Thermodynamics thermochemical cycles

Moreover, the use of heat-flow calorimetry in heterogeneous catalysis research is not limited to the measurement of differential heats of adsorption. Surface interactions between adsorbed species or between gases and adsorbed species, similar to the interactions which either constitute some of the steps of the reaction mechanisms or produce, during the catalytic reaction, the inhibition of the catalyst, may also be studied by this experimental technique. The calorimetric results, compared to thermodynamic data in thermochemical cycles, yield, in the favorable cases, useful information concerning the most probable reaction mechanisms or the fraction of the energy spectrum of surface sites which is really active during the catalytic reaction. Some of the conclusions of these investigations may be controlled directly by the calorimetric studies of the catalytic reaction itself. 

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 ... 

DuBois et al. carried out extensive studies on the thermodynamic hydricity of a series of metal hydrides [13, 15-19]. The determination of thermodynamic hydricity generally requires several measurements (coupled with known thermochemical data) to constitute a complete thermochemical cycle. As with other thermodynamic cycles, obtaining reliable values in an appropriate solvent can be a difficult challenge, and this is sometimes coupled with problems in obtaining reversible electrochemical data. Scheme 7.2 illustrates an example in which the hydricity of cationic monohydrides have been determined. 

Eq. (8) requires determination of the two-electron oxidation potential of L M by electrochemical methods. When combined with the two-electron reduction of protons in Eq. (9), the sum provides Eq. (10), the AGh- values of which can be compared for a series of metal hydrides. Another way to determine the AGh-entails the thermochemical cycle is shown in Scheme 7.3. This method requires measurement of the K of Eq. (11) for a metal complex capable of heterolytic cleavage of H2, using a base (B), where the pK., of BH+ must be known in the solvent in which the other measurements are conducted. In several cases, Du-Bois et al. were able to demonstrate that the two methods gave the same results. The thermodynamic hydricity data (AGh- in CH3CN) for a series of metal hydrides are listed in Table 7.4. Transition metal hydrides exhibit a remarkably large range of thermodynamic hydricity, spanning some 30 kcal mol-1. 

Thermodynamic properties related to RsSiH can be obtained from negative-ion gas-phase studies. The following thermochemical cycle (cf. Scheme 2.1) ... 

These values were compared with independent estimates of the °roor/ro ,ro values from thermochemical cycles, where data were available to evaluate them. In the case of di-cumyl peroxide, for example, the E° obtained experimentally differs from the result of a thermodynamic calculation by only 30 mV. It is of interest to note that the uncorrected a data would have led to °roor/rovro values only slightly negative to the corrected ones (0.06-0.07 V). The good agreement in these cases was used as the basis to support the use of the convolution approach to estimate °roor/rovro for systems where the necessary values for thermochemical estimates are not available. This has been particularly useful in the study of endoperoxides and was used to estimate the standard reduction potential of the antimalarial agent, artemisinin. ... 

Thermochemical cycles can also be used to provide us with information on the thermodynamic properties of compounds with metals in unusual oxidation states that... 

M—H bond dissociation energies, 1, 287 photochemistry, 1, 251 single crystal neutron diffraction, 1, 578 stability toward disproportionation, 1, 301 Metal—hydrogen bonds bond dissociation energy in 1,2-dichloroethane, 1, 289 stable metal hydrides in acetonitrile, 1, 287 thermochemical cycle, 1, 286 in THF and dichloromethane, 1, 289 olefin insertion thermodynamics, 1, 629 in Zr(IV) bis-Cp complexes, 4, 878 Metal—hydrogen hydricity data, 1, 292... 

Galvez M. E. Halmann M. Steinfeld A. Ammonia production via a two-step A1203/A1N thermochemical cycle. 1. Thermodynamic, environmental, and economic analyses. Ind. 

The purpose of this article is to study the viability of the copper chloride thermochemical cycle by studying the hydrolysis reaction of CuCl2 which is not favoured thermodynamically. To better understand the occurrence of possible side reactions, together with a good control of the kinetics of the hydrolysis reaction, the use of optical absorption spectrometries, UV visible spectrometry to detect molecular chlorine which may be formed in side reactions, FTIR spectrometry to follow the concentrations of H20 and HCl is proposed. 

Initial thermodynamic and experimental studies have found a new thermochemical cycle based on uranium. The operating conditions are mild and most of the steps are commercially used in the uranium processing industry. For several of the process steps, there are multiple process options. Additional analysis and experimental work is required before engineering viability (versus scientific proof of principle) can be determined with reasonably credible estimates of efficiencies and economics. 

Despite the importance of keto/enol tautomers [57] only a small amount of work has been devoted to the study of enol radical cations in condensed phase. This is directly related to the fact that simple enols as the thermodynamically less stable tautomers [58] are usually not isolable, sinre the kinetic barrier for ketonization is rather low [59,60]. Much more is known about the chemistry of enol and keto radical cations in the gas-phase [61]. For details the reader is referred to recent comprehensive reviews [62]. The only available data on the thermodynamics of enol/keto radical cations in solution stem from a recent study [63]. Using stable dimesityl substituted enols the relative stabilities were determined by a thermochemical cycle approach. 

Irrespective of the electrophone system involved in one-electron transfer-initiated bond dissociation one can easily derive the thermodynamic driving force for a such process by use of thermochemical cycle calculations [15], Such estimates are particularly valuable as experimental numbers, because bond-dissociation data are scarce. 

We have applied the thermochemical cycle that results in Eq. 13 to a wide range of organotransition-metal hydrides that have thermodynamic acidities suitable for de-... 

Thermochemical cycles, and in particular the Sulphur-Iodine cycle, have been the subject of renewed intense interest in the last years. The accurate evaluation of their industrial potential is difficult, as it involves many aspects, from scientific questions such as the knowledge of thermodynamic data to safety, acceptability and economic assessments. 

These results (both in kcal mol 1, Chap. 2) permit one to construct a thermochemical cycle with one unknown AhyiH, shown in Fig. 1.1 as a dashed arrow. By the first law of thermodynamics, the closed path around the cycle has AH = 0, so... 

There are several ways of converting H29% to Af//29g and subsequently to Ahyd//298. The atomization method is illustrated in Fig. 3.9, which shows a thermochemical cycle for determination of Af//29g of a hydrocarbon. The top horizontal line represents the thermodynamic state of nuclei and electrons, the bottom horizontal line represents elements in their standard states and the verticals, of which there are six, represent enthalpy changes. Each of the three total enthalpy changes in the top half of the figure H29i represents a fall from the top state to the state of... 

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