Heat of combustion determination

Two now classical papers 61MI40400, 62JCS2927) contain almost all the experimental thermochemical data on pyrazole and indazole. Heats of combustion determined by Zimmerman (61MI40400) have been used by Dewar to calculate the heats of atomization (Table 25) (69JA796). Quantum mechanical calculations, carried out by Dewar (69JA796) or Olivella 81JHC1189), gave accurate empirical values. 

By analysis and by measurement, we can determine H2 — H and express it in terms of values at 25 °C and 1 atm. Since Q is large for combustion reactions, these pressure corrections usually have a small effect on the accuracy of the heat of combustion determined in this way. 

Heating value. The heat of combustion determination may be waived at the option of the Inspector if the aniline-gravity product of the fuel is not less than the numerical values specified in Table I. The aniline-gravity product... 

Calorimetric studies were undertaken to determine whether the acetylenic linkage had remained intact during the polymerization. If the linkage were affected, heats of combustion determined experimentally should agree with the theoretical values calculated for acetylenic polymers. The heats of combustion of the acetylenic polymers were measured in a... 

The heat evolved on burning an alkane increases with the number of car bon atoms The relative stability of isomers may be determined by com paring their respective heats of combustion The more stable of two iso mers has the lower heat of combustion... 

FIGURE 3 20 The enthalpy difference between as- and trans 1 2 dimethylcyclopropane can be determined from their heats of combustion Van der Waals strain between methyl groups on the same side of the ring make the cis isomer less stable than the trans... 

The pattern of alkene stability determined from heats of hydrogenation parallels exactly the pattern deduced from heats of combustion... 

The stmcture of DPXN was determined in 1953 from x-ray diffraction studies (22). There is considerable strain energy in the buckled aromatic rings and distorted bond angles. The strain has been experimentally quantified at 130 kj/mol (31 kcal/mol) by careful determination of the formation enthalpy through heat of combustion measurements (23). The release of this strain energy is doubtiess the principal reason for success in the particularly convenient preparation of monomer in the parylene process. 

The impact that variations in coke content and burning conditions can have on the overall heat of coke combustion is shown in Table 2. Because the heat balance dictates the amount of heat that is required from burning coke, the heat of combustion then determines the amount of coke that must be burned. 

Many finely divided metal powders in suspension in air are potential e] losion hazards, and causes for ignition of such dust clouds are numerous [Hartmann and Greenwald, Min. MetalL, 26, 331 (1945)]. Concentration of the dust in air and its particle size are important fac tors that determine explosibility. Below a lower Umit of concentration, no explosion can result because the heat of combustion is insufficient to propagate it. Above a maximum limiting concentration, an explosion cannot be produced because insufficient oxygen is available. The finer the particles, the more easily is ignition accomplished and the more rapid is the rate of combustion. This is illustrated in Fig. 20-7. 

It follows that the efficiency of the Carnot engine is entirely determined by the temperatures of the two isothermal processes. The Otto cycle, being a real process, does not have ideal isothermal or adiabatic expansion and contraction of the gas phase due to the finite thermal losses of the combustion chamber and resistance to the movement of the piston, and because the product gases are not at tlrermodynamic equilibrium. Furthermore the heat of combustion is mainly evolved during a short time, after the gas has been compressed by the piston. This gives rise to an additional increase in temperature which is not accompanied by a large change in volume due to the constraint applied by tire piston. The efficiency, QE, expressed as a function of the compression ratio (r) can only be assumed therefore to be an approximation to the ideal gas Carnot cycle. 

To make the necessary thermodynamic calculations, plausible reaction equations are written and balanced for production of the stated molar flows of all reactor products. Given the heat of reaction for each applicable reaction, the overall heat of reaction can be determined and compared to that claimed. However, often the individual heats of reaction are not all readily available. Those that are not available can be determined from heats of combustion by combining combustion equations in such a way as to obtain the desired reaction equations by difference. It is a worthwhile exercise to verify this basic part of the process. 

To overcome this problem, they proposed a working-fluid heat-addition model. This model implies that the gas dynamics are not computed on the basis of real values for heat of combustion and specific heat ratio of the combustion products, but on the basis of effective values. Effective values for the heat addition and product specific heat ratios were determined for six different stoichiometric fuel-air mixtures. Using this numerical model, Luckritz (1977) and Strehlow et al. (1979) systematically registered the properties of blast generated by spherical, constant-velocity deflagrations over a large range of flame speeds. 

In this section you have seen how heats of combustion can be used to determine relative stabilities of isomeric alkanes. In later sections we shall expand our scope to include the experimentally determined heats of certain other reactions, such as bond dissociation energies (Section 4.16) and heats of hydrogenation (Section 6.2), to see how AH° values from various sources can aid our understanding of structure and reactivity. 

Techniques such as Raman spectroscopy and the determination of heate of combustion and reaction could possibly be applied to the study of tautomerism, but so far they have not become important. 

Explosives and propellants are mixtures of fuel and oxidizer. The intensity of combustion is determined by the heat of combustion per pound of material, the material s density, the gas volume generated per volume of material, and the rate of deflagration or detonation. The latter, the most important variable, is determined by the speed at which fuel and oxidizer molecules combine. 

For reactions involving only organic compounds as reactants, AH°can be determined using heats of combustion data. 

The porphyrin ring system (the parent compound 1 is also known as porphin) consists of four pyrrole-type subunits joined by four methine ( = CH-) bridges to give a macrotetracycle. The macrocycle contains 227i-electrons from which 1871-electrons form a delocalized aromatic system according to Huckel s 4n + 2 rule for aromaticity. The aromaticity of the porphyrin determines the characteristic physical and chemical properties of this class of compounds. The aromatic character of porphyrins has been confirmed by determination of their heats of combustion.1"3 X-ray investigations4 of numerous porphyrins have shown the planarity of the nucleus which is a prerequisite for the aromatic character. 

Several exptl studies are quoted for the heat of nitrating one OH group. On a per mole basis these range from 1.7 to 2.lkcal/mole. However, Miles (Ref 44) computes a lower value, 1.2kcal( mole, on the basis of accurately determined heats of combustion of NC... 

All this is in line with the most recent finding for triquinacene, for which the direct determination of its AHy(g) from its experimentally measured heat of combustion finally corroborated the results of the most advanced computational studies that triquinacene is not homoaromatic [35]. Evidently, heat of combustion measurements should also be carried out for some representative [nlpericyclines to finally settle the quest for their neutral homoaromaticity. 

C06-0093. Solid urea, (NH2)2 CO, bums to give CO2, N2, and liquid H2 O. Its heat of combustion is -632.2 kJ/mol. (a) Write the balanced combustion equation, (b) Calculate the heat generated per mole of H2 O formed, (c) Using this heat of combustion and the appropriate thermod3uamic data, determine the heat of formation of urea. 

Figure 1.24. Heat of reaction determination. AHp r and AHp p are the heats of formation of reactants and products, respectively. AHc,R and AH p are the heats of combustion of reactants and products, respectively.

The heat of combustion of a compound — AH° is the standard heat of reaction for complete combustion of the compound with oxygen. Heats of combustion are relatively easy to determine experimentally. The heats of other reactions can be easily calculated from the heats of combustion of the reactants and products. 

For compounds containing nitrogen, the nitrogen will not be oxidised to any significant extent in combustion and is taken to be unchanged in determining the heat of combustion. 

The process is first divided into a number of units which are assessed individually. The dominant material for each unit is then selected and its material factor determined. The material factor in the Mond index is a function of the energy content per unit weight (the heat of combustion). 

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