Methane standard enthalpy

With all components in the ideal gas state, the standard enthalpy of the process is exothermic by —165 kJ (—39.4 kcal) per mole of methane formed. Biomass can serve as the original source of hydrogen, which then effectively acts as an energy carrier from the biomass to carbon dioxide, to produce substitute (or synthetic) natural gas (SNG) (see Euels, synthetic). 

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... 

C22-0003. Use standard enthalpies of formation to calculate the change in mass that occurs when 1.00 mol of methane (CH4) is burned to form gaseous products. 

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 of formation of methane ... 

Calculate the standard enthalpy change for the oxidation of methanol to methanal ... 

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. 

The previous example (which incidentally is not entirely fictional) shows that a discrepancy of 0.4 kJ mol-1 in the standard enthalpy of formation of methane has some economic impact. To avoid disputes like this one, the International Organization for Standardization (ISO) has issued an International Standard on the subject [34], The 46-page publication includes a detailed discussion of the... 

Taking methane, CH4, as a reference compound, its standard enthalpy of formation is defined by the reaction ... 

Note several points about this definition. First, the "reaction" to form a substance from its constituent elements can be (and often is) hypothetical. We can t combine carbon and hydrogen in the laboratory to make methane, for instance, yet the heat of formation for methane is A H°f = —74.8kJ/mol, which corresponds to the standard enthalpy change for the hypothetical reaction... 

An estimate of the standard enthalpy change for the formation of the methyl anion and a hydrogen cation from methane may be obtained by a calculation based on Hess s Law, as shown in Table 1. 

A value of about — 7 cal.deg-1.mole-1 for the smaller carbon acids seems reasonable3 (that for the dissociation of HCN in aqueous solution is — 7.4), and combination of this value with the standard enthalpy change yields a value of 56 kcal.mole-1 for the standard free energy change at 298 °K for reaction (1), and hence an equilibrium constant of approximately 10-41. This is in excellent agreement with the pAT of methane of 40 on the MSAD scale (see Section 2., p. 7) and lends support to the calculations shown in Table 1. 

The standard enthalpy of formation of a compound, AHf, is defined as the increment in enthalpy associated with the reaction of forming a given compound from its elements, with each substance in its thermodynamic standard state at the given temperature.2 The thermodynamic cycle for the enthalpy of formation of methane (CH4) from the standard states of carbon and hydrogen (graphite and hydrogen molecules) is shown in Figure 1. 

The H values for some common substances are shown in Table 9.4. More values are found in Appendix 4. The importance of tabulated AH values is that enthalpies for many reactions can be calculated using these numbers. To see how this is done, we will calculate the standard enthalpy change for the combustion of methane ... 

The calibration and application of a heat flux DSC in the study of heterogeneous reactions has been discussed in the literature (248). The possibilities and limitations of this technique were demonstrated for methanation and methanol synthesis on Cu/ZnO catalysts. More recently, Rejai and Gonzalez (222, 223) used a DSC to investigate the reduction of Pt02, PtCl2, and H2PtCl6, the decomposition of calcium oxalate, and the formation of supported Pt-Ru bimetallic catalysts. The results were consistent with values based on standard enthalpies of formation reported in the literature. This work illustrates the power of calorimetry for studying the important processes involved in catalyst preparation and treatment. 

One such important measurable quantity is the enthalpy change when a bond is broken in the gas phase, called the bond enthalpy. This is invariably positive because heat must be added to a collection of stable molecules to break their bonds. For example, the bond enthalpy of a C—H bond in methane is 438 kj mol , measured as the standard enthalpy change for the reaction... 

For example, by combining the heats of combustion of carbon, hydrogen, and methane, we obtain the standard enthalpy of formation of methane, which as we noted above, cannot be determined directly ... 

As an example let us consider the enthalpy change when methane is formed from its elements, both reactants and product being in their standard states. This is called the standard enthalpy of formation and written AH°. 

Let s say we are interested in the standard enthalpy of formation of methane (CH4). We might represent the synthesis of CH4 from its elements as... 

The standard enthalpy of formation given in Table 1 is based on the experimental value of the adiabatic ionization potential of the methyl radical and has been adjusted to a series of fundamental enthalpies of formation. Very recently,the onset of the formation of CH from methane was determined quite accurately by means of the PFI-PEPICO technique. This datum, combined with other experimental results, yields the accurate 0 K dissociation energies (Dq) for CH4 and CH, Do(H-CH3) = 103.479 0.023 and Do(H-CH ) = 39.321 0.092 kcal mol ... 

Comments The value of the standard enthalpy of reaction depends on the stoichiometry adopted, and for this reason, the stoichiometry should be indicated when the enthalpy is reported. To avoid ambiguities, the result could be reported as 205, 813 J per mole of methane reacted, or as 68, 604 J per mol of hydrogen produced. ... 

The standard enthalpy of reaction should be understood as a calculation between two very specific states, that of the pure reactants before reactions, and the pure products after reaction. The calculation can be visualized as a process, as shown schematically in Fieure id-i for the water-gas shift reaction. The pure reactants, methane and water in the ideal-gas state at 25 °C and 1 bar, are mixed, possibly compressed, and heated to reaction temperature. The products are cooled and separated to be delivered as pure gases at 25 °C, 1 bar. The standard enthalpy corresponds to the difference between states A and G. Since we are dealing with a state function, the enthalpy of reaction depends only on the end states but is independent of the internal details of the process. 

Fig. 7.6. Standard enthalpy of solution zl//s°(II) (in kcal/mole) of methane as a function of mole fraction of ethanol at two temperatures.

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