the same system without further experimentation. For example, we saw in Chapter 6 that it is possible to calculate the enthalpy of reaction from the standard enthalpies of formation of the reactant and product molecules. This chapter introduces the second law of thermodynamics and the Gibbs free-energy function. It also discusses the relationship between Gibbs free energy and chemical equihbrium. 

In Chapter 6 we encountered the first of three laws of thermodynamics, which says that energy can be converted from one form to another, but it cannot be created or destroyed. One measure of these changes is the amount of heat given off or absorbed by a system during a constant-pressure process, which chemists define as a change in enthalpy (AH). 

The second law of thermodynamics explains why chemical processes tend to favor one direction. The third law is an extension of the second law and will be examined briefly in Section 18.4. 

A spontaneous reaction does not necessarily mean an instantaneous reaction. 

For us to cause a reduction in entropy in one part of the universe, the principles of entropy, free energy, and equilihrium require that we generate an even greater increase in entropy somewhere else. 

Overcoming nature s tendency toward disorder, as we do when we harvest oranges from a grove and arrange them in neatly stacked crates, requires an enormous input of energy. 

The conditions that nx t often are specified stetempaature. pressure, and in the case of a solution, coTKentrBtion. 

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Chapter 17 Thermodynamics Entropy, Free Energy, and Equilibrium... 

Tables of heat capacities, enthalpies, entropies, free energies, and equilibrium constants.)... 

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