Spontaneous changes

The general theory behind the process is that the hypohalite will convert the amide to a haloamide. This then spontaneously changes to the isocyanate when heated and decomposes to the amine from the water present. In effect, all that happens is that a Carbonyl (CO) group is stripped off the starting amide to yield the corresponding amine. Yields pre- purification are around 80%, post-purification average around 65%. Certain uses of the result-... 

The properties of a system at equilibrium do not change with time, and time therefore is not a thermodynamic variable. An unconstrained system not in its equilibrium state spontaneously changes with time, so experimental and theoretical studies of these changes involve time as a variable. The presence of time as a factor in chemical kinetics adds both interest and difficulty to this branch of chemistry. 

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,... 

The development presented in Example 17.6 is an important one from a practical standpoint It tells us the temperature at which the direction of spontaneity changes. In the reduction of Fe203 by hydrogen, this temperature is approximately 700 K. At lower temperatures, the reaction does not occur at standard conditions recall from Example 17.5 that AG° at 500 K is +27.6 kj. At temperatures above 700 K, AG° has a negative sign and the reaction... 

At the point a (bp < 0) the directions of change are reversed, i.e.y all spontaneous changes increase the volume. 

At the point y (8T <0) the direction of the spontaneous change is reversed, i.e., it occurs with evolution of heat on the curve. 

A spontaneous change is a change that has a tendency to occur without needing to be driven by an external influence. A simple example is the cooling of a block of hot metal to the temperature of its surroundings (Fig. 7.1). The reverse change, a... 

In science, we look for patterns to discover nature s laws. What is the pattern common to all spontaneous changes To find a pattern, it is often best to start with very simple examples, because then the pattern is likely to be more obvious. So, let s think about two simple spontaneous changes—the cooling of a hot metal and the expansion of a gas—at a molecular level and search for their common feature. 

As we have already emphasized, to use the entropy to judge the direction of spontaneous change, we must consider the change in the entropy of the system plus the entropy change in the surroundings ... 

Now consider an isolated system consisting of both the system that interests us and its surroundings (again like that in Fig. 7.15). For any spontaneous change in this isolated system, we know from Eq. lib that ASun > 0. If we calculate for a particular hypothetical process that A5tot < 0, we can conclude that the reverse of that process is spontaneous. 

FIGURE 7.24 At constant temperature and pressure, the direction of spontaneous change is toward lower Gibbs free energy. The equilibrium state of a system corresponds to the lowest point on the curve. 

The decrease in Gibbs free energy as a signpost of spontaneous change and AG = 0 as a criterion of equilibrium are applicable to any kind of process, provided that it is occurring at constant temperature and pressure. Because chemical reactions are our principal interest in chemistry, we now concentrate on them and look for a way to calculate AG for a reaction. 

Show that, if two copper blocks with different temperatures are placed in contact, then the direction of spontaneous change is toward the equalization of temperatures. Do so by considering the transfer of 1 J of energy as heat from one to the other and assessing the sign of the entropy change. Assume that the temperatures of the blocks remain constant. 

The following pictures show a molecular visualization of a system undergoing a spontaneous change. Account for the spontaneity of the process in terms of the entropy changes in the system and the surroundings. 

There are two principal chemical concepts we will cover that are important for studying the natural environment. The first is thermodynamics, which describes whether a system is at equilibrium or if it can spontaneously change by undergoing chemical reaction. We review the main first principles and extend the discussion to electrochemistry. The second main concept is how fast chemical reactions take place if they start. This study of the rate of chemical change is called chemical kinetics. We examine selected natural systems in which the rate of change helps determine the state of the system. Finally, we briefly go over some natural examples where both thermodynamic and kinetic factors are important. This brief chapter cannot provide the depth of treatment found in a textbook fully devoted to these physical chemical subjects. Those who wish a more detailed discussion of these concepts might turn to one of the following texts Atkins (1994), Levine (1995), Alberty and Silbey (1997). 

The second entropy increases during spontaneous changes, . ... 

In the equilibrium Second Law, the first entropy increases during spontaneous changes in structure, and when the structure stabilizes (i.e., change ceases), the first entropy is a maximum. This state is called the equilibrium state. Similarly, in the nonequilibrium Second Law, the second entropy increases during spontaneous changes in flux, and when the flux stabilizes, the second entropy is a maximum. This state is called the steady state. The present nonequilibrium Second Law has the potential to provide the same basis for the steady state that Clausius Second Law has provided for the equilibrium state. 

Of course, depending on the system, the optimum state identified by the second entropy may be the state with zero net transitions, which is just the equilibrium state. So in this sense the nonequilibrium Second Law encompasses Clausius Second Law. The real novelty of the nonequilibrium Second Law is not so much that it deals with the steady state but rather that it invokes the speed of time quantitatively. In this sense it is not restricted to steady-state problems, but can in principle be formulated to include transient and harmonic effects, where the thermodynamic or mechanical driving forces change with time. The concept of transitions in the present law is readily generalized to, for example, transitions between velocity macrostates, which would be called an acceleration, and spontaneous changes in such accelerations would be accompanied by an increase in the corresponding entropy. Even more generally it can be applied to a path of macrostates in time. 

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