Standard enthalpy combustion

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... 

STRATEGY We expect a strongly negative value because all combustions are exothermic and this oxidation is like an incomplete combustion. First, add up the individual standard enthalpies of formation of the products, multiplying each value by the appropriate number of moles from the balanced equation. Remember that the standard enthalpy of formation of an element in its most stable form is zero. Then, calculate the total standard enthalpy of formation of the reactants in the same way and use Eq. 20 to calculate the standard reaction enthalpy. 

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. 

Standard enthalpies of formation are commonly determined from combustion data by using Eq. 20. The procedure is the same, but the standard reaction enthalpy is known and the unknown value is one of the standard enthalpies of formation. 

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. 

The enthalpy of combustion of octane (Cg Hig)is -5.5 X 10 kJ/mol. Using tabulated standard enthalpies of formation in Appendix D, determine the standard enthalpy of formation of octane. 

Again, it is convenient to follow the seven-step procedure to solve this problem. We are asked to find an enthalpy of formation. Because enthalpy is a state function, we can visualize the reaction as occurring through decomposition and formation reactions. Appendix D lists enthalpies of formation, and the experimental heat of combustion is provided. We can use Equation to relate the enthalpy of combustion to the standard enthalpy of formation for octane. 

When one considers the potential high-energy release on rupture of a carborane unit, together with the thermodynamic stability of combustion products, it is hardly surprising that there is a body of literature that reports on the use of carbo-ranes within propellant compositions. Their use in energetic applications is to be expected when the enthalpy of formation (AH/) data for the products of combustion for boron are compared to those of carbon. Thermodynamic data for the enthalpy of formation of o-carborane and of typical boron and carbon combustion products is shown in Table 4. Measurements of the standard enthalpy of combustion32 for crystalline samples of ortho-carborane show that complete combustion is a highly exothermic reaction, AH = — 8994 KJmol. ... 

There is considerable variation in the heat of reaction data employed in different articles in the literature that deals with this reaction. Cited values differ by more than an order of magnitude. If we utilize heat of combustion data for naphthalene and phthalic anhydride and correct for the fact that water will be a gas instead of a liquid at the conditions of interest, we find that for the first reaction (equation 13.2.3) the standard enthalpy change will be approximately — 429 kcal/g mole for the second reaction it will be approximately — 760 kcal/g mole. These values will be used as appropriate for the temperature range of interest. Any variation of these parameters with temperature may be neglected. 

Standard enthalpy of combustion, AH%, is the enthalpy change when one mole of a substance is completely burned in oxygen, under standard conditions. 

Example The standard enthalpy of combustion for methane refers to the reaction CFLug) + 2C>2(g)--> CC>2(g) + 2H20(p... 

Example Calculate the standard enthalpy change for the oxidation of ethanol (C2H5OH) to ethanal (CH3CHO), given the following standard enthalpies of combustion ... 

T. Suzuki, Empirical Relationship Between Lower Flammability Limits and Standard Enthalpies of Combustion of Organic Compounds, Fire and Materials (1994), 18 333-336. 

Note that each r in this sum is positive, but, in general, each AH, may be positive or negative (in this example, each A77R is negative). As a standard enthalpy of reaction, each is obtained, at a particular T, in the usual way from standard enthalpies of formation (Am) or of combustion (A H°c ) of the species, such as are provided by Stull et al. (1969) or by JANAF (1986) ... 

Ha Hi AHr AH m AH°fi AHS, AHvaP A I see Dimensionless Groups Henry s law constant for species Pa m3 mol-1, equation 9.2-8 enthalpy of reaction for reaction as written, J standard enthalpy of reaction, J standard enthalpy of combustion of species i J mol-1 standard enthalpy of formation of species i, J mol-1 standard molar enthalpy of reaction with respect to species i, J mol-1 enthalpy of vaporization, J mol-1 standard molar enthalpy of activation (TST), J mol-1 initiator (species) inhibitor, inert species... 

Suzuki [Suzuki, Empirical Relationship between Lower Flammability Limits and Standard Enthalpies of Combustion of Organic Compounds, Fire and Materials, 18 333-336 (1994) Suzuki and Koide, Correlation between Upper Flammability Limits and Thermochemical Properties of Organic Compounds, Fire and Materials, 18 pp. 393-397 (1994)] provides more detailed correlations for the UFL and LFL in terms of the heat of combustion. 

This enthalpy has a special name we call it the enthalpy of combustion, and define it as the change in enthalpy accompanying the burning of methane, and symbolize it as A77(CombUstion) or just AHc.ln fact, we rarely perform calculations with AHc but with the standard enthalpy of combustion AH, where the plimsoll symbol implies s.t.p. conditions. 

Table 3.2 Standard enthalpies of combustion A7/c° for a few organic compounds (all values are at 298 K)...

SAQ 3.9 Calculate the standard enthalpy of combustion A He for burning S-D-glucose, C6Hi206. The required values of AHf may be found in Table 3.1. 

A database is not usually a mere collection of values quoted from the literature. It commonly involves some critical assessment of those values and an effort to present a consistent set of data. It is important to clarify what we mean by consistency. Suppose that 40 years ago, somebody made a careful determination of the standard enthalpy of combustion of anthracene (reaction 2.20) and obtained Ac//°(Ci4Hio, cr) = X kJ mol-1. 

Though the old and the new experiments led to exactly the same value of the enthalpy of combustion (X) of anthracene, the reported standard enthalpies of... 

Equation 2.26 is also used to evaluate the uncertainty of a standard enthalpy of formation, calculated from a reaction enthalpy. Consider, for instance, a selected value for the standard enthalpy of combustion of ferrocene, A CH° [Fe(r 5-C5H5)2, cr] = —5891.5 4.2 kJ mol-1 [31],... 

In the bomb process, reactants at the initial pressure pi and temperature 7 are converted to products at the final pressure pf and temperature Tf. The primary goal of a combustion calorimetric experiment, however, is to obtain the change of internal energy, Ac//°(7r), associated with the reaction under study, with all reactants and products in their standard states pi = pf = O.IMPa) and under isothermal conditions at a reference temperature 7r (usually 298.15 K). Once AC//°(298.15K) is known, it is possible to derive the standard enthalpy of combustion, AC77°(298.15K), and subsequently calculate the standard enthalpy of formation of the compound of interest from the known standard enthalpies of formation of the products and other reactants. 

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