Spontaneous and Nonspontaneous Processes

A fundamental goal of thermodynamics is to predict spontaneity. For example, will rust spontaneously form when iron comes into contact with oxygen Will water spontaneously decompose into hydrogen and oxygen A spontaneous process is one that occurs without ongoing outside intervention (such as the performance of work by some external force). For example, when you drop a book in a gravitational field, it spontaneously drops to the floor. When you place a ball on a slope, it spontaneously rolls down the slope. For simple mechanical systems, such as the 

We must not confuse the spontaneity of a chemical reaction with the speed of a chemical reaction. In thermodynamics, we study the spontaneity of a reaction—the direction in which and extent to which a chemical reaction proceeds. In kinetics, we 

One last word about nonspontaneity— nonspontaneous process is not impossible. For example, the extraction of iron metal from iron ore is a nonspontaneous process it does not happen if the iron ore is left to itself, but that does not mean it is impossible. As we will see later in this chapter, a nonspontaneous process can be made spontaneous by coupling it to another process that is spontaneous or by supplying energy from an external source. Iron can be separated from its ore if external energy is supplied, usually by means of another reaction (that is itself highly spontaneous). 

M FIGURE 17.4 Uiermodynamics and KineOcs Thennodynamics deals with the relative chemical potentials 

The first candidate in our search for a chanical potential might be enthalpy, which we defined in Chapter 6. Periiaps, just as a mechanical system proceeds in the direction of lowest potential energy, so a chanical system might proceed in the direction of lowest enthalpy. If this were the case, all exothermic reactions would be spontaneous and all endothermic reactions would not. However, although most spontaneous processes are exothermic, some spontaneous processes are endothermic. For example, above 0 °C, ice spontaneously melts (an endothermic process). So enthalpy must not be the sole criterion for spontaneity. 

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C14-0080. Two children on opposite ends of a seesaw can be used as an analogy for a coupled reaction. Describe the coupling of spontaneous and nonspontaneous processes during the actions of a seesaw. 

The second law of thermodynamics, as formulated by Kelvin, states that no process is possible in which the sole result is the absorption of heat from a reservoir and its complete conversion into work in other words, in any natural process involving a transfer of energy, some energy is converted irreversibly into heat that cannot be involved in further exchange. The second law of thermodynamics, therefore, is a recognition of spontaneous and nonspontaneous processes and the fact that natural processes have a sense of direction. 

Explain what is meant by a spontaneous process. Give two examples each of spontaneous and nonspontaneous processes. 

We know other spontaneous and nonspontaneous processes that relate more directly to our study of chemistry. For example, a gas spontaneously expands into a vacuum ( FIGU RE 19.2), but the reverse process, in which the gas moves back entirely into one of the flasks, does not happen. In other words, expansion of the gas is spontaneous, but the reverse process is nonspontaneous. In general, processes that are spontaneous in one direction are nonspontaneous in the opposite direction. 

Electrochemistry is the study of the relationships between electricity and chemical reactions. It includes the study of both spontaneous and nonspontaneous processes. 

As discussed in Chapter 4, oxidation is the loss of electrons in a chemical reaction, and reduction is the gain of electrons, ooo (Section 4.4) Thus, oxidation-reduction (redox) reactions occur when electrons are transferred from an atom that is oxidized to an atom that is reduced. Redox reactions are involved not only in the operation of batteries but also in a wide variety of important natural processes, including the rusting of iron, the browning of foods, and the respiration of animals. Electrochemistry is the study of the relationships between electricity and chemical reactions. It includes the study of both spontaneous and nonspontaneous processes. 

Spontaneous and Nonspontaneous Processes 815 Heating with Natural Gas... 

Both spontaneous and nonspontaneous processes can occur, but only spontaneous processes can take place without outside intervention. 

Explain the difference between a spontaneous and nonspontaneous process, and explain why the enthalpy change is not a reliable criterion for predicting the direction of spontaneous change. 

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