The spontaneity of chemical reactions

To calculate the entropy change in the surroundings when a reaction takes place at constant pressure, we use eqn 2.6, interpreting the AH in that expression as the reaction enthalpy. For example, for the formation of the NAD -enzyme complex discussed above, with AjH = -24.2 kj mol , the change in entropy of the surroundings (which are maintained at 25 C, the same temperature as the reaction mixture) is 

This calculation confirms that the reaction is spontaneous. In this case, the spontaneity is a result of the dispersal of energy that the reaction generates in the surroundings the complex is dragged into existence, even though it has a lower entropy than the separated reactants, by the tendency of energy to disperse into the surroundings. 

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Gibbs free energy describes the spontaneity of chemical reactions in terms of enthalpy, entropy, and temperature. Negative values signify a spontaneous reaction, while positive values are nonspontaneous. A free energy of zero denotes equilibrium conditions. 

You will explain how changes in enthalpy, entropy, and free energy affect the spontaneity of chemical reactions and other processes. 

Entropy may also influence the spontaneity of chemical reactions. In general terms the entropy represents the disorder or randomness associated with a particular process. The Gibbs free energy of reaction includes both the enthalpy and entropy associated with chemical processes, and is defined as... 

Constant temperature and pressure are the most common circumstances in the laboratory, so Eq. (4.1-18) is the most useM criterion for the spontaneity of chemical reactions. Thermodynamics does not distinguish between chemical and physical processes, and Eq. (4.1-17) is valid for physical processes such as phase transitions as well as for chemical reactions. 

Thus far we have discussed whether a chemical reaction will occur spontaneously or only with the addition of energy. We have said nothing about the rate of chemical reactions—how fast they occur. If we need to release the energy stored in our food to power the pumping of our heart and allow us to move, we need to release that energy rapidly. We cannot afford to wait hours nr days for the energy-releasing reactions to occur. 

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. 

Notice that the word spontaneous has a different meaning in thermodynamics than it does in everyday speech. Ordinarily, spontaneous refers to an event that takes place without any effort or premeditation. For example, a crowd cheers spontaneously for an outstanding performance. In thermodynamics, spontaneous refers only to the natural direction of a process, without regard to whether it occurs rapidly and easily. Chemical kinetics, which we introduce in Chapter 15, describes the factors that determine the speeds of chemical reactions. Thermodynamic spontaneity refers to the direction that a process will take if left alone and given enough time. 

Whether a reaction is spontaneous or not depends on thermodynamics. The cocktail of chemicals and the variety of chemical reactions possible depend on the local environmental conditions temperature, pressure, phase, composition and electrochemical potential. A unified description of all of these conditions of state is provided by thermodynamics and a property called the Gibbs free energy, G. Allowing for the influx of chemicals into the reaction system defines an open system with a change in the internal energy dt/ given by ... 

The most important clue to explain the essence of chemical reaction has probably been the discovery of catalysis, without which a chemical industry would not exist. Catalysis is not understood in detail but it clearly dictates the course of a reaction by re-inforcing the spontaneous fluctuations that occur in a reaction medium. The most dramatic effects are produced in autocatalysis where a reaction product acts as a catalyst. Such feedback... 

The infinite potential barrier, shown schematically in figure 10 corresponds to a superselection rule that operates below the critical temperature [133]. Above the critical temperature the quantum-mechanical superposition principle applies, but below that temperature the system behaves classically. The system bifurcates spontaneously at the critical point. The bifurcation, like second-order phase transformation is caused by some interaction that becomes dominant at that point. In the case of chemical reactions the interaction leads to the rearrangement of chemical bonds. The essential difference between chemical reaction and second-order phase transition is therefore epitomized by the formation of chemically different species rather than different states of aggregation, when the symmetry is spontaneously broken at a critical point. 

Note that the interference of chemical reactions may alter the effective rate constant of the secondary reaction and break the independence principle of elementary chemical reactions. Moreover, under these conditions non-spontaneous reactions may proceed, which are eliminated in parallel and consecutive reactions. 

Chemical reactions, like the metathesis reaction that produces lead (II) iodide, can occur spontaneously, just like physical processes. It was once believed that only exothermic processes occurred spontaneously however, it has been shown that many endothermic reactions can occur spontaneously as well. Another factor that must be considered when determining the spontaneity of a reaction is entropy. 

Michael Faraday (1791-1867) was the first to realize that electrochemical processes are stoichiometrically associated with straightforward chemical reactions. Consider the cell of Figure 3.1.5. The two electrode reactions of the cell have been represented previously in Equations (2a) and (7). Thus, the spontaneous overall chemical reaction is... 

Once the thermod3mamics of chemical reaction is determined as spontaneous, the reaction kinetics will establish the importance of this reaction to the degradation of the ceramic powder in the solvent. Reaction kinetics of this t3rpe between a solid and a (liquid) fluid were discussed in Chapter 5. Under some conditions the reaction kinetics are very slow, limited by either a slow surface reaction or a slow product layer diflusion. As a result, this reaction can be n ected in its importance to the ceramic powder s chemical stability. Unfortunately little information is found in the literature on the reaction kinetics for ceramic powders reacting with organic solvents. Therefore, trial and error seems to be the only dependable way to determine the chemical stability of ceramic powders in nonaqueous solvents. This is the way that the chemical decomposition of YBa2Cu3Q,. in alcohols was determined. 

This chapter develops the thermodynamic methods for predicting whether a reaction is spontaneous, and Chapter 14 uses these results to determine the equilibrium ratio of products and reactants. Chapter 18 discusses the rates of chemical reactions. Manipulating conditions to optimize the yield of chemical reactions in practical applications requires the concepts from all three chapters. 

When processes are conducted at constant T and P, the criteria for spontaneity and for equilibrium are stated more conveniently in terms of another state function called the Gibbs free energy (denoted by G), which is derived from S. Because chemical reactions are usually conducted at constant T and constant P, their thermodynamic description is based on AG rather than AS. This chapter concludes by restating the criteria for spontaneity of chemical reactions in terms of AG. Chapter 14 shows how to identify the equilibrium state of a reaction, and calculate the equilibrium constant from AG. 

Of the thousands of chemical reactions that make up metabolism, virtually none would occur without an appropriate catalyst, or enzyme. Even a reaction such as the combination of carbon dioxide with water to form carbonic acid, which has the rare distinction of taking place spontaneously at a significant rate, is catalyzed by an enzyme (carbonic anhydrase), which accelerates the reaction more than a million-fold. 

From a study of the examples mentioned and many more cases, we come to the following conclusion Exothermicity favors the spontaneity of a reaction bnt does not guarantee it. Just as it is possible for an endothermic reaction to be spontaneous, it is possible for an exothermic reaction to be nonspontaneous. In other words, we cannot decide whether or not a chemical reaction will occur spontaneously solely on the basis of energy changes in the system. To make this kind of prediction we need another thermodynamic qnantity, which turns ont to be entropy. 

The problem of spontaneity, as outlined above, occupied many eminent scientists in the latter half of the nineteenth century. In the case of chemical reactions carried out under the usual conditions of constant temperature and pressure, it was initially assumed that if and only if a reaction gave out heat, it occurred spontaneously. It turns out that a second factor is involved, and that both factors must be considered. By considering a number of examples in which AH is negative, or zero, or positive, this second factor, which is best described as a tendency to randomness , will become more familiar. 

The first law of thermodynamics considers the enthalpy of chemical reactions. The second law states that the universe spontaneously tends toward increasing disorder or randomness. 

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