Enthalpy of, combustion

From a thermodynamical point of view, combustion can be seen as an open system inside which the fuel and the oxidant enter at a given rate and react together, while the products of the reaction exit at the same mciss flow rate. If the system is at constant pressure, the 

Methane (CH,) Water gas Reacting steam over coke 

The proportions of the various elements, n. n. n. n. n, must be rounded to the nearest integer to obtain the following empirical chemical formula C Hj,N O,S. Therefore, the relative molecular mass of the fuel, denoted M, can be easily calculated from the relative atomic molar mass of each chemical element, A, using the following equation  

While in dry air, assuming that the theoretical chemical composition of dry air is 78.08 vol.% 

95 vol.% Oj and 0.97 vol.% Ar (i.e., a nitrogen to oxygen ratio of 3.727) and that again nitrogen gas is the only nitrogen-containing species present in the combustion products, the overall scheme of the reaction of combustion can be written as follows  

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The enthalpy of fomiation is obtained from enthalpies of combustion, usually made at 298.15 K while the standard entropy at 298.15 K is derived by integration of the heat capacity as a function of temperature from T = 0 K to 298.15 K according to equation (B 1.27.16). The Gibbs-FIehiiholtz relation gives the variation of the Gibbs energy with temperature... 

This reaction has been carefully studied with the aim of obtaining the enthalpy of combustion as electrical energy, and successful hydrazine-air fuel cells have been developed using potassium hydroxide as the electrolyte. The hydrazine fuel, however, has the disadvantage that it is expensive and poisonous. 

Physical properties of the acid and its anhydride are summarized in Table 1. Other references for more data on specific physical properties of succinic acid are as follows solubiUty in water at 278.15—338.15 K (12) water-enhanced solubiUty in organic solvents (13) dissociation constants in water—acetone (10 vol %) at 30—60°C (14), water—methanol mixtures (10—50 vol %) at 25°C (15,16), water—dioxane mixtures (10—50 vol %) at 25°C (15), and water—dioxane—methanol mixtures at 25°C (17) nucleation and crystal growth (18—20) calculation of the enthalpy of formation using semiempitical methods (21) enthalpy of solution (22,23) and enthalpy of dilution (23). For succinic anhydride, the enthalpies of combustion and sublimation have been reported (24). 

The enthalpy of combustion of isoxazole was only determined several years ago (78MI41615). For isoxazole, AH°c (298.15 K) =-(1649.85 0.50) kJ mol , from which the entropy of formation in the gas phase was derived as AH tig) = 78.50 0.54 kJ moF. The enthalpies of combustion of 3-amino-5-methylisoxazole and 5-amino-3,4-dimethyl-isoxazole have also been determined (73MI41606). 

Enthalpies and Gibbs Energies of Formation, Entropies, and Net Enthalpies of Combustion of Inorganic and... 

Cmpd. no. Name Formula CAS no. Mol wt Ideal gas enthalpy of formation, J/kmol X lE-07 Ideal gas Gihhs energy of formation, J/kmol X lE-07 Ideal gas entropy, J/(kmol-K) X lE-05 Standard net enthalpy of combustion, J/kmol X lE-09... 

Compiled from Daubert, T. E., R. R Danner, H. M. Sibiil, and C. C. Stebbins, DIPPR Data Compilation of Pure Compound Properties, Project 801 Sponsor Release, July, 1993, Design Institute for Physical Property Data, AlChE, New York, NY and from Thermodynamics Research Center, Selected Values of Properties of Hydrocarbons and Related Compounds, Thermodynamics Research Center Hydrocarbon Project, Texas A M University, College Station, Texas (extant 1994). The compounds are considered to be formed from the elements in their standard states at 298.15 K and 101,325 P. These include C (graphite) and S (rhombic). Enthalpy of combustion is the net value for the compound in its standard state at 298.15K and 101,325 Pa. 

Figure 1. Typical enthalpy of combustion gases for a natural gas fuel and 20% excess air.

The second reaction is the enthalpy of combustion of diamond. It does not directly give Af// of C02, since diamond is not the stable form of carbon. We can, however, get Af// for diamond by subtracting the two reactions 1... 

Starting with the AfH°m for H20(1) and for C02(g), Ar//° for many organic compounds can be determined. For example, Bastos et air have measured the enthalpy of combustion of glycerol OHrO.v They obtain a value at T= 298.15 K of... 

Almost all of the directly measured thermochemical data for the sulfoxides, sulfones, sulfites and sulfates are due to the work of Busfield and Mackle and their coworkers at the University of Leeds and The Queens University, Belfast1-14. This work involved measurement of enthalpies of combustion, fusion and vaporization. It is the basis of the subsequent compilations of Benson and coworkers15, Cox and Pilcher16 and Pedley, Naylor and Kirby11. The data given by the latter are used as the basic data set in the present work. Corrections and omissions are noted in the next section. Data on additional compounds were sought by searching the IUPAC Bulletin of Thermochemistry and Thermodynamics for the years 1980 198318, and by searches of Chemical Abstracts. 

It is difficult to measure the enthalpy change of this reaction. However, standard enthalpies of combustion reactions are easy to measure. Calculate the standard enthalpy of this reaction from the following experimental data ... 

The standard enthalpy of combustion, AH°, is the change in enthalpy per mole of a substance that is burned in a combustion reaction under standard conditions. The products of the combustion of an organic compound are carbon dioxide... 

TABLE 6.4 Substance Standard Enthalpies of Combustion at 25°C (kl-mol ) Formula AH°... 

Standard enthalpies of combustion are listed in Table 6.4 and Appendix 2A. We have seen in Toolbox 6.1 how to use enthalpies of combustion to obtain the standard enthalpies of reactions. Here we consider another practical application— the choice of a fuel. For example, suppose we want to know the heat output from the combustion of 150. g of methane. The thermochemical equation allows us to write the following relation... 

Self-Test 6.15A Calculate the standard enthalpy of combustion of glucose from the standard enthalpies of formation in Table 6.5 and Appendix 2A. 

Self-Test 6.15B You have an inspiration maybe diamonds would make a great fuel Calculate the standard enthalpy of combustion of diamonds from the information in Appendix 2A. 

EXAMPLE 6.12 Using the enthalpy of combustion to calculate an enthalpy of formation... 

Use the information in Table 6.5 and the enthalpy of combustion of propane gas to calculate the enthalpy of formation of propane, a gas commonly used for camping stoves and outdoor barbecues. 

A student rides a bicycle to class every day, a lO.-mile round trip that takes 30. minutes in each direction. The student burns 1250 kj-h 1 cycling. The same round trip in an automobile would require 0.40 gallons of gasoline. Assume that the student goes to class 150 days per year and that the enthalpy of combustion of gasoline can be approximated by that of octane, which has a density of 0.702 g-cm-3 (3.785 F = 1.000 gal). 

Elemental sulfur exists in several forms, with rhombic-sulfur the most stable under normal conditions and monoclinic sulfur slightly less stable. The standard enthalpies of combustion of the two forms (to sulfur dioxide) are —296.83 kj-mol 1 and —297.16 kj-mol, respectively. Calculate the enthalpy of the rhombic —> monoclinic transition. 

The standard enthalpy of formation of solid ammonium acetate is —616.14 kJ mol l. Calculate its standard enthalpy of combustion. 

Robert Curl, Richard Smalley, and Harold Kroto were awarded the Nobel prize in chemistry in 1996 for the discovery of the soccer-ball-shaped molecule C60. This fundamental molecule was the first of a new series of molecular allotropes of carbon. The enthalpy of combustion of C60 is —25 937 kj-mol, and its enthalpy of sublimation is +233 kj-mol There are 90 bonds in C60, of which 60 are single bonds and 30 are double bonds. Like benzene, C60 has... 

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