Revision and Extension of a Generally Applicable Group-Additivity Method for the Calculation of the Standard Heat of Combustion and Formation of Organic Molecules.

The calculation of the heats of combustion ΔH°c and formation ΔH°f of organic molecules at standard conditions is presented using a commonly applicable computer algorithm based on the group-additivity method. This work is a continuation and extension of an earlier publication. The method rests on the complete breakdown of the molecules into their constituting atoms, these being further characterized by their immediate neighbor atoms. The group contributions are calculated by means of a fast Gauss-Seidel fitting calculus using the experimental data of 5030 molecules from literature. The applicability of this method has been tested by a subsequent ten-fold cross-validation procedure, which confirmed the extraordinary accuracy of the prediction of ΔH°c with a correlation coefficient R2 and a cross-validated correlation coefficient Q2 of 1, a standard deviation σ of 18.12 kJ/mol, a cross-validated standard deviation S of 19.16 kJ/mol, and a mean absolute deviation of 0.4%. The heat of formation ΔH°f has been calculated from ΔH°c using the standard enthalpies of combustion for the elements, yielding a correlation coefficient R2 for ΔH°f of 0.9979 and a corresponding standard deviation σ of 18.14 kJ/mol.

A Generally Applicable Computer Algorithm Based on the Group Additivity Method for the Calculation of Seven Molecular Descriptors: Heat of Combustion, LogPO/W, LogS, Refractivity, Polarizability, Toxicity and LogBB of Organic Compounds; Scope and Limits of Applicability.

A generally applicable computer algorithm for the calculation of the seven molecular descriptors heat of combustion, logPoctanol/water, logS (water solubility), molar refractivity, molecular polarizability, aqueous toxicity (protozoan growth inhibition) and logBB (log (cblood/cbrain)) is presented. The method, an extendable form of the group-additivity method, is based on the complete break-down of the molecules into their constituting atoms and their immediate neighbourhood. The contribution of the resulting atom groups to the descriptor values is calculated using the Gauss-Seidel fitting method, based on experimental data gathered from literature. The plausibility of the method was tested for each descriptor by means of a k-fold cross-validation procedure demonstrating good to excellent predictive power for the former six descriptors and low reliability of logBB predictions. The goodness of fit (Q²) and the standard deviation of the 10-fold cross-validation calculation was >0.9999 and 25.2 kJ/mol, respectively, (based on N = 1965 test compounds) for the heat of combustion, 0.9451 and 0.51 (N = 2640) for logP, 0.8838 and 0.74 (N = 1419) for logS, 0.9987 and 0.74 (N = 4045) for the molar refractivity, 0.9897 and 0.77 (N = 308) for the molecular polarizability, 0.8404 and 0.42 (N = 810) for the toxicity and 0.4709 and 0.53 (N = 383) for logBB. The latter descriptor revealing a very low Q² for the test molecules (R² was 0.7068 and standard deviation 0.38 for N = 413 training molecules) is included as an example to show the limits of the group-additivity method. An eighth molecular descriptor, the heat of formation, was indirectly calculated from the heat of combustion data and correlated with published experimental heat of formation data with a correlation coefficient R² of 0.9974 (N = 2031).

Thermodynamic and spectroscopic data suggesting hypothetical humic fractions evolution on carbon cycle in soils.

Soil organic matter is considered by soil scientists as the interlayer that connect alive with mineral sides of the soil. In addition, microorganisms have in soil organic matter a source of carbon as well as a source of energy. We can observe a duality that can be analyzed from a biological, physicochemical, or even thermodynamic sense. From this last point of view carbon cycle follows its evolution on burial soil, and under certain temperature and pression conditions, up to fossil fuels or coals through kerogen being humic substances the ending point of biologically linked structures. When biological aspects are minimized, physicochemical aspects are maximized and carbonaceous structures are a source of energy but resilient to microorganism actions. Under these premises, we have isolated, purified, and analyzed different humic fractions. Heat of combustion of these humic fractions here analyzed reflects this situation and fitted the list of evolution stage of carbonaceous materials that step by step accumulates energy. Theoretical value of this parameter calculated from studied humic fractions, and by combination of its biochemical macromolecules yielded an exaggerated value in comparison to the real and measured value indicating a complexity of these humic structures, more than simpler molecules. Heat of combustion and excitation-emission matrices by fluorescence spectroscopy of isolated and purified grey and brown humic materials revealed different values for each fraction. Grey fractions showed a higher heat of combustion values and shorter λexc/λem, whereas brown fractions showed a lower heat of combustion and a larger λexc/λem. These data together with previous chemical analysis indicated a deep structural differentiation that can be observed by the Pyrolysis MS-GC data of the studied samples. Authors hypothesized that this incipient distinction between aliphatic and aromatic cores could evolve independently up to fossil fuel on one hand and coals on the other hand but separately.

Physico-Chemical and Sensory Profiles of Enriched Linz Biscuits.

The aim of the present study was to determine the physico-chemical properties (dry matter content, combustion heat, electrical properties, total protein, ash, fat and crude fibre contents, selected amino acids, and trace elements), antioxidant content, and sensory profile of Linz biscuits. They were enriched by the addition of powdered carrot, nettle leaves and elderberry fruit, which is 3% of the product. For comparison of results, a control variant without the addition of these components was also prepared. The enriched biscuits showed slightly higher total ash and crude fibre contents in comparison to the control samples. Results for the antioxidant activity and total polyphenol, flavonoid, and phenolic acid contents of the enriched biscuits were higher in all observed parameters than in the control sample with the best results obtained for Linz biscuits enriched with elderberry and nettle powder. In enriched biscuits, higher contents of iron, zinc, and manganese were measured, especially in biscuits with nettle. Linz biscuits with nettle had higher combustion heat values than control samples; the other two sample types had lower values. We found that the resistance, capacitance, and relative permittivity of the enriched biscuits decreased with frequency according to the power regression function. On the contrary, the conductivity increased with an increasing frequency. Electrical properties were mainly influenced by the water content but also by added components.

Heats of combustion of the main carbohydrates contained in plant-source foods.

In a previous review, the experiments of American chemist W.O. Atwater were critically examined, with the findings demonstrating certain weaknesses that could compromise the validity of the values currently used for metabolizable energy. An examination of published works on the heat of combustion of carbohydrates reveals 2 types of weaknesses: the inaccuracy and imprecision of the calorimetric data used, and the averaging procedure employed to estimate such representative values. The present review focuses on the first type of weakness, namely the inaccuracy and imprecision of the calorimetric data used in previous studies. An exhaustive bibliographic search yielded almost 100 heat of combustion values for some of the 6 main carbohydrates contained in plant-source foods (glucose, fructose, sucrose, maltose, starch, and cellulose). These heats of combustion were subjected to rigorous statistical analysis to propose the following for each carbohydrate: (1) an interval (termed a bibliographic interval) that very likely includes the actual heat of combustion value and (2) a "representative value" (calculated to produce the minimum level of inaccuracy). In addition, an estimation of the maximum level of inaccuracy that could be expected when using such a representative value is reported.

Effect of heat treatment on hydrophobic silica aerogel.

Hydrophobic silica aerogels were heat treated under various conditions. Physical and chemical analyses were conducted to study the effect of the heat treatments on the silica aerogels. The O/Si and C/Si values in the hydrophobic silica aerogels increased and decreased, respectively, with the increase in the heating temperature. C-O, -OH, and CO were detected during pyrolysis. Pyrolysis of the silica aerogels in air could be divided into 3 steps: the hydroxylation of methyl groups, the splitting of the alcoholic hydroxyl, and the oxidisation of CO. When the heat treatment temperature was lower than 350 °C, the properties of the silica aerogels showed little change. With further increase in the heat treatment temperature, the variation in the relevant parameters became more prominent. The secondary particles coalesced with one another, and the mesopores were destroyed. Consequently, the thermal conductivity and bulk density rose greatly. The carbon within the silica aerogels was released after heat treatment. As a result, the heat released in the thermal gravimetry and oxygen bomb analyses dropped remarkably with the increase in the heat treatment temperature.

Comparative valorisation of agricultural and industrial biowastes by combustion and pyrolysis.

Combustion and pyrolysis processes were assessed and compared for two types of lignocellulosic biowastes: agricultural (Eucalyptus bark, grape seeds, peach stones, walnut shells, olive waste and peanut shells) and industrial (primary and biological paper mill sludge) biowastes. They were characterized by elemental, proximate and thermal analyses; the pyrolysis behaviour was studied by thermogravimetric analysis and the gases produced were identified using mass spectrometry. Agricultural biowastes showed the highest calorific values, close to the fossil fuel values (20-30MJkg(-1)) and, in general, emission of gases containing the carbon element (CH4, C2H2, CO and CO2) was higher than that of the tested industrial biowastes, making the agricultural biowastes highly competitive for combustion applications such as gas fuel. Further, the solid product which resulted from the pyrolysis of industrial biowastes is a material with large specific surface area, which is a good characteristic for possible applications as adsorbent in water remediation.

Comparison of leaf construction costs in woody species with differing leaf life-spans in contrasting ecosystems.

• The construction costs (CC) are reported of leaves from 162 wild woody species from 14 contrasting environments (desert to rain forest) and with different leaf life-spans. • Calorimetric methods were used to estimate the CC of deciduous, semideciduous and evergreen leaves. • Leaf CC showed a wide range (78%) between species, and deciduous species showed a slightly lower CC (6%) than both semideciduous and evergreen species. Mean leaf CC differed between ecosystems, with the highest and lowest CC in the tundra and rain forest, respectively. Leaf CC was positively correlated with lipid concentration. Leaf size (log) and specific leaf area (SLA, leaf area per leaf dry mass) were negatively correlated with leaf CC. Leaf CC did not show differences between different leaf life-spans or ecosystems when leaf size (log) or SLA were included as covariates. • The small differences in leaf CC among leaf life-span types and ecosystems (6% and 23%, respectively) suggest that SLA is more important in determining differences in the carbon balance between species than leaf CC. Leaf size is shown to be an important trait associated with other leaf characteristics. ABBREVIATIONS: A, ash concentration; CC, construction cost per unit dry mass; CCA , construction cost per unit area; Eg, growth efficiency; Hc, ash free heat of combustion; N, nitrogen; SLA, specific leaf area.

The Enthalpies of Combustion and Formation of Linear Polyethylene.

The enthalpies of combustion and formation of two samples of linear polyethylene which differ only in the degree of crystallinity have been determined in an oxygen bomb calorimeter. For the two samples the degree of crystallinity, the enthalpy of combustion at 298.15 K, and the enthalpy of formation at 298.15 K, were respectively: 72 percent, -651.16 ± 0.12 kJ · mol-1, -28.18 ± 0.13 kJ · mol-1 for the less crystalline sample; and 96 percent, -650.27 ± 0.12 kJ · mol-1 and -29.08 ± 0.12 kJ · mol-1 for the more crystalline sample. The values are per mole of CH2. Uncertainties listed are estimates of accuracy of approximate 95 percent confidence limits. The results of previous determinations by other investigators are discussed briefly.

The Heat of Formation of Boron Carbide.

The standard heat of combustion in fluorine of a boron carbide sample having the composition B4.222C was determined from the heats of combustion of polytetrarluoroethylene and of boron carbide-polytetrafluoroethylene mixtures. The energy of the combustion reaction was measured in an isothermal-jacket bomb calorimeter. From the experimental data, we calculate -17.1 kcal mol-1 for the heat of formation of boron carbide. By combining all probable errors, we estimate our overall experimental uncertainty to be 2.7 kcal mol-1. The value for the heat of formation of boron carbide is for the phase represented by the formula B4.222C.

Studies in Bomb Calorimetry. A New Determination of the Energy of Combustion of Benzoic Acid in Terms of Electrical Units.

The heat of combustion of NBS Standard Sample 39i of benzoic acid under standard bomb conditions has been determined in terms of electrical units. A value of -26,434.0 J g-1 was obtained. The total uncertainty in our determination is estimated to be ±3.3 J g-1. The uncertainty due to random errors was 1.7 J g-1 and is based on the appropriate factors for the Student t distribution at the 95 percent confidence limits for eleven determinations of the energy equivalent of the calorimeter and six determinations of the heat of combustion of benzoic acid. The principal systematic error, neglect of surface temperature correction for our calorimeter, has been assigned a value of ±2.6 J g-1 until more reliable estimates of the correction can be made. Particular emphasis was placed on improving the precision of a calorimetric measurement over those previously obtained in this laboratory by the use of more sensitive auxiliary measuring equipment and more accurate procedures to evaluate the corrected temperature rise.

The Heats of Combustion of Polytetrafluoroethylene (Teflon) and Graphite in Elemental Fluorine.

Bomb calorimetric measurements are reported for the combustion in fluorine of polytetrafluoroethylene (Teflon) and graphite-polytetrafluoroethylene mixtures. Mass spectrometric examination of the product gases showed CF4 to be the only major product with C2F6(g) present in only very small amounts. The completeness of combustion of the graphite was determined by chemical analysis of combustion residues and found to range from 97 to better than 99 percent. From the combustion data, the heats of formation Δ H f 298 ° [ CF 4 ( g ) ] and Δ H f 298 ° [ C 2 F 4 ( solid polymer ) ] were determined to be -222.87 ±0.38 kcal mol-1 and -197.82 ±0.39 kcal (gfw C2F4)-1, respectively. The uncertainties are estimates of the overall experimental errors. A previously reported value for the heat of formation of AlF3(c) is adjusted to be consistent with the present work. An evaluation of other data on CF4 is presented. The heat of formation of CF4(g) is combined with other work to derive the heats of formation of HF solutions at three specific concentrations.

Heats of Formation of Aluminum Diboride and α-Aluminum Dodecaboride.

The energies of combustion of AlB2 and α-AlB12 were measured in a bomb calorimeter using fluorine as the oxidant. Major problems of this investigation were the assessment of the state and distribution of impurities in the samples and the establishment of the stoichiornetry of the aluminum boride phase. We obtain -16±3 kcal mol-1 and -48±10 kcal mol-1 for the heats of formation of AlB2 and α-AlB12, respectively. The uncertainties cited are the overall experimental errors. Their magnitudes are chiefly due to uncertainties in the impurity correction applied and the uncertainties in the heats of formation of the combustion products.