Methane energy changes

When you ignite methane in a Bunsen burner, the amount of heat evolved is very close to 885 kj/mol. There is a small work effect, due to the decrease in volume that occurs when the reaction takes place (Fig. 8.11, p. 216), but this amounts to less than 1% of the energy change. 

The thermodynamic properties of a chemical substance are dependent upon its state and, therefore, it is important to indicate conditions when writing chemical reactions. For example, in the burning of methane to form carbon dioxide and water, it is important to specify whether each reactant and product are solid, liquid, or gaseous since different changes in the thermodynamic property will occur depending upon the state of each substance. Thus, different volume and energy changes occur in the reactions... 

Fig. 17.7), is therefore the nucleus of an atom of a different element. For example, when a radon-222 nucleus emits an a particle, a polonium-218 nucleus is formed. In this case, a nuclear transmutation, the conversion of one element into another, has taken place. Another important difference between nuclear and chemical reactions is that energy changes are very much greater for nuclear reactions than for chemical reactions. For example, the combustion of 1.0 g of methane produces about 52 kj of energy as heat. In contrast, a nuclear reaction of 1.0 g of uranium-235 produces about 8.2 X 10 kj of energy, more than a million times as much. 

Calculations used to obtain a crude idea of the solvent influence by means of the Huron-Claverie method show that due to the transition from the gas phase to the solvent CH2C12, a distinct reduction of the energy gain occurs during the recombination of the free ions into neutral species. This means that the free ions are stabilized (AE stands for the energy change during the transfer from the gas phase to di-chloro methane solution) ... 

Figure 6-13 shows three different paths for the combustion reaction of methane. One path, indicated with the blue arrow, is the path that might occur when natural gas bums on a stove burner. As CH4 and O2 combine in a flame, all sorts of chemical species can form, including OH, CH3 O, and so on. This is not a convenient path for calculating the energy change for the net reaction, because the process involves many steps and several unstable chemical species. 

If a substance is to be dissolved, its ions or molecules must first move apart and then force their way between the solvent molecules which interact with the solute particles. If an ionic crystal is dissolved, electrostatic interaction forces must be overcome between the ions. The higher the dielectric constant of the solvent, the more effective this process is. The solvent-solute interaction is termed ion solvation (ion hydration in aqueous solutions). The importance of this phenomenon follows from comparison of the energy changes accompanying solvation of ions and uncharged molecules for monovalent ions, the enthalpy of hydration is about 400 kJ mol-1, and equals about 12 kJ mol-1 for simple non-polar species such as argon or methane. 

Gibbs free energy change, AG°, for methane transformation reactions (10)... 

Fig. 6 Free energy changes in the steam reforming of acetaldehyde, ethylene and methane.

Fig. 9 Free energy changes in the partial oxidation of ethanol, acetaldehyde and methane. The data of CO oxidation also is included.

The standard free energy change for methanogenesis from hydrogen and CO2 is more exergonic than that of acetogenesis (AG o = -135.6kJ per mole methane and AG o = -104.6kJ per mole acetate, respectively). However, certain conditions compromise or inhibit methanogenesis (Fig. 13.5). In a complex ecosystem, the metabolic interactions of the anaerobic populations... 

Since other possible transformations, such as, formation of dimethyl ether, higher alcohols, and hydrocarbons, are accompanied with higher negative free-energy change, methanol is thermodynamically a less probable product. Therefore, solely on a thermodynamic basis, these compounds as well as methane should be formed in preference to methanol. To avoid the formation of the former compounds, the synthesis of methanol requires selective catalysts and suitable reaction conditions. Under such conditions, methanol is the predominant product. This indicates that the transformations leading to the formation of the other compounds are kinetically controlled. In the methanol-to-hydrocarbon conversion, dimethyl ether generally is converted similarly to methanol. 

Initially, we will be concerned with the physical properties of alkanes and how these properties can be correlated by the important concept of homology. This will be followed by a brief survey of the occurrence and uses of hydrocarbons, with special reference to the petroleum industry. Chemical reactions of alkanes then will be discussed, with special emphasis on combustion and substitution reactions. These reactions are employed to illustrate how we can predict and use energy changes — particularly AH, the heat evolved or absorbed by a reacting system, which often can be estimated from bond energies. Then we consider some of the problems involved in predicting reaction rates in the context of a specific reaction, the chlorination of methane. The example is complex, but it has the virtue that we are able to break the overall reaction into quite simple steps. 

Energy change = D (Bonds broken) — D (Bonds formed) Use the data in Table 7.1 to calculate an energy change for the reaction of methane with chlorine. 

The energy change accompanying a nuclear reaction is far greater than that accompanying a chemical reaction. The nuclear transformation of 1.0 g of uranium-235 releases 8.2 X 107 kj, for example, whereas the chemical combustion of 1.0 g of methane releases only 56 kj. 

Pulse electron-beam mass spectrometry was applied by Kebarle, Hiraoka, and co-workers766,772 to study the existence and structure of CH5+(CH4) cluster ions in the gas phase. These CH5+(CH4) clusters were previously observed by mass spectrometry by Field and Beggs.773 The enthalpy and free energy changes measured are compatible with the Cs symmetrical structure. Electron ionization mass spectrometry has been recently used by Jung and co-workers774 to explore ion-molecule reactions within ionized methane clusters. The most abundant CH5+(CH4) cluster is supposed to be the product of the intracluster ion-molecule reaction depicted in Eq. (3.120) involving the methane dimer ion 424. 

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. 

There are two fermentative processes that at first appear to be quite similar to oxygen and nitrate-dependent respirations the reduction of C02 to methane and of sulfate to sulfide. However, on closer examination, it is clear that they bear little resemblance to the process of denitrification. In the first place, the reduction of C02 and of sulfate is carried out by strict anaerobes, whereas nitrate reduction is carried out by aerobes only if oxygen is unavailable. Equally important, nitrate respirers contain a true respiratory chain sulfate and C02 reducers do not. Furthermore, the energetics of these processes are very different. Whereas the free energy changes of 02 and nitrate reduction are about the same, the values are much lower for C02 and sulfate reduction. In fact, the values are so low that the formation of one ATP per H2 or NADH oxidized cannot be expected. Consequently, not all the reduction steps in methane and sulfide formation can be coupled to ATP synthesis. Only the reduction of one or two intermediates may yield ATP by electron transport phosphorylation, and the ATP gain is therefore small, as is typical of fermentative reactions. 

Sample Calculate the standard free energy change for the complete combustion of methane, CH4, at 25°C. 

Since the maximum attainable temperature is sought, we assume complete adiabatic (Q = 0) combustion. With the additional assumptions that the kinetic- and potential-energy changes are negligible and that there is no shaft work, the overall energy balance for the process reduces to AH = 0. For purposes of calculation of the final temperature, any convenient path between the initial and final states may be used. The path chosen is indicated in the diagram. With one mole of methane burned as the basis for all calculations,... 

Assume that kinetic and potential energy changes are negligible, and the system is at steady state. Assume that the natural gas is pure methane gas, and the surroundings are at 298.15 K. 

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