Molar heat of combustion

The temperature at which decarboxylation occurs is of particular interest in manufacturing processes based on polymerisation in the molten state where reaction temperatures may be near the point at which decomposition of the diacid occurs. Decarboxylation temperatures are tabulated in Table 2 along with molar heats of combustion. The diacids become more heat stable at carbon number four with even-numbered acids always more stable. Thermal decomposition is strongly influenced by trace constituents, surface effects, and other environmental factors actual stabiUties in reaction systems may therefore be lower. 

Table 2. Decarboxylation Temperatures and Molar Heats of Combustion of Dicarboxylic Acids...

Dicarboxyhc acid Decarboxylation temp, °C Molar heat of combustion, kJ /mol... 

Before we examine the details of this rather strange looking equation, let us focus our attention on the + energy term. The numerical value is of the order of 4.5 X 109 kcal/mole of uranium. Look at that figure again and compare it to the molar heat of combustion of carbon. Roughly, what is the ratio of these two energies It is 107, or 10 million ... 

As anticipated, SA conversion increases with increasing residence time (1/LHSV) and with increasing temperature to a maximum of about 98%. This limit is most likely caused by equihbrium. This limit and thus the equilibrium constant were not affected by the temperature range studied, consistent with a low heat of reaction. The sum of the molar heats of combustion of stearic acid (11320 kJ/mol) and methanol (720 kJ/mol) is almost the same as the heat of combustion of methyl stearate (12010 kJ/mol), meaning that the change in enthalpy of this reaction is nearly zero and that the equihbrium constant is essentially temperature independent. 

The MIE of gas — air or vapour—air mixtures can be determined from the structural formula and the molar heat of combustion of the compounds studied, and equations for the calculation are presented. The method is stated to give more accurate results than conventional methods used to assess flammability of mixtures of gas or vapour with air [1], It is claimed that in oxygen MIEs are about a hundredfold lower than in air [2], A study of the ignition behaviour of dusts, including correlation of electrical and mechanical minimum ignition spark energies and ignition temperature is made [3],... 

Sventoslavsky (Ref 2a) developed in 1908 a method of calculation of heats of combustion which later proved to be in agreement with the method developed by Kharash (Ref 4). Kassatkin Planovsky gave a good description of Sventoslavsky s method (Ref 7, p 31). Later, Thornton (Ref 2b) has shown that the molar heat of combustion at constant volume of any saturated hydrocarbon at room temperature is approximately 52.7 kcal for each atomic weight of oxygen required to burn it. For example, methane, which burns according to the equation ... 

What is the molar heat of combustion of propene (C3Hg), if 321 kJ heat is liberated when 7 g of CjHg is burnt ... 

The molar heat of combustion of the reaction is the amount of heat when 1 mol of propene (42 g/mol) is burnt. So,... 

When 0.5 g of benzene (CgHg) is burnt in a bomb calorimeter, the temperature of the calorimeter rises from 25 °C to 33.52 °C. If the molar heat of combustion of benzene is 1955 kJ, find the heat capacity of the calorimeter ... 

This detector, invented by Scott, consists of a thermocouple placed directly above a flame (48). When a compound is eluted, an increase in temperature produces a signal. The device is simple and inexpensive and has the capability of detecting microgram quantities. The signal from the detector is proportional to the molar heat of combustion of the eluted compound. To improve the stability of the detector nitrogen is used as the carrier gas. 

This AH value tells us that when 1 mole of methane is burned in oxygen, 728 kj of energy are released. This value is called the enthalpy of combustion (or molar heat of combustion) of methane. 

The magnitudes of the mechanical and thermal energy flows for each of the cases are given in Table 1. Also shown in Table 1 are the values of the overall conversion efficiency, which is defined as the ratio of total work output (wT + w t) to the molar heat of combustion of liquid methanol (727 Kj). sc... 

By approximation other quantities are additive as well, such as the molar volume, molar heat capacity, molar heat of combustion and formation, molar refraction, etc. 

The molar heat of combustion can be calculated from the difference between the heat of formation of the carbon dioxide and water formed by complete combustion and the heat of formation of the substance combusted. The data for this calculation are provided in Chap. 20. 

The molar heat of combustion of cyclohexane is nearly twice that of cyclopropane, simply because cyclohexane contains twice as many methylene (CH2) groups per mole. To compare the relative stabilities of cycloalkanes, we divide the heat of combustion by the number of methylene (CH2) groups. The result is the energy per CH2 group. These normalized energies allow us to compare the relative amounts of ring strain (per methylene group) in the cycloalkanes. 

Ring Size Cycloalkane Molar Heat of Combustion Heat of Combustion per CH2 Group Ring Strain per CH2 Group Total Ring Strain... 

The product is described as a powder, white to dark red in colour according to the time of heating at 250° C. The density was 2-5. The nitride was tasteless, odourless, and chemically inactive at ordinary temperatures. The heat of formation (from white phosphorus) is given as +81-5 Cals, per mol.3 The molar heat of combustion was 474-7 Cals, (at constant pressure). The nitride dissociated into its elements in a vacuum at about 760° C.4 It was reduced to phosphorus and ammonia by hydrogen at a red heat, and burned when heated in oxygen or chlorine. It was hydrolysed by boiling water, thus... 

The cycle efficiency is the net power produced divided by the heating rate of the fuel. The heating rate is the molar flow of fuel multiplied by the standard molar heat of combustion ... 

In its simplest form, the method proposed by M. S. Kharasch (1929) is virtually the same as that just described. The molar heat of combustion of a liquid compound at constant pressure is equal to — 26.05x kcal., where X is the number of valence electrons of carbon not shared with oxygen in the original substance, but which are shared with oxygen, i.e., in carbon dioxide, when combustion is complete. In general, x is equal to twice the number n of oxygen atoms utilized in the combustion of a molecule, so that this rule is equivalent to stating that the heat of combustion is — 52. In kcal. per mole. However, Kharasch has realized the necessity for including allowances for various types of structure in the compound, and by the use of these correction factors results have been obtained which are within one per cent, or less, of the experimental heats of combustion. 

Melting point Specific gravity Index of refraction Molecular volume Molar heat of combustion Specific rotation... 

A quantity of 1.435 g of naphthalene (CioHg), a pungent-smeUing substance used in moth repellents, was burned in a constant-volume bomb calorimeter. Consequently, the temperature of the water rose from 20.17°C to 25.84°C. If the mass of water surrounding the calorimeter was exactly 2000 g and the heat capacity of the bomb calorimeter was 1.80 kJ/°C, calculate the heat of combustion of naphthalene on a molar basis that is, find the molar heat of combustion. 

The molar heat of combustion of gaseous cyclopropane is —2089 kj/mol that for gaseous cyclopentane is —3317 kj/mol. Calculate the heat of combustion per CH2 group in the two cases, and account for the difference. 

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