Enthalpy Change Of Reaction Research Articles

Thermal behavior and decomposition kinetics of cinnarizine under isothermal and non-isothermal conditions

Cinnarizine (CIN) is an anti-histaminic drug which is mainly used for the control of vomiting due to motion sickness. The thermal behavior of CIN was investigated using thermogravimetry/derivative thermogravimetry and differential thermal analysis. The kinetic parameters were evaluated by isothermal and non-isothermal conditions including Ozawa’s conventional method, Ozawa–Flynn–Wall and Friedman isoconversional methods. The kinetic parameters were determined using the thermogravimetric curves of the decomposition process. The activation energy values obtained were 88.38 and 90.12 kJ mol−1 for the isothermal and non-isothermal conditions, respectively, with the conventional method. The activation energy values obtained by isoconversional method were practically equal; hence, the reaction involves a single step. Finally, the values of entropy (∆S), enthalpy (∆H), and Gibbs free energy change (∆G) of decomposition reaction were calculated and compared.

Determining Students’ Conceptual Understanding Level of Thermodynamics

Science students find heat, temperature, enthalpy and energy in chemical reactions to be some of the most difficult subjects. It is crucial to define their conceptual understanding level in these subjects so that educators can build upon this knowledge and introduce new thermodynamics concepts. This paper reports conceptual understanding levels of high school students of common thermodynamics subjects such as heat, temperature, enthalpy and energy changes in chemical reactions. The sample of the study was composed of 418 randomly selected students from 11 different high schools. A cross sectional survey model was employed in this study. Data were collected through the Conceptual Understanding Test (CUT), which consists of 16 questions. Cronbach's alpha reliability coefficient for the test was 0.72. Besides, interviews were conducted with 10 students to get qualitative data about their beliefs on the subject. The results of the study indicated that students’ level of conceptual understanding in thermodynamics concepts is very low. It was observed to be especially low for the concepts of (1) relationship between energy, enthalpy and bonds in chemical reactions, (2) energy and catalysts in chemical reactions, (3) changes in heat, temperature and enthalpy during change of state, and (4) the relationship between heat, temperature, mass and specific heat. Most of the students in this study do not fully understand common thermodynamics concepts although they have been studying these topics since primary school.

Temperature Aspect of CH3OH effect on the rate of cyclohexene hydrocarbomethoxylation catalyzed by the Pd(OAc)2–PPh3–p-toluenesulfonic acid system

In the temperature range of 353–368 K, quantitative characteristics of the effect of the CH3OH concentration on cyclohexene hydrocarbomethoxylation catalyzed by the Pd(OAc)2–PPh3–p-toluenesulfonic acid system are determined. It is shown that, in the temperature range covered, the dependence of the reaction rate on the CH3OH concentration passes through a maximum. The results are interpreted in terms of a catalytic cycle involving hydride–, alkyl–and acyl–palladium complexes of cationic type as intermediates, supplemented by ligand exchange reactions that transfer part of the catalyst into an inactive form. The effective constants for the earlier obtained kinetic equation are estimated. Based on the temperature dependences of the effective constants and the activated complex theory, the effective activation energies, as well as the enthalpy, entropy, and Gibbs energy changes for the ligand exchange reactions between the Pd(CH3OH)2(PPh3)2 and Pd(CO)2(PPh3)2 complexes are estimated. It is established that, at 373 K, this reaction is close to the state of equilibrium.

Thermodynamic Analysis of the Experimental Equilibria for the Liquid-Phase Etherification of Isobutene with C1 to C4 Linear Primary Alcohols

The chemical equilibrium of the liquid-phase syntheses of 2-methoxy-2-methylpropane (MTBE), 2-ethoxy-2-methylpropane (ETBE), 2-methyl-2-propoxypropane (PTBE), and 1-tert-butoxybutane (BTBE) by reaction of isobutene with methanol, ethanol, 1-propanol, and 1-butanol, respectively, has been studied. Four different ion exchange resins as the catalysts, and two different reactor systems, namely, a batch reactor and a setup of tubular reactors, were used. Temperature and pressure were in the range 313–383 K and 1.5–2.0 MPa, respectively. MTBE and ETBE synthesis reactions experiments were carried out mainly to validate the reliability of the reaction systems. Experiments in PTBE and BTBE etherifications allowed estimating thermodynamic properties for those reactions and involved species, namely, molar standard enthalpy and entropy changes of reaction and molar enthalpy change of formation of the four ethers. A comparison of estimated reaction thermodynamic values among the homologous series of linear alcohols, and with results quoted in the literature, when available, has been made.

Dissociation Enthalpies of Chloride Adducts of Nitrate and Nitrite Explosives Determined by Ion Mobility Spectrometry.

The kinetics for thermal dissociations of the chloride adducts of the nitrate explosives 1,3-dinitroglycerin (1,3-NG), 1,2-dinitroglycerin (1,2-NG), the nitrite explosive 3,4-dinitrotoluene (3,4-DNT), and the explosive taggant 2,3-dimethyl-2,3-dinitrobutane (DMNB) have been studied by atmospheric pressure ion mobility spectrometry. Both 1,3-NG·Cl(-) and1,2-NG·Cl(-) decompose in a gas-phase SN2 reaction in which Cl(-) displaces NO3(-) while 3,4-DNT·Cl(-) and DMNB·Cl(-) decompose by loss of Cl(-). The determined activation energy (kJ mol(-1)) and pre-exponential factor (s(-1)) values for the dissociations respectively are 1,3-NG·Cl(-), 86 ± 2 and 2.2 × 10(12); 1,2-NG·Cl(-), 97 ± 2 and 3.5 × 10(12); 3,4-DNT·Cl(-), 81 ± 2 and 4.8 × 10(13); and DMNB·Cl(-), 68 ± 2 and 9.7 × 10(11). Calculations by density functional theory show the structures of the nitrate ester adducts involve three hydrogen bonds: one from the hydroxyl group and the other two from the two nitrated carbons. The relative Cl(-) dissociation energies of the nitrates together with the previously reported smaller value for glycerol trinitrate and the calculated highest value for glycerol 1-mononitrate are explicable in terms of the number of hydroxyl hydrogen bond participants. The theoretical enthalpy changes for the nitrate ester displacement reactions are in agreement with those derived from the experimental activation energies but considerably higher for the nitro compounds.

A thermodynamic study of interaction of Ag+, Mg2+, Ca2+, and K+ cations with 4-hydroxyphenyl-2,5-bis(2-benzofuranyl)pyridine in some binary mixed non-aqueous solvents

In the present work the complexation process between Ag+ and Mg2+ cations and 4-hydroxyphenyl-2,5-bis(2-benzofuranyl)pyridine (HBFPY) ligand was studied in pure dimethylformamide (DMF), ethanol (EtOH), acetonitrile (AN) and in (DMF-EtOH), (AN-EtOH) and (DMF-AN) binary mixed solvent solutions at different temperatures using the conductometric method. Also in this work the complexation reaction between Ca2+, K+ cations and HBFPY ligand, was studied in pure dimethylformamide (DMF), propanol (PrOH), 1,4-dioxane (DOX), ethanol (EtOH) and in DMF-PrOH, DMF-DOX and DMF-EtOH binary mixed solvent solutions at different temperatures using the conductometric method. The conductance data show that the stoichiometry of the complexes formed between this ligand and the studied cations is 1 : 1 [ML]. In most cases, addition of HBFPY to solutions of these cations, causes a continuous increase in the molar conductivities which indicates that the mobility of complexed cations is more than the uncomplexed ones. The stability constants of the complexes were obtained from fitting of molar conductivity curves using a computer program, GENPLOT. The stability constant of [Mg(HBFPY)]2+ complex in various neat solvents at 15°C decreases in order: EtOH > DMF > AN and the stability constant of [Ag(HBFPY)]+ complex in various neat solvents at 35°C decreases in order: DMF > EtOH. The values of standard enthalpy changes (ΔH°c) for complexation reactions were obtained from the slope of the Van’t Hoff plots and the changes in standard entropy (ΔS°c) were calculated from the relationship ΔH°c,295.15= ΔH°c–298.15ΔS°c.

Computational study on stereoselective synthesis of substituted 1H-tetrazoles via a click reaction: DFT and QTAIM approaches

In the present research, the observed stereoselectivity in catalyst-free click synthesis of 1H-tetrazole-5-yl-acrylonitriles has been modeled by means of density functional theory (DFT) employed to investigate the structural and thermochemical aspects of this synthesis in the gas and solution phases. Comparison of our calculated structural parameters of the title compound with the available X-ray crystallographical data demonstrated a reliable agreement. Then, the effect of five different solvents, benzene, chloroform, ethanol, methanol and water was examined via polarized continuum model (PCM) calculations, showing a significant decrease in the computed values of the reaction enthalpy and free energy changes compared with the gas phase results and also the thermodynamical preference in using water solution in comparison with nonpolar solvents such as benzene and other protic and aprotic polar ones such as methanol, ethanol and chloroform that is in confirmation with the experimental observations. Additionally, we have concentrated on origins of the observed stereoselectivity by means of topological electronic indices via quantum theory of atoms in molecule (QTAIM) approach. In this respect, we have assessed the variation of electronic density and its Laplacian at some selected bond critical points of stereoisomer products, their corresponded transition states and intermediate and also construe the experimental elucidations.

Effects of functional group modification on the thermal properties of nano-carbon clusters

In this paper, the thermal properties including thermal stability, thermal decomposition activation energy and the thermal enthalpy of nano-carbon clusters (NCCs, including fullerene[60](C60, with a diameter of 0.71 nm), multi-walled carbon nanotubes(MWCNTs, with a diameter of 10–30 nm and a length of 1–2 µm), single-walled carbon nanotubes (SWCNTs, with a diameter of ~2 nm and a length of 5–15 µm), ligands of NCC-based terpyridine (NCC-tpy), and NCC-based ruthenium complexes (NCC-tpyRuCl3) were systematically studied by method of simultaneous thermogravimetric and differential thermal analysis. The results show that the modification of NCCs with terpyridine leads to a decrease in the thermal stability and in the thermal decomposition activation energy (the thermal decomposition activation energy decreased from 174.4 for C60, 144.9 for MWCNTs and 161.2 kJ/mol for SWCNTs to 166.2 for C60-tpy, 119.7 for MWCNT-tpy and 85.0 kJ/mol for SWCNT-tpy). But the modification of NCCs with terpyridine results in an increase in the enthalpy change of NCC thermal decomposition reaction. The introduction of the metal ions through complexation further decreases the thermal stability and the thermal decomposition activation energy of NCC-tpyRuCl3 due to the catalytic oxidation of Ru(III) ions (the activation energy decreases to 124.1 for C60-tpyRuCl3, 106.4 for MWCNT-tpyRuCl3 and 41.2 kJ/mol for SWCNT-tpyRuCl3). The introduction of the metal ions also leads to a decrease in the enthalpy change of the thermal decomposition reaction.

Thermodynamic equilibrium for the dehydration of 1-butanol to di-n-butyl ether

The thermodynamic equilibrium of the bimolecular dehydration of 1-butanol to di-n-butyl ether (DNBE) and water in the liquid phase was studied. Equilibrium experiments were performed at 4MPa and in the temperature range of 413–463K over the ion exchange resin Amberlyst-70. The thermodynamic equilibrium for the side reactions (dehydration to 1-butene, olefins isomerization, olefins hydration and branched ether formation) was also studied. The equilibrium constant for the dehydration reaction of 1-butanol to di-n-butyl ether and water was found to be independent of the operating temperature, within the limits of the experimental error (±5.2%). The experimental equilibrium constants at 413–463K allows to estimate the standard enthalpy change of reaction (ΔrH0(l)=−0.3±2.9kJmol−1) and the standard entropy change of reaction (ΔrS0(l)=26.8±6.7Jmol−1K−1). From these values the standard formation enthalpy (ΔfH0DNBE,(l)) and the molar entropy of DNBE (S0DNBE,(l)) at 298.15K were computed to be −370.5±10.9kJmol−1 and 408.3±6.8Jmol−1K−1, respectively.

Ionic liquids tailored amine aqueous solution for pre-combustion CO2 capture: Role of imidazolium-based ionic liquids

The desirable properties of room temperature ionic liquid (RTIL) with low heat capacity, low corrosive, nonvolatile and good CO2 solubility make it possible to tailor the amine aqueous solution for capture of CO2 with low energy consumption. However, the detailed thermal analysis, which is helpful for understanding deeply the energy saving potential for ionic liquids (ILs)-based solvent, is scant. In this work, three different ILs with the same cation, 1-butyl-3-methylimidazolium ([Bmim]), and different anions, including tetrafluoroborate ([BF4]), nitrate ([NO3]), chloride ([Cl]), are used to tailor the N-methyldieth anolamine/piperazine (MDEA/PZ) aqueous solution to assess their role in CO2 capture performance. The results showed that the investigated ILs affect the physical solubility of CO2. Adding [Bmim][BE4] into MDEA/PZ showed the largest CO2 cyclic capacities, whereas, it is the smallest for MDEA/PZ/[Bmim][NO3], which indicates that anion species of the ILs could tune the sensible heat. Based on the calorimeter measured results and modeling estimated results, the reaction enthalpy changes with the addition of different ILs were also observed. The addition of [Bmim][BF4] could reduce the average enthalpy of CO2 dissolution and the sensible heat at 313 K by 30.1% and 20.3%, respectively. Finally, the studies revealed that not only the latent heat of vaporization but also the sensible heat and reaction heat of CO2 capture can be tailored by the choice of ILs. (C) 2015 Elsevier Ltd. All rights reserved.

Kinetic aspects of the influence of concentrations of methanol and the trans-2,3-bis(diphenylphosphinomethyl)norbornane promoting additive on the hydrocarbomethoxylation of cyclohexene catalyzed by the Pd(OAc)2/p-toluenesulfonic acid system

The effects of temperature, the concentrations of methanol and the trans-2,3-bis(diphenylphosphinomethyl)norbornane (TBDPN) promoting additive on the rate of cyclohexene hydrocarbomethoxylation catalyzed by the Pd(OAc)2/p-toluenesulfonic acid system were studied. It was found that in the 358–383 K temperature range, the increase in the CH3OH concentration from 0 to 0.15 mol/L induces a virtually linear increase in the reaction rate, which slows down as the methanol concentration further increases. In the temperature range of 343–373 K, the dependences of the reaction rate on the TBDPN concentration pass through maxima at [TBDPN] = (3.0–4.0) × 10−3 mol/L. The results were interpreted in terms of the hydride mechanism that included diphosphinepalladium complexes as intermediates and was supplemented by ligand exchange reactions, resulting in a decrease in the activity of the palladium catalyst. The effective constants of the previously derived kinetic equation in the temperature range of 343–383 K were estimated by the least-squares method. The effective activation energies were determined and used to evaluate the enthalpy change in the ligand exchange reaction between the complexes Pd(TBDPN)3 and [HPd(TBDPN)(CH3OH)]OTs. The zerovalent complex Pd(TBDPN)3 was concluded to be more stable than the hydride complex [HPd(TBDPN)(CH3OH)]OTs.

On the negative activation energy for limestone calcination at high temperatures nearby equilibrium

The study of limestone calcination under high CO2 pressure and high temperature has gained an extraordinary practical importance due to the recently emerged Ca-looping technology for post-combustion CO2 capture, which uses natural limestone (CaCO3) as precursor of the CO2 solid sorbent (CaO). A critical issue of this promising process is the necessity of producing CaO by calcination of limestone under high CO2 partial pressure. Experimental measurements on the kinetics of limestone calcination usually show that the rate of the endothermic calcination reaction is increased with temperature according to an Arrhenius law with an activation energy similar to the reaction enthalpy change. In situ microscopic observations have demonstrated that the reaction involves a crystallographic structural transformation of CaO. Calcination is started by chemical decomposition leading to the development of metastable CaO⁎ nanocrystals after which CO2 is desorbed while stable CaO cubic crystals grow. In calcination environments with low CO2 partial pressure P (as compared to the equilibrium pressure: P/Peq≪1), desorption of CO2 and the exothermic structural transformation of CaO⁎ to its stable CaO form occur extremely fast and do not play a role on the reaction kinetics, which is just determined by chemical decomposition. However, at high values of P/Peq the reaction rate may be significantly influenced by the structural transformation above a critical temperature. As a main consequence, the reaction mechanism proposed in the present paper shows that, above a critical temperature, limestone calcination is characterized by a negative activation energy in agreement with the experimental results shown.

Temperature sensitivity of nanochannel electrical conductance.

Electrical measurement is a widely used technique for the characterization of nanofluidic devices. The electrical conductivity of electrolytes is known to be dependent on temperature. However, the similarity of the temperature sensitivity of the electrical conductivity for bulk and nanochannels has not been validated. In this work, we present the results from experimental measurements as well as analytical modeling that show the significant difference between bulk and nanoscale. The temperature sensitivity of the electrical conductance of nanochannel is higher at low ionic concentration where the nanofluidic transport is governed by the electrostatic effects from the wall. Neglecting this effect can result in significant errors for high temperature measurements. Additionally, the temperature sensitivity of the nanochannel conductance allows to measure the enthalpy change of surface reactions at low ionic concentrations.

Comparison of enthalpy, entropy, and volume changes of chemical reactions

The enthalpy, entropy, and volume changes were compared for 63 reactions. The changes in entropy and volume are proportional to a significant extent, whereas no proportionality between the changes in enthalpy and volume of the reactions are observed.

A Thermodynamic Study of Simultaneous Removal of SO2 and NO by a KMnO4/Ammonia Solution

Molar reaction Gibbs function change, molar reaction enthalpy change, equilibrium constant, and the equilibrium partial pressure of SO2 and NO for the reaction of simultaneous removal of SO2 and NO by using aqueous KMnO4/ammonia solution are calculated based on the principle of chemical thermodynamics. The results show that simultaneous removal of SO2 and NO by using KMnO4/ ammonia solution is available.

PH Dependent Electrochemical Characterization, Computational Studies and Evaluation of Thermodynamic, Kinetic and Analytical Parameters of Two Phenazines

The electrochemical response of two biologically important phenazine derivatives was examined by using cyclic, differential pulse and square wave voltammetry in a wide pH range of 2.0 – 12.0. Cyclic voltammetry was employed for the determination of heterogeneous electron transfer rate constant and diffusion coefficient of the analytes. The value of electron transfer rate constant was used for evaluation of Gibbs free energy, enthalpy and entropy changes of redox reactions of the compounds. Differential pulse voltammetry was used for the determination of number of protons and electrons involved in the redox processes. Limits of detection and quantification were assessed by square wave voltammetry. The detailed pH dependent redox mechanisms of the compounds were proposed on the basis of experimental results which were further supported by theoretical calculations. These redox mechanistic pathways are expected to play a key role in understanding the necessary aspects of structure-activity relationships and exploring the hidden routes by which this class of compounds exert its biochemical actions.

Thermodynamic study of complex formation between Kryptofix-5 and Sn2+ in several individual and binary non-aqueous solvents using a conductometric method

The complex formation between 1,13-bis(8-quinolyl)-1,4,7,10,13-pentaoxatridecane (Kryptofix-5) and Sn2+ ions was studied in pure acetonitrile (AN), dimethylformamide (DMF), 1,4-dioxane (DOX), and methanol (MeOH) and in acetonitrile-1,4-dioxane (AN-DOX), acetonitrile-dichloromethane (AN-DCM), acetonitrile-methanol (AN-MeOH), and acetonitrile-dimethylformamide (AN-DMF) binary mixed solvent solutions at different temperatures using conductometric method. 1: 1 [ML] complex is formed between the metal cation and ligand in most solvent systems but in the cases of AN-MeOH (MeOH = 90 mol %) binary mixture and in pure MeOH a 2: 1 [M2L] complex was observed, that is the stoichiometry of complexes may be changed by the nature of the medium. The stability order of the (Kryptofix-5·Sn)2+ complex in the studied binary mixed solvent solutions at 25°C was found to be AN-DOX > AN-DCM > AN-MeOH > AN-DMF and in the case of pure solvents at 25°C the sequence was the following: AN > DMF > DOX. A non-linear behavior was observed for changes of logK f of (Kryptofix-5·Sn)2+ complex versus the composition of the binary mixed solvents, which was explained in terms of solvent-solvent intractions and also by the preferential solvation of the f species involved in the complexation reaction. The values of standard enthalpy changes (ΔH c ○ ) for complexation reactions were obtained from the slope of the Van’t Hoff plots and the changes in standard entropy (ΔS c ○ ) were calculated from the relationship ΔG c, 298.15 ○ = ΔH c ○ − 298.15ΔS c ○ . The results show that in most cases, the (Kryptofix-5·Sn)2+ complex is both enthalpy and entropy stabilized.

Synthesis, thermochemical and thermokinetic studies on the mononuclear copper Valen Schiff base complex

A mononuclear salen-type complex [Cu( vanen )(H 2 O)] was synthesized from the ligand [H 2 vanen = N , N - ethylene-bis(3-methoxysalicylideneimine)] and Cu(OAc) 2 ·H 2 O. Based on Hess law, a reasonable thermochemical cycle was designed. According to the cycle, the standard molar enthalpies of dissolution of samples in the calorimetric solvent of DMF ( N , N -dimethylformamide) were determined, respectively. The standard molar enthalpy change of the coordination reaction (Δ r H m θ ) was calculated to be -(16.61 ± 0.54) kJ/mol. Combined with some relevant literature data, the standard molar enthalpy of formation (Δ f H m θ ) of the complex was obtained to be -(756.5 ± 2.4) kJ/mol. At the temperature of 32 ℃, the metabolic thermogenetic curves of S. pombe under the influence of the complex with different concentration were determined by using a TAM air isothermal calorimeter. The relationships of the concentration of the complex with the microbial growth rate constant ( k ) of S. pombe , inhibition ratio (I), the half inhibition concentration (IC 50 ), the maximum heat output power ( P max ) and the time it took ( t max ) were analyzed. The results indicated that the complex possessed the Hormesis effect, which promoted the growth of S. pombe at lower concentration, but inhibited the growth at higher concentration.

New systematic methodology for incorporating dynamic heat transfer modelling in multi-phase biochemical reactors

This paper presents a new modelling methodology for dynamically predicting the heat produced or consumed in the transformations of any biological reactor using Hess's law. Starting from a complete description of model components stoichiometry and formation enthalpies, the proposed modelling methodology has integrated successfully the simultaneous calculation of both the conventional mass balances and the enthalpy change of reaction in an expandable multi-phase matrix structure, which facilitates a detailed prediction of the main heat fluxes in the biochemical reactors. The methodology has been implemented in a plant-wide modelling methodology in order to facilitate the dynamic description of mass and heat throughout the plant. After validation with literature data, as illustrative examples of the capability of the methodology, two case studies have been described. In the first one, a predenitrification-nitrification dynamic process has been analysed, with the aim of demonstrating the easy integration of the methodology in any system. In the second case study, the simulation of a thermal model for an ATAD has shown the potential of the proposed methodology for analysing the effect of ventilation and influent characterization.

Interaction of thermal and kinetic parameters for a liquid–liquid reaction system: Application to vegetable oils epoxidation by peroxycarboxylic acid

A mathematical model was developed to analyze an exothermic liquid–liquid reaction system using epoxidation of oleic acid by peroxyformic acid formed in situ as an example. Kinetic and thermal parameters were included, mass transfer parameters were eliminated from the model and evaporation/condensation was taken into account. A calorimetric semi-batch reactor under isoperibolic mode was used in the experimental work. Different initial aqueous-phase concentrations of H2O2 [6.5–8.8mol/l], water [44–45mol/l], molar flow rate of formic acid [0.02–0.54mol/min], initial reaction temperature [50–70°C] and amount of organic phase [34–46wt.%] were studied. A non-linear regression method was used to estimate kinetic (e.g., rate constant at average temperature and activation energy) and thermal parameters (e.g., reaction enthalpy) of the epoxidation and ring-opening reactions. The standard reaction enthalpy changes were estimated to be −116kJ/mol for epoxidation reaction and −50kJ/mol for the ring opening.