Free Energy Change Of Reaction Research Articles

Catalytic influence of the environment on outer-sphere electron-transfer reactions in aqueous solutions

The continuous-flow method with integrating observation (CFMIO) has been used to investigate irreversible electron-transfer (ET) reactions between negatively charged, substitution-inert transition metal complexes. Special attention has been paid in order to distinguish between the different contributions to the energy of activation such as size of reactants, long-range charge interactions, influence of the free energy of reactions (difference in redox potential) and the composition (electrolytic content) of the solutions. We selected the ET reaction between Fe(CN)6Hx–4x and IrCl2–6 to demonstrate the catalytic rate enhancement caused by the addition of mono-, di- and tri-valent cations.Increasing protonation of Fe(CN)6Hx–4x decreases the rate of ET; strong association with M2+ and M3+(where M indicates the metal) has no catalytic effect. All alkali-metal ions show an increasing catalytic effect with increasing size; the four tetra-alkylammonium ions show the opposite. The Arrhenius plot of the above-mentioned ET reactions in the presence of cations is strongly curved; the decreasing slope at higher temperatures indicates a complex reaction mechanism.In the ET reaction between silvertetraphenylporphyrin tetrasulphonate and IrCl2–6, ET occurs at the axial position of the complex, far away from the negatively charged sulphonate groups. A catalytic effect similar to that found in the reaction with Fe(CN)4–6 is observed; this result precludes the cations having a bridge-like function during ET. Monovalent cations of varying size show a maximum rate enhancement when their ionic radius is ca. 0.23 nm. If long-range Coulomb interactions are shielded, and a situation in which the free-energy change of reaction is zero is simulated, we extrapolate a maximum for the ET rate constant of 1011 dm3 mol–1 s–1.

Excited-state electron-transfer quenching by a series of water photoreduction mediators

Rate constants k q for the quenching of the excited state of Ru(bipy) 3 2+ by a series of viologen salts having different redox potential E 1 2 have been determined in deaerated aqueous solutions at pH = 5 by laser flash photolysis. The k q values are found to decrease with increasing — E 1 2 and to correlate with the reaction free-energy change Δ G. Such a correlation is shown to be consistent with the Rehm—Weller model for electron-transfer reactions.

ChemInform Abstract: A TEMPERATURE‐JUMP STUDY OF ELECTRON‐TRANSFER REACTIONS INVOLVING ION RADICALS‐ REACTIONS OF THE CATION RADICALS OF PHENOTHIAZINE AND AROMATIC P‐DIAMINES

AbstractEs wird die Elektronenübertragung z.B. zwischen Phenothiazin und N,N,N′,N′‐Tetramethylbzw.

On the free-energy changes in the synthesis and degradation of nucleic acids.

Standard free-energy changes for reactions involving single- and double-stranded nucleic acids have been related to that for polynucleotide synthesis from ribonucleoside diphosphates for which deltaG degrees' approximately O. For polynucleotide formation from triphosphates this quantity is about -1 kcal. In the replication reaction the base pairing interactions are quantitatively of comparable importance. Production of a hydrolytic break in a double strand is substantially less favorable than in a single strand. The resealing of breaks utilizing ATP and NAD+ have similar free-energy changes and are entropy driven processes. The highly exergonic hydrolysis of pyrophosphate is maintained to be of significance for both in vivo and in vitro polymerizations.

Thermochemical production of hydrogen via multistage water splitting processes

This paper presents and reviews the fundamental thermodynamic principles underlying thermochemical water splitting processes. The overall system is considered first and the temperature limitation in process thermal efficiency is developed. The relationship to an ideal water electrolysis cell is described and the nature of efficient multistage reaction processes is discussed. The importance of the reaction entropy change and the relation of the reaction free energy change to the work of separation is described. A procedure for analyzing thermochemical water splitting processes is presented and its use to calculate individual state efficiency is demonstrated. A number of processes are used to illustrate the concepts and procedures.

Joint destruction and chondrocalcinosis in patients with generalised osteoarthrosis.

<h3>Abstract</h3> Microbial acclimation to different temperature conditions can involve broad changes in cell composition and metabolic efficiency. A systems-level view of these metabolic responses in non-mesophilic organisms, however, is currently missing. In this study, thermodynamically-constrained genome-scale models were applied to simulate the metabolic responses of a deep-sea psychrophilic bacterium, <i>Shewanella psychrophila</i> WP2, under suboptimal (4°C), optimal (15°C), and supraoptimal (20°C) growth temperatures. The models were calibrated with experimentally determined growth rates of WP2. Gibbs free energy change of reactions (<i>Δ_rG’</i>), metabolic fluxes, and metabolite concentrations were predicted using random simulations to characterize temperature-dependent changes in the metabolism. The modeling revealed the highest metabolic efficiency at the optimal temperature, and it suggested distinct patterns of ATP production and consumption that could lead to the lower metabolic efficiency under suboptimal or supraoptimal temperatures. The modeling also predicted rearrangement of fluxes through multiple metabolic pathways, including the glycolysis pathway, Entner-Doudoroff pathway, tricarboxylic acid (TCA) cycle, and the electron transport system, and these predictions were corroborated through comparisons to WP2 transcriptomes. Furthermore, predictions of metabolite concentrations revealed the potential conservation of reducing equivalents and ATP in the suboptimal temperature, consistent with experimental observations from other psychrophiles. Taken together, the WP2 models provided mechanistic insights into the metabolism of a psychrophile in response to different temperatures. <h3>Importance</h3> Metabolic flexibility is a central component of any organism’s ability to survive and adapt to changes in environmental conditions. This study represents the first application of thermodynamically-constrained genome-scale models in simulating the metabolic responses of a deep-sea psychrophilic bacterium to varying temperatures. The models predicted differences in metabolic efficiency that were attributed to changes in metabolic pathway utilization and metabolite concentration during growth under optimal and non-optimal temperatures. Experimental growth measurements were used for model calibration, and temperature-dependent transcriptomic changes corroborated the model-predicted rearrangement of metabolic fluxes. Overall, this study highlights the utility of modeling approaches in studying the temperature-driven metabolic responses of an extremophilic organism.

A Temperature-Jump Study of Electron-Transfer Reactions Involving Ion Radicals: Reactions of the Cation Radicals of Phenothiazine and Aromatic p-Diamines

Abstract The reversible one-electron-transfer reactions of the following cation radicals were studied by temperature-jump and stopped-flow methods: phenothiazine (PhZ)/N,N,N′,N′-tetramethyl-p-phenylenediamine cation (TMPD\underset.+), PhZ/N,N-dimethyl-p-phenylenediamine cation (DMPD\underset.+), DMPD/TMPD\underset.+ and DMPD/N,N,N′,N′-tetramethylbenzidine cation (TMBZ\underset.+). The solvent used was either 1 : 3 ethanol-water or acetonitrile. For PhZ/TMPD\underset.+, PhZ/DMPD\underset.+ in ethanol-water and also for DMPD/TMPD\underset.+ in acetonitrile, the backward reaction corresponding to the negative free energy change of reaction (ΔF°&amp;lt;0) proceeds almost at the diffusion controlled rate. The backward rate constant for DMPD/TMPD\underset.+ in ethanol-water, (6±1)×108 M−1 s−1, is slightly less than a diffusion controlled one even for ΔF°&amp;lt;0. The results are discussed as compared with the previous study on 7,7,8,8-tetracyanoquinodimethane/substituted-p-benzoquinone anions.

Some Thermodynamic Properties of Zn3As2, Cd3As2, and ZnP2

By analogy with the known mode of vaporization of , and , the Knudsen cell pressure measurements on of Nesmeyanov et al. between 601° and 751°K have been reinterpreted according to the reaction The derived free energy change for this reaction, given by is consistent with the fragmentary thermochemical data for . In addition, the free energy change for the dissociation reaction has been estimated as Finally, the entropies of fusion of and have been evaluated by applying the Clausius‐Clapeyron equation to recent data, yielding 28.7 and 29.5 eu/g‐mole, respectively.

Thermodynamics of silicon sulphides and zinc sulphide

The equilibrium constant and standard free energy change for the reactions: 2 ZnS(s)⇌2 Zn(g)+ S2(g)(1), ZnS(s)+ Si(s)⇌Zn(g)+ SiS(g)(2) have been measured around 1000°C by a transportation method. The following free energies were obtained ΔG°= 178 900–92.4 T cal mol–1 1293 K (1b), ΔG°= 101 700–65.7 T cal mol–1 1293 K. (2b) By combination, the free energy change for reaction (3) is obtained: 2Si(s)+ S2(g)= 2 SiS(g)(3), ΔG°= 24 500–39.0 T cal mol–1 Combined with results of other investigators the free energy of formation of SiS2(s, l, g) is estimated.

Free energy changes and metabolic regulation in steady-state photosynthetic carbon reduction

The standard physiological free energy changes of reactions of glycolysis, the reductive pentose phosphate cycle (photosynthetic carbon reduction cycle) and the oxidative pentose phosphate cycle have been calculated from available data. The concentrations of metabolites of the photosynthetic carbon reduction cycle, measured during steady-state photosynthesis in Chlorella pyrenoidosa in the presence of radioactive tracers, and the concentrations of some intermediates of the oxidative pentose phosphate cycle measured during a subsequent dark period, have been employed to calculate the free energy changes of each reaction of the reductive cycle and of some of the reactions of the oxidative cycle during steady-state light and dark conditions. With respect to the magnitude of the negative free energy change, at steady state, such reactions have been found to be of two types. Those with high negative free energy changes (−6 to −11 kcal) are in each case reactions from which there exists independent evidence of a role in metabolic regulation. Those with small negative free energy changes (o to −2 kcal) are not regulated reactions and are highly reversible. Thus most of the negative free energy change occurring under steady-state conditions in this metabolic system is dissipated for purposes of control. By the criterion of negative free energy change, ribulosediphosphate carboxylase, and fructose- and heptosediphosphatases are regulated enzymes. The activities of these enzymes are known to be high in the light and low in the dark. Phosphoribulokinase, which mediates the one reaction with an intermediate negative free energy change (−3.82 kcal) also may be a regulated enzyme with greater activity in the light than in the dark. In the oxidative cycle, the reaction mediated by glucose-6-phosphate dehydrogenase has a very high negative free energy change and appears to be active in the dark and inactive in the light. One function of these controls is thought to be the exclusive operation of the reductive cycle in the light and the oxidative cycle in the dark.

The Transport of Gallium Arsenide in the Vapor Phase by Chemical Reaction

The heat contents and entropies have been derived as a function of temperature for gallium arsenide and the gaseous species which exist in equilibrium with it, in the temperature range 800°–1200°K when iodine or hydrogen chloride and hydrogen are also present. The important gaseous species are shown to be , , , , , I2, I, , As4, and H2. Under certain conditions the species As2, , and Ga (liq.) may also be present. The free energy changes for reactions among the above species are derived. They are shown to be consistent with those isothermal equilibria that have been directly measured. The free energy changes are used, together with the conservation equations for nondepositing components (I, Cl, H, and As in excess of the stoichiometric ) to give the partial pressures of all the important species at any temperature in a temperature gradient in a constant pressure system. The results are applicable to the calculation of the flux of gallium arsenide in open flow or closed tube (diffusive transport) systems, and they are in good agreement with experimentally measured rates of transport in the system in an open flow apparatus.

Some Semiempirical Methods for Estimating the Entropies of Oxides, Silicates, and the Prediction of Reaction Equilibria and Yields

Several methods are reviewed for computing the entropies of crystalline inorganic oxides, silicates, and titanates. A method relating entropies with molecular weights is used for oxides. The entropies of silicates and titanates may be estimated with the sums of the entropies of oxides with which they may be considered to be formed. Calculations are suggested to obtain free energy changes for reactions in systems of ceramic materials. Free energy calculations will allow for the prediction of reaction equilibria and yields. An example is given for the reaction at 1100°C. between calcia and silica to form calcium silicate.