Enthalpy species formation

Data on proton affiri itics (gas ph asc) of m any differen t com poti u ds (see Table 2) deni on strate Lh e h igh level of accuracy possible in determ in in g energies of related species. In th is report by Dew-ar and Dieter , the enthalpy of formation of II is the experimental value (367.2 kcal/moll. The calculated value for H is unreliable. 

Several portions of Section 4, Properties of Atoms, Radicals, and Bonds, have been significantly enlarged. For example, the entries under Ionization Energy of Molecular and Radical Species now number 740 and have an additional column with the enthalpy of formation of the ions. Likewise, the table on Electron Affinities of the Elements, Molecules, and Radicals now contains about 225 entries. The Table of Nuclides has material on additional radionuclides, their radiations, and the neutron capture cross sections. 

Quantities AiT are enthalpies of formation of the species the densitiese of S1H2 and SiH species in the plasma have been shown to be larger than other species. 

You will note from Table 8.3 that there are no entries for elemental species such as Br2(l) and O2(g). This is a consequence of the way in which enthalpies of formation are defined. In effect, the enthalpy of formation of an element in its stable state at 25°C and 1 atm is taken to be zero. That is,... 

In recent years, aqueous solutions of Xe03 have been used to oxidize a species in solution, from which A[H°m can be calculated when AH for the oxidation reaction is combined with AH for other reactions. The noble gas oxide Xe03 is used as an oxidant because of its stability and the fact that the final reaction product is Xe(g), which has a zero enthalpy of formation and is easily removed from the reaction mixture. As an example, O Hare4 has reported AfHcm for UCI4. We will not go through the details of his procedure, but the critical step involved measuring A TH for the reaction... 

Complex chlorides of plutonium (34, 41) do not present such a wide range of formulae as the complex fTuorides but we have at hand thermodynamic information on two important species which have also been characterized with other actinides. In table II we have disregarded the complex halides for which no thermodynamic data are available. The enthalpy of formation of Cs2NaPuClg(c) (55) and Cs2PuClg(c) (56) have been obtained from enthalpy of solution measurements."The selected (8) values are AHf(Cs2NaPuCl6,c) =... 

Self-Test 7.20A Calculate the standard Gibbs free energy of formation of NH,(g) at 25°C from the enthalpy of formation and the molar entropies of the species taking part in its formation. 

When the partial pressure of each gaseous reagent is 1 bar and the concentration of each species in solution is 1 M, the conditions are defined to be standard. Under these conditions, the enthalpy change in a formation reaction is the standard enthalpy of formation (A... 

Considering that the standard enthalpies of formation of various oxides, sulfides, and halides of As, Sb, and Bi are very close to each other, the similarity between this value and the standard enthalpy for the surface formation of As(OH)3, A//f = —680 + 20 kJ/mol, was taken as an indication that, indeed, the As redox process on Pt(l 11) involves the formation of hydroxide species [Blais et al., 2001]. For comparison with A/7 for the surface formation of Bi(OH)2, the strategy followed... 

It is observed that higher potential values for the adatom redox process are correlated with a lower energy of the M—O bond, i.e., lower (less negative) enthalpy of formation of the adatom oxygenated species. In this regard, the discrepant behavior of Ge-Pt(lOO) may be related to the dilute nature of this adlayer, with a maximum coverage of only 0.25. 

The enthalpy change for the reaction is given by the difference in the standard enthalpies of formation of the species, which can be found from the table in Appendix C ... 

The thermochemistry of totally cumulated trienes, i.e. species with the C=C=C=C substructure, is very limited. Indeed, the sole examples we know are those reported by Roth, namely (Z)- and ( )-2,3,4-hexatrienes MeCH=C=C=CHMe, species 17 and 18. Their enthalpies of formation are identical to within experimental error, 265 kJ mol-1. This equality is altogether reasonable given the small Me—Me interaction across the 4-carbon, linear, cumulene chain in contradistinction to the 4.3 kJ mol-1 difference that is found for the isomeric (Z)-and (E)-2-butenes with their significantly smaller Me...Me distance. Are cumulated trienes unstable relative to cumulated dienes much as cumulated dienes are unstable relative to simple olefins Briefly regressing to cumulated dienes, this assertion is corroborated by the finding that species 3, i.e. 1,3-dimethylallene, has an enthalpy of decarbonization 18 of 144.5 kJmol-1 (reaction 12)... 

The liquid phase enthalpy of formation of this species is from Pedley the necessary enthalpy of vaporization was estimated. 

Data are sparse. Let us thus relax the earlier phase restriction to the gas phase. We therefore briefly discuss some conjugated dienes for which we have enthalpy of formation data solely in the condensed phase. The first pair of species are the isomeric (Z,Z)-and... 

What about measurements of enthalpies of combustion of condensed phase species 49 and 50 and accompanying enthalpies of vaporization Enthalpies of formation of the gaseous hydrocarbons can be directly obtained from these studies as well. There are two recent studies that provide us with useful information. The first42 results in the values of 104.6 0.6 and 104.8 0.6 kJmol-1 respectively. The second accompanies the earlier cited cyclic bisallene (and polycyclic monoolefin) study, in which the authors20... 

From enthalpy of formation data of 63 from Roth, and for the other species from Pedley, we find reaction 30 is exothermic by 6 kJ mol 1. Consider now the isomeric 1,3-and 1,4-dimethylenecyclohexane, 65 and 66 no thermochemical data for its 1,2-isomer are seemingly available. We can write the related formal reactions 31a and 31b. 

From enthalpy of formation data of 65 from Roth, of 66 from Reference 46, and for the other species from Pedley, we find reactions 31a and 31b are exothermic by nearly 19 and 9 kJ mol-1 respectively. We have no understanding of why 1,3-dimethylenecyclohexane is so stable relative to its 1,4-isomer, or how to predict stabilization of any bismethylenecy-cloalkane as a function of ring size. 

There are many bicyclic dienes and polyenes. If for no other reason than to show that seemingly homologous series often show profound complications, in Table 3 we present the enthalpies of formation of the bicyclo[2.2.n]alka-2,5-dienes, bicyclo[2.2.n]alk-2-enes and bicyclo[2.2.n]alkanes, species 68, 69 and 70, respectively, wherein we limit our attention to the cases of n = 0, 1 and 2. It is seen that the enthalpies of formation of the bicycloalkadiene, bicycloalkene and bicycloalkane always become more negative in that... 

Consider now the 1,1,6,6-tetramethylated derivative of (Z)-l,3,5-hexatriene (83), a species more properly named (Z)-2,6-dimethyl-2,4,6-octatriene and occasionally and trivially called ds-allo-ocimene . To estimate its enthalpy of formation, let us use simple olefin additivity along with ... 

Relatedly, one would have expected 1,3,5-cyclooctatriene to have a more negative enthalpy of formation than tropilidene by the same —20.6 kJmol-1. By contrast, the difference for these enthalpies of formation of species 86 and 87 as derived from experimentally measured enthalpies of formation is ca +12 kJ mol-1. From this we may deduce that tropilidene enjoys considerable stabilization due to homoaromatic interactions. While this conclusion is not new64, nonetheless we find it encouraging to see it corroborated. 

It is interesting that attachment of — ( 2)4— and —CH2CH=CHCH2— to benzene results in nearly the same enthalpy of formation change but it is not obvious how fortuitous this equality is we have reasons for considerable skepticism of its validity68. That formation of naphthalene from benzene is accompanied by a lessened enthalpy of formation increase than that of l,6-methano[10]annulene (yet another name for species 90) from tropilidene would appear to be more of a strain than a resonance derived effect. From Roth, we find the resonance energy increase on going from tropilidene to l,6-methano[10]annulene is 55 kJmol-1 and from benzene to naphthalene the increase is nearly the same, nearly 59 kJmol-1. By contrast, the l,5-methano[10]annulene (99) is less stable by 77 kJmol 1 than the species it appears most naturally to be compared with, namely the isomeric 90. 

This class of compounds is defined to have some of the three conjugated double bonds found in the ring and others not. This class includes the isomeric 3,3 -bis(cyclohexenylidenes), 100 and 101. Roth shows us that the two isomers have the same enthalpy of formation within ca 1 kJmol-1, a difference somewhat smaller than the 4 kJmol-1 found for the totally acyclic 1,3,5-hexatrienes, 79 and 80 respectively. Naively these two sets of trienes should have the same (E)/(Z) enthalpy difference. Given experimental uncertainties, we will not attempt to explain the difference69. We may compare 100 and 101 with phenylcyclohexane, 102, an isomeric species which also has the same carbon skeleton. There is nearly a 110 kJ mol-1 enthalpy of formation difference between the semicyclic and cyclic trienes. We are not surprised, for the word cyclic is customarily replaced by aromatic when in the context of the previous sentence. 

While we know of no experimental thermochemical data for 123, Roth informs us that the enthalpy of formation of 124 is 259 kJmol-1. There are no experimental thermochemical data for 125 either, but it is easy to estimate the desired enthalpy of formation. We may either use the standard olefin approach with ethylene, 1,3-butadiene and (E)-l,3,5-hexatriene (i.e. with CH2=CH2, 33 and 79) or linearly extrapolate these three unsaturated hydrocarbons. From either of these approaches, we find a value of ca 225 kJ mol-1. Cross-conjugation costs some 35 kJ mol-1 in the current case. Interestingly, the directly measured cross-conjugated 1,1-diphenylethylene (126) is only ca 10 kJmol-1 less stable than its directly measured conjugated (E)- 1,2-isomer (40) despite the expected strain effects that would additionally destabilize the former species. 

The r2 is but 0.998 with a standard deviation of ca 1.0 kJ mol 1 (Figure 2). Again the diphenyl species 129 is sorely deviant, this time above the line by some 59 kJmol-1, i.e. in the opposite direction. Despite the nearly 15 kJ mol 1 uncertainty reported for the measurement for enthalpy of formation of diphenylfulvene, these results suggest the... 

From the enthalpies of formation from Roth for the fulvenes and from Pedley for the other hydrocarbons in equations 37 and 38, we find the former reaction is exothermic by 12 kJmol-1 while the latter is endothermic by 12 kJmol-1. Ionic resonance structures analogous to 128 are expected to be of less importance for the ring alkylated species than for the parent species 103 negatively charged carbon is destabilized by adjacent... 

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