Thermodynamically allowed

Redox reactions are particularly instructive. If all thermodynamically allowed reactions in liquid NH3 were kinetically rapid, then no oxidizing agent more powerful than N2 and no reducing agent more powerful than H2 could exist in this solvent. Using data for solutions at 25° ... 

One can write for Eq. (7-49) an expression for the equilibrium constant. Statistical thermodynamics allows its formulation in terms of partition functions ... 

Examples of the utility and need for new, advanced-performance materials are numerous. For example, in turbine engines today, the need is to be able to increase operating temperatures by 100—150 °C. The laws of thermodynamics allow significant fuel efficiency to be gained as the temperature increases. However, the material, particularly for the turbine blades, must be able to handle these increased temperatures. This material need illustrates the connection between product application and material performance. 

Thus far we have explored the field of classical thermodynamics. As mentioned previously, this field describes large systems consisting of billions of molecules. The understanding that we gain from thermodynamics allows us to predict whether or not a reaction will occur, the amount of heat that will be generated, the equilibrium position of the reaction, and ways to drive a reaction to produce higher yields. This otherwise powerful tool does not allow us to accurately describe events at a molecular scale. It is at the molecular scale that we can explore mechanisms and reaction rates. Events at the molecular scale are defined by what occurs at the atomic and subatomic scale. What we need is a way to connect these different scales into a cohesive picture so that we can describe everything about a system. The field that connects the atomic and molecular descriptions of matter with thermodynamics is known as statistical thermodynamics. 

The second law of thermodynamics allows us to represent the entropy change of a... 

The metal hydride bond is stronger than a metal carbon bond and the insertion of carbon monoxide into a metal hydride is thermodynamically most often uphill. Alkene insertion into a metal hydride is thermodynamically allowed and often reversible. 

Monsanto developed the rhodium-catalysed process for the carbonylation of methanol to produce acetic acid in the late sixties. It is a large-scale operation employing a rhodium/iodide catalyst converting methanol and carbon monoxide into acetic acid. At standard conditions the reaction is thermodynamically allowed,... 

Lipids with a suitable hydrophilic-lipophilic balance (HLB) are known to spread on the surface of water to form monolayer films. It is obvious that if the lipid-like molecule is highly soluble in water, it will disappear into the bulk phase (as observed for SDS). Thus, the criteria for a monolayer formation are that it exhibits very low solubility in water. The alkyl part of the lipid points away from the water surface. The polar group is attracted to the water molecules and is inside this phase at the surface. This means that the solid crystal, when placed on the surface of water, is in equilibrium with the him spread on the surface. A detailed analysis of this equilibrium has been given in the literature (Gaines, 1966 Adamson and Gast, 1997 Birdi, 2009). The thermodynamics allows one to obtain extensive physical data on this system. It is thus apparent that, by studying only one monolayer of the substance, the effect of temperature can be very evident. 

A powerful theoretical tool for cosmochemical models is thermodynamics. This formalism considers a system in a state of equilibrium, a consequence of which is that observable properties of a system undergo no net change with time. (We offer a somewhat more rigorous discussion of thermodynamics in Chapter 7). The tools of thermodynamics are not useful for asking questions about a system s evolutionary history. However, with the appropriate equations, we are able to estimate what a system at equilibrium would look like under any environmental conditions. The methods of thermodynamics allow the use of temperature and pressure, plus the system s bulk composition, to predict which minerals will be stable and in what relative amounts they will be present. In this way, the thermodynamic approach to a cosmochemical system can help us measure its stability and predict the direction in which it will change if environmental parameters change. 

The photochemical dimerization of unsaturated hydrocarbons such as olefins and aromatics, cycloaddition reactions including the addition of 02 ( A ) to form endoperoxides and photochemical Diels-Alders reaction can be rationalized by the Woodward-Hoffman Rule. The rule is based on the principle that the symmetry of the reactants must be conserved in the products. From the analysis of the orbital and state symmetries of the initial and final state, a state correlation diagram can be set up which immediately helps to make predictions regarding the feasibility of the reaction. If a reaction is not allowed by the rule for the conservation of symmetry, it may not occur even if thermodynamically allowed. 

Biochemical transformations of organic compounds are especially important because many reactions, although thermodynamically feasible, occur extremely slowly due to kinetic limitations. For example, we might be interested in the question of whether benzene can be biodegraded under naturally occurring methanogenic conditions (see Illustrative Example 17.1). Such natural attenuation of this toxic aromatic substance may be thermodynamically allowed under the perceived conditions. But these conditions may not be accurate (e.g., the benzene and methane chemical activities in the system). Also other environmental factors may cause the rate to be unobservably slow. One possibility is that the relevant microorganisms are simply not active in the environment of interest. 

The foregoing discussion has neglected most of the details of thermochemical properties of the adsorbed species, for example, tacitly taking the entropy of the surface species Ss to be zero. Statistical thermodynamics allows a more rigorous treatment of surface processes such as these, discussed next. 

Because energy underlies all chemical change, thermodynamics—the study of the transformations of energy—is central to chemistry. Thermodynamics explains why reactions occur at all. It also lets us predict the heat released or required by chemical reactions. Heat output is an essential part of assessing the usefulness of compounds as fuels and foods, and the first law of thermodynamics allows us to discuss these topics systematically. The material in this chapter provides the foundation for the following chapters, in particular Chapter 7, which deals with the driving force of chemical reactions—why they occur and in which direction they can be expected to go. 

When 97 is sufficiently dilute its polymerization is no longer thermodynamically allowed, but, in the presence of 8, a mixture of cyclic dimers, cc (84%), ct (14%) and tt (2%), is readily formed, in equilibrium with the monomer. The cc dimer has been isolated and its crystal structure determined247. These cyclic dimers can also be made by metathesis degradation of the polymers or by ADMET of diallyldiphenylsilane (Section VILA.6). 

The first and the second law of thermodynamics allow the description of a reversible fuel cell, whereas in particular the second law of thermodynamics governs the reversibility of the transport processes. The fuel and the air are separated within the fuel cell as non-mixed gases consisting of the different components. The assumption of a reversible operating fuel cell presupposes that the chemical potentials of the fluids at the anode and the cathode are converted into electrical potentials at each specific gas composition. This implies that no diffusion occurs in the gaseous phases. The reactants deliver the total enthalpy J2 ni Hi to the fuel cell and the total enthalpy J2 ni Hj leaves the cell (Figure 2.1). 

The C60H36 hydride contains 4.76 mass% of hydrogen. It can be used as hydrogen accumulator because in the temperature of 400-600 K the hydrogenation-dehydrogenation cycles are thermodynamically allowable. The thermodynamic parameters and equilibrium hydrogen pressures for the reaction... 

Equilibrium and nonequilibrium thermodynamics can be combined to give a complete thermodynamic description of a process or process step. Equilibrium thermodynamics allows us to calculate the changes in thermodynamic properties with the change in process conditions. Nonequilibrium thermodynamics allows us to calculate unambiguously the work that is lost associated with the process taking place. It relates this loss to the process s flows and forces driving these flows and identifies the various friction factors as a function of the relationship between flows and forces. 

We turn to Figure 6.2, which shows a specific mass, let us say 1 mol, of air that is out of equilibrium with the environment only with respect to its temperature T, which has been chosen to be larger than T0, the temperature of the environment. As has been shown before in this chapter, Equation 6.11, thermodynamics allows us to calculate the minimum amount of work required to bring this mass out of equilibrium with the environment Weired Weired... 

In Figure 6.2, we studied an arbitrary mass of air that was out of equilibrium with its environment because its temperature T > T0. We mentioned that thermodynamics allows us to calculate the minimum amount of work, Wr " / that it takes to create this nonequilibrium situation. Well considered, this amount of air creates a structure within its environment, in which a different... 

Table 2.1 lists equilibrium ratios for the reduction of selected metal oxides [4], while Figure 2.2 provides a complete phase diagram for the reduction of iron oxide at different temperatures [3, 5], In order to reduce bulk iron oxide to metallic iron at 600 K, the water content of the hydrogen gas above the sample must be below a few percent, which is easily achieved. However, in order to reduce Cr2C>3, the water content should be as low as a few parts per billion, which is much more difficult to realize. The data in Table 2.1 also illustrate that, in many cases, only partial reduction to a lower oxide may be expected. Reduction of Mn2C>3 to MnO is thermodynamically allowed at relatively high water contents, but further reduction to manganese is unlikely. 

The results of nucleophilic substitutions in solvated ionic aromatic cations (RX+) are strongly dependent on the nature of the halogen and on the solvent cluster size. The energetics of these systems have been studied and it has been shown that the reactions are thermodynamically allowed for each solvent, even for the 1-1 complexes. The observed different behavior related to cluster size is probably governed by kinetic reasons (existence of barriers to the reactions). Two types of substitution reaction have been identified—leading either to X radical or to HX molecule formation. 

---