Temperature Dependence Of Equilibrium Constants Research Articles

Modeling Differential Enthalpy of Absorption of CO2 with Piperazine as a Function of Temperature.

Temperature-dependent correlations for equilibrium constants (ln K) and heat of absorption (ΔHabs) of different reactions (i.e., deprotonation, double deprotonation, carbamate formation, protonated carbamate formation, dicarbamate formation) involved in the piperazine (PZ)/CO2/H2O system have been calculated using computational chemistry based ln K values input to the Gibbs–Helmholtz equation. This work also presents an extensive study of gaseous phase free energy and enthalpy for different reactions using composite (G3MP2B3, G3MP2, CBS-QB3, and G4MP2) and density functional theory [B3LYP/6-311++G(d,p)] methods. The explicit solvation shell (ESS) model and SM8T solvation free energy coupled with gaseous phase density functional theory calculations give temperature-dependent reaction equilibrium constants for different reactions. Calculated individual and overall reaction equilibrium constants and enthalpies of different reactions involved in CO2 absorption in piperazine solution are compared against experimental data, where available, in the temperature range 273.15–373 K. Postcombustion CO2 capture (PCC) is a temperature swing absorption–desorption process. The enthalpy of the solution directly correlates with the steam requirement of the amine regeneration step. Temperature-dependent correlations for ln K and ΔHabs calculated using computational chemistry tools can help evaluate potential PCC solvents’ thermodynamics and cost-efficiency. These correlations can also be employed in thermodynamic models (e.g., e-UNIQUAC, e-NRTL) to better understand postcombustion CO2 capture solvent chemistry.

Thermodynamic analysis of strontium deoxidizing ability in liquid iron at presence of aluminum

Phase diagram of the ternary oxide system FeO - SrO -Al2O3 was constructed for the first time. In this system, the following compounds can be formed: hercynite FeAl2O4 and five strontium aluminates - Sr4Al2O7 , Sr3Al2O6 , SrAl2O4 , SrAl4O7 , SrAl12O19 . According to the calculations performed, solid solutions of oxides are not formed in the system, as it is confirmed by the literature data. In the course of modeling, the optimal energy parameters of the theory of subregular ionic solutions were selected for the components of the oxide melt (FeO, SrO, Al2O3 ). Thermodynamic analysis of strontium deoxidizing ability in liquid iron at presence of aluminum was carried out using the technique for constructing the surface of solubility of strontium and aluminum in metal for steelmaking temperatures (1550 and 1600 °C) and carbon concentrations of 0.1 and 0.4 %. The equilibrium constants of the reactions of formation of strontium aluminates Sr3Al2O6 and SrAl2O4 from the components of the metal melt were calculated for the temperature range of 1550 - 1650 °C. It was found that the rest of strontium aluminates can be formed in liquid metal only at temperatures above 1750 °C. The base of thermodynamic data for the studied systems is given: temperature dependences of equilibrium constants for reactions occurring between components; values of interaction parameters of the first order (according to Wagner) for elements in liquid iron; values of energy parameters of the theory of subregular ionic solutions (for oxide melt). It follows from the calculations that the formation of strontium monoaluminate SrAl2O4 and corundum Al2O3 is most probable as the interaction products in Fe -Al - Sr - O and Fe -Al - Sr - C - O systems.

Hetero Diels-Alder Reactions with a Dicationic Urea Azine Derived Azo Dienophile and Their Use for the Synthesis of an Electron-Rich Pentacene.

Herein, the first hetero Diels–Alder (DA) reactions with a stable, dicationic urea azine derived azo dienophile, synthesized by two‐electron oxidation of a neutral urea azine are reported. Several charged DA products were synthesized in good yield and fully characterized. The DA adduct of anthracene is in thermal equilibrium with the reactants at room temperature, and the reaction enthalpy and entropy were determined from the temperature‐dependent equilibrium constant. Furthermore, base addition to solutions of the pentacene DA product led to deprotonation, cleavage of the N−N bond, and formation of an electron‐rich 6,13‐bisguanidinyl‐substituted pentacene. The redox and optical properties of this new pentacene derivative were studied. Furthermore, the dication resulting from its two‐electron oxidation was synthesized and fully characterized. The results disclose a new elegant route to electron‐rich pentacene derivatives.

Standard Gibbs energy of metabolic reactions: VI. Glyceraldehyde 3-phosphate dehydrogenase reaction

Glycolysis is a very central metabolic pathway for many organisms because it represents a key component in their energy production. For this reason, it has always been an extensively studied pathway. The glyceraldehyde 3-phosphate dehydrogenase (GDH) reaction is an important reaction of glycolysis yielding nicotinamide adenine dinucleotide (NADH). The aim of this work is to investigate the thermodynamics of the GDH reaction and determine the standard Gibbs energy of reaction ΔRg'0 and standard enthalpy of reaction ΔRh'0. Currently, so-called ‘standard’ data exist in the literature that depend on the conditions they were measured at. In this work, ΔRg'0 and ΔRh'0 values were determined that are independent from reaction conditions by accounting for the activity coefficients of the reacting substances. Therefore, the equation of state electrolyte Perturbed-Chain Statistical Associating Fluid Theory (ePC-SAFT) was used. The required ePC-SAFT parameters were taken from literature or fitted to new experimental osmotic coefficients. A value of ΔRg'0 = 51.5 ± 0.4 kJ mol−1 was determined at 298.15 K. This value deviates by up to 10 kJ mol−1 from existing literature values, caused by activity coefficients in the reaction medium. It can be used to determine the Gibbs energy of reaction ΔRg', which will allow statements concerning the feasibility of the GDH reaction. Further, a method is presented to predict influences of pH, initial substrate concentration and Mg2+ concentration on the reaction equilibrium. Finally, we measured the standard reaction enthalpy for the GDH reaction ΔRh'0 by titration calorimetric measurements (ΔRh'0 = 4.6 ± 0.1 kJ mol−1). This value was within van ’t Hoff evaluated ΔRh'0 (9 ± 16 kJ mol−1) using temperature-dependent equilibrium constants from equilibrium measurements corrected by ePC-SAFT predicted activity coefficients.

Thermodynamic Analysis of Liquid Steel Refining by Complex La–Ce–Al-Based Alloy

Alloys containing additions of rare-earth metals (REMs) are increasingly used in the production of steels and alloys. After modification and microalloying of steel with REMs, the structure of a primary metal becomes finer, its mechanical characteristics are improved, and the oxygen concentration dissolved in liquid iron decreases significantly. The size of the forming nonmetallic inclusions does not exceed several microns in size, and they have a predominantly globular shape. Due to relatively high cost of REMs, it is proposed to calculate their consumption at each technological process stage to determine optimal amount required for deoxidation or modification. In this work, thermodynamic modeling of phase equilibrium in liquid metal of the Fe–La–Ce–Al–O system has been performed. We present a thermodynamic database of this system: temperature dependences of equilibrium constants found in reactions between components, the first-order parameters of interaction (according to Wagner) for elements in liquid iron, the energy parameter values of the theory of subregular solutions (for oxide melt) and regular ionic solutions (for solid solution). Based on coordinates of the calculated isothermal (1600°C) isostructural (0.01 wt % Al) section found on the solubility surface of metal components (SSSM) of the Fe–La–Ce–Al–O system, we have plotted the consumption diagrams of La and Ce for various initial oxygen concentrations (metal is pre-deoxidized with aluminum). The consumption of expensive REMs used in microalloying and metal modification has been shown to strongly depend on the initial metal composition.

Standard Gibbs energy of metabolic reactions: V. Enolase reaction

The glycolytic pathway is one of the most important pathways for living organisms, due to its role in energy production and as supplier of precursors for biosynthesis in living cells. This work focuses on determination of the standard Gibbs energy of reaction ΔRg′0 of the enolase reaction, the ninth reaction in the glycolysis pathway. Exact ΔRg′0 values are required to predict the thermodynamic feasibility of single metabolic reactions or even of metabolic reaction sequences under cytosolic conditions. So-called “apparent” standard data from literature are only valid at specific conditions. Nevertheless, such data are often used in pathway analyses, which might lead to misinterpretation of the results. In this work, equilibrium measurements were combined with activity coefficients in order to obtain new standard values ΔRg′0 for the enolase reaction that are independent of the cytosolic conditions. Reaction equilibria were measured at different initial substrate concentrations and temperatures of 298.15 K, 305.15 K and 310.15 K at pH 7. The activity coefficients were predicted using the equation of state electrolyte Perturbed-Chain Statistical Associating Fluid Theory (ePC-SAFT). The ePC-SAFT parameters were taken from literature or fitted to new experimentally determined osmotic coefficients and densities. At 298.15 K and pH 7, a ΔRg′0(298.15 K, pH 7) value of −2.8 ± 0.2 kJ mol−1 was obtained. This value differs by up to 5 kJ mol−1 from literature data. Reasons are the poorly defined “standard” conditions and partly undefined reaction conditions of literature works. Finally, using temperature-dependent equilibrium constants and the van ‘t Hoff equation, the standard enthalpy of reaction of ΔRh′0(298.15 K, pH 7) = 27 ± 10 kJ mol−1 was determined, and a similar value was found by quantum-chemistry calculations.

Visible light spectroscopic analysis of Methylene Blue in water; the resonance virtual equilibrium hypothesis

After a critical approach to the centennial literature on Methylene Blue self-aggregation in water, accurate visible-light absorption spectra and thermodynamic equilibrium constants for the monomeric and dimeric forms of this dye in water at different temperatures were obtained by means of a novel procedure based on the Beer-Lambert equation. A thermo-solvatochromic effect was clearly detected in the monomer spectra, which we believe to be the main source of inaccuracy in all previous works. The only published explanation for this phenomenon, the existence of a colorless form of the monomer, is refuted here. Instead, a new notion has been introduced; the resonance virtual equilibrium hypothesis. According to this hypothesis the temperature-dependent absorption behavior of the monomer is provoked by the change in its electron charge distribution with the variation in the temperature-dependent dielectric constant of water. The monomer charge distribution stands between those of the virtual resonance forms, being the absorption spectrum of the monomer a composition of the theoretical spectra for the virtual mesomers, whose proportion is established by a temperature-dependent equilibrium constant. The application of this hypothesis allowed the virtual equilibrium constants and spectra for the mesomers to be evaluated with remarkable accuracy.

Adsorption enthalpy of lead(II) and phenol on coals and activated carbon in the view of thermodynamic analysis and calorimetric measurements

Comparison among adsorption enthalpies of thermodynamic analysis and these of direct calorimetric measurements is reported for sorption of lead(II) and phenol on natural coals and activated carbon. Thermodynamic calculations came out from developments of equilibrium adsorption isotherms recorded at 303K, 333K and 353K. Standard adsorption enthalpies ΔH0 were calculated from temperature dependences of equilibrium constants of adsorption Ka, with values of Ka being evaluated directly from experimental data and from fitted courses of the Langmuir and Freundlich isotherms. In addition, dependences of isosteric enthalpy on surface coverage were evaluated. Values of the exothermic enthalpies (ranging from −6kJmol−1 to −50kJmol−1) were obtained, however exclusive endothermic effect for lead(II) adsorption on a sample of natural coal (24kJmol−1) was also ascertained. Direct calorimetric measurements using flow and immersion techniques then proved that all the studied adsorptions proceed with exothermic effects from −2kJmol−1 to −42kJmol−1. It was found that experimental adsorption enthalpies correspond better to mean isosteric enthalpies in the interval 0–1molkg−1 of the surface coverages than to the standard adsorption enthalpies ΔH0 of the thermodynamic calculations.

Melting phase relations in the systems Mg2SiO4–H2O and MgSiO3–H2O and the formation of hydrous melts in the upper mantle

High-pressure and high-temperature melting experiments were conducted in the systems Mg2SiO4–H2O and MgSiO3–H2O at 6 and 13GPa and between 1150 and 1900°C in order to investigate the effect of H2O on melting relations of forsterite and enstatite. The liquidus curves in both binary systems were constrained and the experimental results were interpreted using a thermodynamic model based on the homogeneous melt speciation equilibrium, H2O+O2−=2OH−, where water in the melt is present as both molecular H2O and OH− groups bonded to silicate polyhedra. The liquidus depression as a function of melt H2O concentration is predicted using a cryoscopic equation with the experimental data being reproduced by adjusting the water speciation equilibrium constant. Application of this model reveals that in hydrous MgSiO3 melts at 6 and 13GPa and in hydrous Mg2SiO4 melts at 6GPa, water mainly dissociates into OH− groups in the melt structure. A temperature dependent equilibrium constant is necessary to reproduce the data, however, implying that molecular H2O becomes more important in the melt with decreasing temperature. The data for hydrous forsterite melting at 13GPa are inconclusive due to uncertainties in the anhydrous melting temperature at these conditions. When applied to results on natural peridotite melt systems at similar conditions, the same model infers the presence mainly of molecular H2O, implying a significant difference in physicochemical behaviour between simple and complex hydrous melt systems. As pressures increase along a typical adiabat towards the base of the upper mantle, both simple and complex melting results imply that a hydrous melt fraction would decrease, given a fixed mantle H2O content. Consequently, the effect of pressure on the depression of melting due to H2O could not cause an increase in the proportion, and hence seismic visibility, of melts towards the base of the upper mantle.

PEaCH4 v.2.0: A modelling platform to predict early diagenetic processes in marine sediments with a focus on biogenic methane – Case study: Offshore Namibia

The modelling of early diagenetic processes in marine sediments is of interest in marine science, and in the oil and gas industry, here, especially with respect to methane occurrence and gas hydrate formation as resources. Early diagenesis in marine sediments evolves from a complex web of intertwining (bio)geochemical reactions. It comprises microbially catalysed reactions and inorganic mineral–water–gas interactions. A model that will describe and consider all of these reactions has to be complex. However, it should be user-friendly, as well as to be applicable for a broad community and not only for experts in the field of marine chemistry. The presented modelling platform PeaCH4 v.2.0 combines both aspects, and is Microsoft Excel©–based. The modelling tool is PHREEQC (version 2), a computer programme for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. The conceptual PEaCH4 model is based on the conversion of sediment-bound degradable organic matter. PEaCH4 v.2.0 was developed to quantify and predict early diagenetic processes in marine sediments with the focus on biogenic methane formation and its phase behaviour, and allows carbon mass balancing. In regard to the irreversible degradation of organic matter, it comprises a “reaction model” and a “kinetic model” to predict methane formation. Both approaches differ in their calculations and outputs as the “kinetic model” considers the modelling time to integrate temperature dependent biogenic methane formation in its calculations, whereas the “reaction model” simply relies on default organic matter degradation. With regard to the inorganic mineral–water–gas interactions, which are triggered by irreversible degradation of organic matter, PEaCH4 v.2.0 is based on chemical equilibrium thermodynamics, appropriate mass-action laws, and their temperature dependent equilibrium constants. The programme is exemplarily presented with the example of upwelling sediments off Namibia, ODP Leg 175, Site 1082. The application demonstrates that the modelling platform PEaCH4 v.2.0 provides a user-friendly, but complex scientific tool that delivers retraceable information about early diagenetic processes and products in marine sediments.

Evidence for SiO 2 ‐NaCl complexing in H 2 O‐NaCl solutions at high pressure and temperature

Experimental studies reveal complex dissolution behavior of quartz in aqueous NaCl solutions at high temperature and pressure, involving variation from salting-in to salting-out that changes with temperature, pressure, and salt concentration. The behavior is not explainable by traditional electrostatic theory. An alternative hypothesis appeals to complexing of SiO2 with NaCl and can explain the observations. However, the hypothesis of complexing, as previously applied, is inadequate in several respects: it neglects polymerization of solute silica, regards the SiO2-NaCl hybrid complex(es) as anhydrous, which seems unlikely, and invokes an incorrect stoichiometry of the hydrated silica monomer, now known to be Si(OH)42H2O. These neglected features can be incorporated into the complexing model in a revised formulation based on a simple thermodynamic analysis using existing quartz solubility data. The analysis leads to a quasi-ideal solution model with silica monomers, dimers, and two distinct hydrous SiO2-NaCl hybrid complexes with overall NaCl:H2O = 1:6, one Na-bearing and one Cl-bearing. Their (equal) molar concentrations (Xhc) are governed by a pressure- and temperature-dependent equilibrium constant, Khc ¼ X 2=ðaNacla 62OÞ, where aNacl and aH2O are the respective activities of the solvent components. The stability of the hybrid complexes (i.e., their concentration) is very sensitive to H2O activity. The entire set of experimental quartz-solubility data at 700°C, 1–15 kbar, is reproduced with high fidelity by the expression logKhc ¼� 4:585 þ 0:2691P � 2:023 � 10 � 3 P 2 (P is pressure in kbar), including the transition from low-pressure salting-in to high pressure salting-out. The results indicate that hybrid SiO2-NaCl complexes are the main hosts for dissolved silica at NaCl concentrations greater than 6 wt%, which are likely common in crustal fluids. At higher temperatures, approaching the critical end point in the system SiO2-H2O, the model becomes progressively inaccurate, probably because polymers higher than the dimer become significant as SiO2 concentration

Carbon speciation in silicate-C-O-H melt and fluid as a function of redox conditions: An experimental study, in situ to 1.7 GPa and 900 C

Carbon speciation in and partitioning among silicate-saturated C-O-H fluids and (C-O-H)-saturated melts have been determined ~1.7 GPa and 900 °C under reducing and oxidizing conditions. The measurements were conducted in situ while the samples were at the conditions of interest. The solution equilibria were (1) 2CH 4 + Q n = 2CH 3 − + H 2 O + Q n+1 and (2) 2CO 3 2− + H 2 O + 2Q n+1 = HCO 3 − + 2Q n , under reducing and oxidizing conditions, and where the superscript, n , in the Q n -species denotes number of bridging oxygen in the silicate species (Q-species). The abundance ratios, CH 3 /CH 4 and HCO 3 − /CO 3 2− , increase with temperature. The enthalpy change associated with the species transformation differs for fluids and melts and also for oxidized and reduced carbon [Reducing: Δ H (1) fluid = 16 ± 5 kJ/mol, Δ H (1) melt = 50 ± 5 kJ/mol; oxidizing Δ H (2) fluid = 81 ± 14 kJ/mol]. For the exchange equilibrium of CH 4 and CH 3 species between fluid and melt, the temperature-dependent equilibrium constant, (X CH 4 /X CH 3 ) fluid /(X CH 4 /X CH 3 ) melt , yields Δ H = 34 ± 3 kJ/mol. Increased abundance ratios, CH 4 /CH 3 and HCO 3 − /CO 3 2− , lead to increased polymerization of silicate+(C-O-H) melt. Because of such relations, melt transport properties (e.g., viscosity) and element partition coefficients between magmatic liquids, C-O-H fluids, and crystalline phases can vary by more than 100% with speciation changes of C-bearing volatiles upper mantle. These structure effects are more pronounced the higher the pressure and the more mafic the magma.

Temperature Dependent Enthalpy of CO2 Absorption for Amines and Amino Acids from Theoretical Calculations at Infinite Dilution.

The thermodynamics of carbamate formation from the reaction of amines or amino acids with CO2 in post combustion CO2 capture processes is studied in this work using computational chemistry calculations. Density functional theoretical calculations are coupled with the implicit and explicit solvation shell model presented by da Silva et al. [da Silva, E. F.; Svendsen, H. F.; Merz, K. M. J Phys Chem A 2009, 113, 6404] to calculate equilibrium constants for the reaction. These calculated temperature dependent equilibrium constants are used to derive temperature dependent heats of absorption for the carbamate formation reaction of amines and amino acids using the Gibbs Helmholtz equation. The computationally derived temperature dependent equilibrium constants and heats of absorption are compared against experimental results.

ExoMol line lists – III. An improved hot rotation-vibration line list for HCN and HNC

A revised rotation-vibration line list for the combined hydrogen cyanide (HCN) / hydrogen isocyanide (HNC) system is presented. The line list uses {\it ab initio} transition intensities calculated previously (Harris et al., ApJ, 2002, 578, 657) and extensive datasets of recently measured experimental energy levels (Mellau, J. Chem. Phys. and J. Mol. Spectrosc. 2010-2011). The resulting line list has significantly more accurate wavelengths than previous ones for these systems. An improved value for the separation between HCN and HNC is adopted leading to an approximately 25%\ lower predicted thermal population of HNC as a function of temperature in the key 2000 to 3000 K region. Temperature-dependent partition functions and equilibrium constants are presented. The line lists are validated by comparison with laboratory spectra and are presented in full as supplementary data to the article and at \url{www.exomol.com}.

Estimation and application of the thermodynamic properties of aqueous phenanthrene and isomers of methylphenanthrene at high temperature

Estimates of standard molal Gibbs energy (ΔGf°) and enthalpy (ΔHf°) of formation, entropy (S°), heat capacity (CP°) and volume (V°) at 25°C and 1bar of aqueous phenanthrene (P) and 1-, 2-, 3-, 4- and 9-methylphenanthrene (1-MP, 2-MP, 3-MP, 4-MP, 9-MP) were made by combining reported standard-state properties of the crystalline compounds, solubilities and enthalpies of phenanthrene and 1-MP, and relative Gibbs energies, enthalpies and entropies of aqueous MP isomers from published quantum chemical simulations. The calculated properties are consistent with greater stabilities of the β isomers (2-MP and 3-MP) relative to the α isomers (1-MP and 9-MP) at 25°C. However, the metastable equilibrium values of the abundance ratios 2-MP/1-MP (MPR) and (2-MP+3-MP)/(1-MP+9-MP) (MPI-3) decrease with temperature, becoming <1 at ∼375–455°C. The thermodynamic model is consistent with observations of reversals of these organic maturity parameters at high temperature in hydrothermal and metamorphic settings. Application of the model to data reported for the Paleoproterozoic Here’s Your Chance (HYC) Pb–Zn–Ag ore deposit (McArthur River, Northern Territory, Australia) indicates a likely effect of high-temperature equilibration on reported values of MPR and MPI-3, but this finding is contingent on the location within the deposit. If metastable equilibrium holds, a third aromatic maturity ratio, 1.5×(2-MP+3-MP)/(P+1-MP+9-MP) (MPI-1), can be used as a proxy for oxidation potential. Values of logaH2aq determined from data reported for HYC and for a sequence of deeply buried source rocks are indicative of more reducing conditions at a given temperature than those inferred from data reported for two sets of samples exposed to contact or regional metamorphism. These results are limiting-case scenarios for the modeled systems that do not account for effects of non-ideal mixing or kinetics, or external sources or transport of the organic matter. Nevertheless, quantifying the temperature dependence of equilibrium constants of organic reactions enables the utilization of organic maturity parameters as relative geothermometers at temperatures higher than the nominal limits of the oil window.

Ligand Coordination and Spin Crossover in a Nickel Porphyrin Anchored to Mesoporous TiO2 Thin Films

The coordination and spin equilibrium of a Ni(II) meso-tetra(4-carboxyphenyl)porphyrin compound, NiP, was quantified both in fluid solution and when anchored to mesoporous, nanocrystalline TiO2 thin films. This comparison provides insights into the relative rate constants for excited-state injection and ligand field population. In the presence of pyridine, the spectroscopic data were consistent with the presence of equilibrium concentrations of a 4-coordinate low-spin S = 0 ((1)A1g) Ni(II) compound and a high-spin S = 1 ((3)B1g) 6-coordinate compound. Temperature-dependent equilibrium constants were consistently smaller for the surface-anchored NiP/TiO2, as were the absolute values of ΔH and ΔS. In the presence of diethylamine (DEA), the ground-state 6-coordinate compound was absent, but evidence for it was present after pulsed light excitation of NiP. Arrhenius analysis of data, measured from -40 to -10 °C, revealed activation energies for ligand dissociation that were the same for the compound in fluid solution and anchored to TiO2, Ea = 6.6 kcal/mol, within experimental error. At higher temperatures, a significantly smaller activation energy of 3.5 kcal/mol was found for NiP(DEA)2/TiO2. A model is proposed wherein the TiO2 surface sterically hinders ligand coordination to NiP. The lack of excited-state electron transfer from Ni(II)P*/TiO2 indicates that internal conversion to ligand field states was at least 10 times greater than that of excited-state injection into TiO2.

Kinetics of the Reversible Reaction of CO2(aq) and HCO3– with Sarcosine Salt in Aqueous Solution

Aqueous sarcosine salts are fast carbon dioxide (CO(2)) absorbents suitable for use in postcombustion CO(2) capture in coal-fired power plants. We have developed a detailed reaction scheme including all the reactions in the sarcosine-CO(2)-water system. All unknown rate and equilibrium constants were obtained by global data fitting. We investigated the temperature-dependent rate and equilibrium constants of the reaction between aqueous CO(2) and sarcosine using stopped-flow spectrophotometry, by following the pH changes over the wavelength range 400-700 nm via coupling to pH indicators. The corresponding rate and equilibrium constants ranged from 15.0 to 45.0 °C and were analyzed in terms of Arrhenius, Eyring, and van't Hoff relationships. The rate constant for the reaction between CO(2) and sarcosine to form the carbamate at 25.0 °C is 18.6(6) × 10(3) M(-1) s(-1), which is very high for an acyclic amine; its activation enthalpy is 59(1) kJ mol(-1) and the entropy is 33(4) J mol(-1) K(-1). In addition, we investigated the slow reaction between bicarbonate and sarcosine using (1)H nuclear magnetic resonance spectroscopy and report the corresponding rate and equilibrium constants at 25.0 °C. This rate constant is 5.9 × 10(-3) M(-1) s(-1).

Thermodynamic study of complex formation process of free base meso-tetraphenylporphyrins with dimethyl and dibutyltin(IV) dichloride: a new algorithm for a single thermometric titration

A novel, fast and easy single sample measurement has been developed based upon temperature dependence of equilibrium constant in order to determine the enthalpy and entropy changes of a complexation reaction using spectrophotometric temperature titration. The method can be used in determination of the formation constant and thermodynamic parameters of the solutions that there are difficulties in their titration where volatile compounds are studying. Knowledge of component spectra is not required for the analysis. The formation constants of the interactions of s-di and tri-brominated meso-tetraphenylporphyrins, and meso-tetrakis(4-methylphenyl) and (4-methoxyphenyl) porphyrins with Me2SnCl2 and Bu2SnCl2, have been determined in range of 0–25 °C utilizing van’t Hoff relation, mass balance and equilibrium constant equations by an iterative least squares method with ΔH0 as adjustable parameter. The outputs of analysis are the equilibrium constants, ligand and adduct spectral profiles, their concentrations as a function of temperature, the adjusted values of the standard enthalpy ΔH0, and entropy ΔS0 changes. The order of formation constants of the resulting 1:1 complexes decreased with increasing number of bromide substituents and increased with adding methyl and methoxy groups, and vary as H2T(4-CH3O)PP > H2T(4-CH3)PP > H2TPP > H2TPPBr2 > H2TPPBr3 and Me2SnCl2 > Bu2SnCl2.

Thermodynamic Investigation of the Formation of Complexes between Norfloxacin and Various Mononucleotides

The fluorescence of norfloxacin was quenched by various nucleotides. The ratio of the fluorescence intensities in the absence and presence of nucleotide was linearly dependent on nucleotide concentration, suggesting that quenching occurred through the formation of nonfluorescent norfloxacin-nucleotide complexes. The gradient of the linear relationship represented the equilibrium constant of complex formation; it decreased with increasing temperature. The slopes of van’t Hoff plots constructed from the temperature-dependent equilibrium constants were positive in all cases, indicating that complex formation was energetically favorable – i.e., exothermic, with negative Gibb’s free energy. The equilibrium constant increased when triphosphate was used instead of monophosphate. It also increased when the oxygen at the C'2 position of the nucleotide was removed. Both enhancements were due to entropic effects: entropy decreased when complexes with AMP or GMP formed, while it increased when norfloxacin complexed with ATP, GTP, dAMP and dGMP.

A calorimetric study of carbamate formation

Post combustion capture of CO 2 (PCC) is currently one of the leading technologies for the reduction of green house gas emissions from power plants. The most common PCC process is based on the absorption of CO 2 into aqueous amine solutions. CO 2 absorption involves several parallel reactions including hydration of CO 2; deprotonation of carbonic acid; protonation of the amine; and formation of carbamate. The extent to which each reaction proceeds is dependent on the conditions of absorption (or desorption), as well as the associated equilibrium constants. All reactions other than carbamate formation have been extensively investigated previously. The investigation of carbamate formation is more complex, as it cannot be studied in the absence of other, simultaneous reactions. In particular, the enthalpy of carbamate formation has been determined previously only from the temperature dependence of equilibrium constants, but this methodology is not robust. In this contribution, we use calorimetry and advanced model-based data analysis methods for the unravelling of the thermo-chemistry relevant to PCC and specifically directly determine the reaction enthalpy for carbamate formation. The reaction enthalpies of carbamate formation and amine protonation were measured in dilute aqueous solution at 298 K using isothermal titration calorimetry (ITC) for monoethanolamine (MEA), diethanolamine (DEA), and ammonia (NH 3). The enthalpy of protonation was also measured for carbonate and bicarbonate. The re-determined protonation constants of the carbonate species and the three amines are in excellent agreement with previously reported results. No measured enthalpies of carbamate formation have been reported previously. For the carbamate formation reaction HCO 3 - + R ′ RNH ↔ R ′ RNCO 2 - , the following reaction enthalpies were determined: MEA, −29.7 ± 0.1 kJ/mol; DEA, −23.7 ± 0.9 kJ/mol, and NH 3, −27.6 ± 0.9 kJ/mol. The results are in good agreement with, but much more precise and robust than estimated values reported in the literature.