Born-Haber process

SCFs will find applications in high cost areas such as fine chemical production. Having said that, marketing can also be an issue. For example, whilst decaffeina-tion of coffee with dichloromethane is possible, the use of scCC>2 can be said to be natural Industrial applications of SCFs have been around for a long time. Decaffeination of coffee is perhaps the use that is best known [16], but of course the Born-Haber process for ammonia synthesis operates under supercritical conditions as does low density polyethylene (LDPE) synthesis which is carried out in supercritical ethene [17]. 

However, in the Born-Haber process, the commercial production of ammonia, the formation of the partially reduced intermediates requires high temperature and pressure. Remarkably, by acquiring energy from ATP hydrolysis and coupling the reaction with hydrogen (H2) formation,... 

Let us consider the formation of sodium chloride from its elements. An energy (enthalpy) diagram (called a Born-Haber cycle) for the reaction of sodium and chlorine is given in Figure 3.7. (As in the energy diagram for the formation of hydrogen chloride, an upward arrow represents an endothermic process and a downward arrow an exothermic process.)... 

When this analysis was first attempted [9-11] very few values of 1 had been obtained from series limits in the third spectra of the lanthanides, and the first comprehensive sets were calculated from Born-Haber cycles [9]. Subsequent spectroscopic values [12] confirmed the early work and are plotted in Eig. 1.1. In all cases they refer to the ionization process... 

Determination of Lattice Breakup Energies from Experimental Data The process of lattice breakup can be split into individual steps for which the energies can be measured. Thus, breaking up the NaCl lattice to form free ions in the gas phase can be described (with a Born-Haber cycle) as... 

The enthalpy of formation of a compound is a so-called thermodynamic state function, which means that the value depends only on the initial and final states of the system. When the formation of crystalline NaCl from the elements is considered, it is possible to consider the process as if it occurred in a series of steps that can be summarized in a thermochemical cycle known as a Born-Haber cycle. In this cycle, the overall heat change is the same regardless of the pathway that is followed between the initial and final states. Although the rate of a reaction depends on the pathway, the enthalpy change is a function of initial and final states only, not the pathway between them. The Born-Haber cycle for the formation of sodium chloride is shown as follows ... 

From the standpoint of energy, the processes of separating the crystal lattice and solvating the ions can be related by means of a thermochemical cycle of the Born-Haber type. For an ionic compound MX, the cycle can be shown as follows ... 

Although a great deal is known about some of the processes, some catalytic processes or at least some of the steps in the process may not be completely understood. It is rather like a Born-Haber cycle that is used to represent the process by which a metal and a halogen are converted into a metal halide. 

Figure 3.8 Born-Haber cycle constructed to obtain the lattice enthalpy A//(E, lce) of sodium chloride. All arrows pointing up represent endothermic processes and arrows pointing down represent exothermic processes (the figure is not drawn to scale)...

The relationships between the two different states and between the enthalpy of formation from the elements at standard state (H°) and the lattice energy (U) are easily understood by referring to the Born-Haber-Fayans thermochemical cycle. In this cycle, the formation of a crystalline compound from isolated atoms in the gaseous state is visualized as a stepwise process connecting the various transformations. Let us follow the condensation process of a crystal MX formed from a metal M and a gaseous molecule X2 ... 

Students have already / v / employed Hess s law when performing Born-Haber cycle calculations in Chapter 6. 8.9 Hess s Law Now that we ve discussed in general terms the energy changes that occur during chemical reactions, let s look at a specific example in detail. In particular, let s look at the Haber process, the industrial method by which approximately 13 million tons of ammonia are produced each year in the United States, primarily for use as fertilizer. The reaction of hydrogen with nitrogen to make ammonia is exothermic, with AH0 = -92.2 kj. 

Now let us estimate the low boundary, Wiow, of the Ore gap in molecular liquids. Because the Ore process is just an electron-transfer reaction, we assume that no rearrangement of molecules occurs and, therefore, the final positronium state will be quasi-free (formation of the bubble requires much longer time). The corresponding Born-Haber cycle is the following ... 

However, this simple chemical equation conceals a more complicated sequence of events in which the reactants undergo various transformations before the product is formed. These may be summarized in a Born-Haber thermodynamic cycle (Figure 3.1). The first stage of the reaction process is the conversion of M and E into gaseous state atoms, requiring an enthalpy of atomization of A// j(M), and, if E is a solid or liquid, the enthalpy of vaporization, of E or... 

Formation of ionic compounds from the elements appears to be one of the simpler overall reactions, but can also be written as a series of steps adding up to the overall reaction. The Born-Haber cycle is the process of considering the series of component reactions that can be imagined as the individual steps in compound formation. For the example of lithium fluoride, the first five reactions added together result in the sixth overall reaction. 

During this period, Wilson Dam was built on the Tennessee River at Muscle Shoals, Alabama, to furnish the power needed for the production of synthetic ammonia by the Haber process. This project was undertaken to insure a domestic source of nitrates for producing munitions and fertilizers. When the Tennessee Valley Authority was organized in 1933, the dam and facilities became a part of the Authority and thus the National Fertilizer Development Center at Muscle Shoals was born. 

In order for an ionic compound to dissolve, the Madelung energy or electrostatic attraction between the ions in the lattice must be overcome. In a solution in which the ions are separated by molecules of a solvent with a high dielectric constant (e,., o = 8l.7e ) the attractive force will be considerably less. The process of solution of an ionic compound in water may be considered by a Born-Haber type of cycle. The overall enthalpy of the process is the sum of two terms, the enthalpy of dissociating the ions from the lattice (the lattice energy) and the enthalpy of introducing the dissociated ions into the solvent (the solvation energy) ... 

Hess s law states that the enthalpy or a reaction is the same whether the reaction takes place in one or several steps it is a necessary consequence of the first law of thermodynamics concerning the conservation of energy. If this were not true, one could manufacture energy by an appropriate cyclic process. Bom and Haber10 applied Hess s law to the enthalpy of formation of an ionic solid. For the formation of an ionic crystal from the elements, the Born-Haber cycle may most simply be depicted as... 

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