Predictions spontaneous change, direction

Clearly, the direction of a spontaneous change is not always determined by the tendency for a system to go to a state of lower energy. There is another natural tendency that must be taken into account to predict the direction of spontaneity. Nature tends to move spontaneously from a state of lower probability to one of higher probability. On as G. N. Lewis put it,... 

Spontaneous processes result in the dispersal of matter and energy, hi many cases, however, the spontaneous direction of a process may not be obvious. Can we use energy changes to predict spontaneity To answer that question, consider two everyday events, the melting of ice at room temperature and the formation of ice in a freezer. 

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. 

Given sufficient time, chemical substances in contact with each other tend to come to chemical equilibrium. Chemical equilibrium is the time-invariant, most stable state of a closed system (the. state of minimum Gibbs free energy). We study chemical equilibrium concepts so as to learn the direction of spontaneous change of chemical reactions in any system, especially for conditions of constant temperature and pressure. We want to be able to compute the hypothetical equilibrium stale of a system. We would like to predict the conditions for equilibrium in different systems and at different temperatures and pressures without having to measure them. 

We need to understand the concepts of Gibbs free enei gy (G) and chemical potential i/i) in order to know the direction of spontaneous change of a reaction or system. These concepts can also be used to define or predict the most stable (equilibrium) assemblage and gas, fluid, or rock compositions expected in a system at a given pressure and temperature. Some phases and aqueous species in a system may be out of equilibrium with that system. Free-energy calculations permit us to decide which substances are out of equilibrium, and, therefore, which concentrations may be governed by chemical kinetics. 

We predict the direction of a spontaneous change from the second iaw of thermodynamics a spontaneous change occurs in the direction that increases the entropy of the universe (system plus surroundings). In other words, a change occurs spontaneously if the energy of the universe becomes more dispersed. 

In other words Gibbs saw that the calculated change in entropy for the system and surroundings predicted the direction of spontaneous change in any chemical reaction. Thus with pencil and paper— and not one drop of solution or sweat— we can calculate whether a laboratory or industrial reaction should occur. 

Gibbs free energy (G) (10.6) A thermodynamic state function that can be used to predict the direction of spontaneous change at constant temperature and pressure. AG = AH - TAS, and AG < 0 for any spontaneous process. 

Reaction quotient (Q) (12.3) Expression identical in form to the equilibrium constant, but in which the concentrations do not correspond to equilibrium values. Comparison of the reaction quotient to the equihbrium constant predicts the direction of spontaneous change. 

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... 

A chemical reaction proceeding toward equilibrium is a spontaneous change. Recall that we can predict the net direction of the reaction—its spontaneous direction— by comparing the reaction quotient (0 with the equilibrium constant (K). But why is there a drive to attain equilibrium And what determines the value of the equilibrium constant And, most importantly, can we predict the direction of a spontaneous change in cases that are not as obvious as burning gasoline or falling books ... 

Given just these few examples, we see that, as for the first law, the sign of AH by itself does not predict the direction of a spontaneous change. 

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. 

Using Enthalpy and Entropy Changes to Predict the Direction of Spontaneous Change... 

This equation can be used to establish a method for predicting the direction of spontaneous change. The method involves comparing the values of Q and K. Again, there are three specific cases to consider. They are described below and summarized in Table 13.6. 

In Section 13, we established that, for a spontaneous process at constant T and constant P, the Gibbs energy of the system always decreases (AG)y p < 0. This idea can also be expressed as (dG)p, p < 0, for a system that undergoes a spontaneous change that is infinitesimally small. We will find this second expression the most useful for developing an equation that can be used for predicting the direction of spontaneous change in a system in which a chemical reaction occurs. 

Equation (13.35) is the key to predicting the direction of spontaneous change, so we must be certain to interpret it properly. It shows clearly that if Aj.G < 0, then we must have reaction occurring in the forward direction d > 0) to ensure that G decreases dG < 0), as required by the second law. On the other hand, if Aj.G > 0, then we must have reaction occurring in the reverse direction d < 0) to ensure that G decreases. Table 13.9 summarizes the various possibilities. 

Because reaction (19.3) is a spontaneous reaction, the displacement of Zn " (aq) by Cu(s)—the reverse of reaction (19.3)—does not occur spontaneously. This is the observation made in Figure 19-1. In Section 19-3, we will discuss how to predict the direction of spontaneous change for oxidation-reduction reactions. 

Our main criterion for spontaneous change is that Aj-G < 0. According to equation (19.14), however, redox reactions have the property that, if Afi < 0, then Eceii > 0. That is, E<.eii must be positive if Afi is to be negative. Predicting the direction of spontaneous change in a redox reaction is a relatively simple matter by using the following ideas ... 

Chapter 13 is a significant revision of Chapter 19 from the tenth edition. It introduces the concept of entropy, the criteria for predicting the direction of spontaneous change, and the thermodynamic equilibrium condition. In Chapters 14-19, we apply and extend concepts introduced in Chapter 13. However, Chapters 14 19 can be taught without explicitly covering, or referring back to. Chapter 13. 

Although thermodynamics can be used to predict the direction and extent of chemical change, it does not tell us how the reaction takes place or how fast. We have seen that some spontaneous reactions—such as the decomposition of benzene into carbon and hydrogen—do not seem to proceed at all, whereas other reactions—such as proton transfer reactions—reach equilibrium very rapidly. In this chapter, we examine the intimate details of how reactions proceed, what determines their rates, and how to control those rates. The study of the rates of chemical reactions is called chemical kinetics. When studying thermodynamics, we consider only the initial and final states of a chemical process (its origin and destination) and ignore what happens between them (the journey itself, with all its obstacles). In chemical kinetics, we are interested only in the journey—the changes that take place in the course of reactions. 

For a scientist, the primary interest in thermodynamics is in predicting the spontaneous direction of natural processes, chemical or physical, in which by spontaneous we mean those changes that occur irreversibly in the absence of restraining forces—for example, the free expansion of a gas or the vaporization of a hquid above its boiling point. The first law of thermodynamics, which is useful in keeping account of heat and energy balances, makes no distinction between reversible and irreversible processes and makes no statement about the natural direction of a chemical or physical transformation. 

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