The Second Law of Thermodynamics Predicting Spontaneous Change

In the last few chapters, we ve posed and answered some essential questions about chemical and physical change How fast does the change occur, and how is this rate affected by concentration and temperature How much product will be present when the net change ceases, and how is this yield affected by concentration and temperature  

Concepts Skills to Review Before You Study This Chapter 

In contrast, for a nonspontaneous change to occur, the surroundings must supply the system with a continuous input of energy. A book falls spontaneously, but it rises only if something else, such as a human hand (or a hurricane-force wind), supplies energy in the form of work. Under a given set of conditions, if a change is spontaneous in one direction, it is not spontaneous in the other. 

Note that the term spontaneous does not mean instantaneous and has nothing to do with how long a process takes to occur it means that, given enough time, the process will happen by itself. Many processes are spontaneous but slow—ripening, rusting, and (happily ) aging. 

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The Second Law of Thermodynamics Predicting Spontaneous Change Lrnitations of the First Law The Sign of AH and Spontaneous Change Freedom of Motion and Dispersal of Energy Entropy and the Number of Microstates Entropy and the Second Law Standard Molar Entropies and the Third Law... 

The deduction of a criterion for the evolution of an open system to its stationary state resembles the classical thermodynamic problem of predict ing the direction of spontaneous irreversible evolution in an isolated system According to the Second Law of thermodynamics, in the latter case the changes go only toward the increase in entropy, the entropy being maximal at the final equilibrium state. 

The second law of thermodynamics tells us that a process will be spontaneous if the entropy of the universe increases when the process occurs. We saw in Section 10.7 that for a process at constant temperature and pressure, we can use the change in free energy of the system to predict the sign of ASuniv and thus the direction in which it is spontaneous. So far we have applied these ideas only to physical processes, such as changes of state and the formation of solutions. However, the main business of chemistry is studying chemical reactions and, therefore, we want to apply the second law to reactions. 

How can we use the fact that any spontaneous process is irreversible to make predictions about the spontaneity of an unfamiliar process Understanding spontaneity requires us to examine the thermodynamic quantity called entropy, which was first mentioned in Section 13.1. In general, entropy is associated either with the extent of randomness in a system or with the extent to which energy is distributed among the various motions of the molecules of the system. In this section we consider how we can relate entropy changes to heat transfer and temperature. Our analysis will bring us to a profound statement about spontaneity that we call the second law of thermodynamics. 

The second law of thermodynamics tells us the essential character of any spontaneous change—it is always accompanied by an increase in the entropy of the universe. We can use this criterion to predict whether a given process is spontaneous or not. Before seeing how this is done, however, we will find it useful to explore entropy from a molecular perspective. 

Many of those who found themselves unable to accept chemical oscillation as a reality based their refusal on the Second Law of Thermodynamics. The power of the Second Law lies in its ability to predict the direction of spontaneous change from the deceptively simple condition that... 

Laws of Thermodynamics The laws of thermodynamics have been successfully applied to the study of chemical and physical processes. The first law of thermodynamics is based on the law of conservation of energy. The second law of thermodynamics deals with natural or spontaneous processes. The function that predicts the spontaneity of a reaction is entropy. The second law states that for a spontaneous process, the change in the entropy of the universe must be positive. The third law enables us to determine absolute entropy values. 

Our interest in chemical potentials (and activities) arises from the fact that the Gibbs energy of a mixture can be expressed in terms of the amounts of the substances and their chemical potentials, as shown by equations (13.31) and (13.32). These two equations, and the second law of thermodynamics, can be used to develop the criterion for predicting the direction of spontaneous chemical change. 

In Chapter 1 we described the fundamental thermodynamic properties internal energy U and entropy S. They are the subjects of the First and Second Laws of Thermodynamics. These laws not only provide the mathematical relationships we need to calculate changes in U, S, H,A, and G, but also allow us to predict spontaneity and the point of equilibrium in a chemical process. The mathematical relationships provided by the laws are numerous, and we want to move ahead now to develop these equations.1... 

Thus our definition of the second law has led to a function, G, which will always decrease to a minimum in spontaneous processes in systems having specified values of T and P. It is an extremely useful thermodynamic potential. All we have to do is to find a way to get measurable values of this function for all pure compounds and solutes, and to find how they change with T, P, and concentration, and we will then be able to predict the equilibrium configuration of any system by minimizing G. 

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