Search Results

You are looking at 1 - 10 of 1,619 items for :

  • All content x
Clear All

Abstract  

Through structural relaxation, the configuration of a viscous liquid changes to allow the Gibbs free energy to be minimum in response to temperature variations. In this review, the practical importance of relaxation in silicate melts is first illustrated by configurational heat capacity and entropy and their connection with viscosity via Adam-Gibbs theory. Relaxation effects on thermal expansion and compressibility are then examined, and the similarity of the kinetics of structural, enthalpy and volume relaxation is pointed out. Turning to microscopic mechanisms, we finally stress the importance of Si-O bond exchange and its decoupling with the motion of network-modifying elements near the glass transition.

Restricted access

Abstract  

The excess molar enthalpy of ternary mixture for 3-diethylaminopropylamine+heptane+cyclohexane were measured using a Calvet microcalorimeter at 303.15 K. Empirical equations, Redlich-Kister, Tsao-Smith, and Kohler and group contribution models, UNIFAC (modified version) and DISQUAC have been applied. A reasonable representation of ternary data is obtained.

Restricted access

Abstract  

Calorimetric enthalpy changes in reactions have been measured for the formation of zinc and cadmium trimethylenediaminetetraacetates at 298.15 K and ionic strengths of 0.1. 0.5, 1.0 mol L−1 (KNO3). The standard thermodynamic parameters of the reactions studied were evaluated from calorimetric and potentiometric measurements under the same conditions. The results obtained were compared with the corresponding data on related compounds.

Restricted access

Abstract  

Reaction enthalpy changes were measured with the use of calorimetric method for the protonation and neutralization of glycyl-β-alanine at 298.15 K and ionic strengths 0.5, 1.0, 1.5 mol L−1 (KNO3), and the corresponding standard thermodynamic parameters were evaluated. The results obtained were compared with the literature data on related compounds.

Restricted access

Abstract  

Three samples of silicon dioxide were syhthesized and their surface areas were measured. A thermo-chemical cycle was designed to calculate the molar formation enthalpy. The molar formation enthalpy, Δf H m Φ, for three amorphous silica with the Langmuir surface area 198.0854, 25.1108 and 11.9821 m2 g−1 gave −895.52, −910.86 and −915.67 kJ mol−1, respectively. With the increasing surface area, the values of Δf H m Φ increased accordingly. The results suggest that the silica with larger surface area is more unstable. The wetting heat was also measured by adding the silica powder into water. With the rehydration of the more SiOH groups on the surface, the larger surface areas of silica lead to the more wetting heat. A smaller particle has the more unstable hydroxyl groups and surface energy.

Restricted access

Abstract  

The enthalpies of absorption and solubility at T=298.15 K of carbon dioxide in aqueous solutions of bis-(3-dimethylaminopropyl)amine (CAS RN: [6711-48-4]) are reported in this paper. It was observed that the saturation loading of the CO2 is α=2.9 mol CO2/mol TMBPA, which is close to the theoretical value, α0=3 mol CO2/mol TMBPA. The molar heat of absorption of CO2 is independent of the polyamine concentration of the solutions and the amount of CO2 absorbed and was calculated to be Δabs H m=–44 (2) kJ mol–1 CO2.

Restricted access

Abstract  

A calorimetric method for determining the enthalpy of the aqueous oxidation of sulfide minerals at high temperatures and oxygen pressures has been developed and evaluated under conditions relevant to industrial pressure oxidation operations. This information is important for heat balance calculation and optimization of the pressure reactor design. Experiments were carried out on a differential scanning calorimeter (DSC) with a commercial mixing cell. Enthalpy measured during oxidative dissolution of pyrite (Valdenegrillos, Spain), pentlandite with pyrrhotite (Sudbury, Ontario, Canada) and impure chalcopyrite (Victoria, Australia) minerals at 150°C and partial oxygen pressures of 3.4 and 5.5 MPa have been performed and found to be consistent with theoretical estimations.

Restricted access
Journal of Thermal Analysis and Calorimetry
Authors: R. Nomen, M. Bartra, J. Sempere, E. Serra, J. Sales, and X. Romero

Summary  

Estimation methods developed over years by S. W. Benson and co-workers for calculation the thermodynamic properties of organic compounds in the gas phase are applied to a pharmaceutical real process with all type of non-idealities. The different strategies used to calculate the reaction enthalpy of a chemical process, in the absence of data for complex molecules, using the Benson group additivity method are presented and also compared with the experimental value of reaction enthalpy obtained using reaction calorimetry (Mettler-Toledo, RC1). We demonstrate that there are some strategies that can be followed to obtain a good estimation of the reaction enthalpy in order to begin the safety assessment of a chemical reaction. This work is part of an industrial project [1] in which the main objective was the risk assessment of chemical real and complex processes using the commonly available tools for the SMEs (with limited resources).

Restricted access

Solution enthalpies of hydroxylic compounds

Study of solvent effects through quantitative structure property relationships

Journal of Thermal Analysis and Calorimetry
Authors: Marina Reis, Luís Moreira, Nelson Nunes, Ruben Leitão, and Filomena Martins

molecular interactions in solution. As such, they enclose valuable information that can be unraveled using QSPR techniques. Formally, the solution enthalpy of a given solute A in a solvent S can be partitioned into three energetic contributions

Restricted access

Abstract  

The curing of composition containing epoxy bond is complicated chemical and technological process where under temperature and pressure conditions a change of its structure occurs. The structure changes are possible to know by thermal methods as DSC, DTA, etc. and also by measurement of dielectrical response under the low frequency electrical field.

Restricted access