Enthalpies of alkanes

Furthermore, an explicit expression of the activation enthalpy of alkane homolysis has also been found. It is written as ... 

A. Warshel and S. Lifson, ]. Chem. Phys., 53, 582 (1970). Consistent Force Field Calculations. II. Crystal Structures Sublimation Energies, Molecular and Lattice Vibrations, Molecular Conformations and Enthalpies of Alkanes. 

CONSISTENT FORCE FIELD CALCULATIONS. II. CRYSTAL STRUCTURES, SUBLIMATION ENERGIES, MOLECULAR AND LATTICE VIBRATIONS, MOLECULAR CONFORMATIONS, AND ENTHALPIES OF ALKANES. 

Warshel, A., Lifson, S. Consistent force field calculations. II. Crystal structures, sublimation energies, molecular and lattice vibrations, molecular cmiRmnations, and enthalpies of alkanes. J. Chem. Phys. 53, 582-594 (1970)... 

Marongiu, B. Excess enthalpies of alkanals (C2-C5) + cyclohexane Int. DATA Sen, Sel. Data Mixtures, Sen A 1984,16,208-211... 

Morawetz E 1972 Enthalpies of vaporization of/i-alkanes from C, 2to C2q J. Chem. Thermodyn. 4 139... 

The bond matrix expresses 2 C—C bonds plus 8 C—H bonds for propane and 3 C—C bonds plus 10 C—H bonds for n-butane. Eaeh enthalpy of atomization is obtained by subtraeting the enthalpy of formation of the alkane from the sum of atomie atomization enthalpies (C 716 H 218 kJ mol ) for that moleeule. For example, the moleeular atomization enthalpy of propane is 3(716) +8(218) — (—104) = 3996 kJ mol . Enthalpies of formation are available from Pedley et al. (1986) or on-line at www.webbook.nist.gov. 

The way in which these factors operate to produce Type III isotherms is best appreciated by reference to actual examples. Perhaps the most straightforward case is given by organic high polymers (e.g. polytetra-fluoroethylene, polyethylene, polymethylmethacrylate or polyacrylonitrile) which give rise to well defined Type III isotherms with water or with alkanes, in consequence of the weak dispersion interactions (Fig. S.2). In some cases the isotherms have been measured at several temperatures so that (f could be calculated in Fig. 5.2(c) the value is initially somewhat below the molar enthalpy of condensation and rises to qi as adsorption proceeds. In Fig. 5.2(d) the higher initial values of q" are ascribed to surface heterogeneity. 

Comparative studies [1028,1052,1053] of the decompositions of Ni, Co and Cu alkanoates from formate to valerate showed that both the cation present and the length of the alkane chain influenced the temperature and enthalpy of decomposition. No such relationship was found [1048], however, between chain length and temperature of reaction of a series of nickel salts between the propionate and the stearate in a study which included some qualitative identifications of the products. Mass... 

Alkanes with long, unbranched chains tend to have higher melting points, boiling points, and enthalpies of vaporization than those of their branched isomers. The difference arises because, compared with unbranched molecules, the atoms of neighboring branched molecules cannot get as close together (Fig. 18.5). As a result, molecules with branched chains have weaker intermolecular forces than their unbranched isomers. 

Very recently, Zavitsas [44] expressed standard enthalpies of formation of alkanes as a simple sum in which each term consists of the number of hydrogen atoms of different types multiplied by an associated coefficient derived from the known enthalpy of formation of typical molecules. 

RoCek et al. have also measured rate coefficients for a series of cyclo-alkanes, (CH2) (/i = 4 to 14), and find the analogue of kcHj in equation (25) to fluctuate with ring size in a manner corresponding exactly to the enthalpy of combustion of the cycloalkane concerned per methylene group, provided n is greater than five, i.e. there exists a direct correlation between reactivity and thermochemical strain. 

Gallicchio, E. Kubo, M. M. Levy, R. M., Enthalpy-entropy and cavity decomposition of alkane hydration free energies numerical results and implications for theories of hydrophobic solvation, J. Phys. Chem. B 2000,104, 6271-6285... 

Piacente, V., Pompili, T., Scardala, P., Ferro, D. (1991) Temperature dependence of the vaporization enthalpies of n-alkanes from vapour-pressure measurements. J. Chem. Thermodyn. 23, 379-396. 

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... 

Thermodynamic properties of alkane chains 13 Thermodynamic properties of ring compounds 15 Enthalpy and entropy effects on ring closure 21... 

The functionalization reaction as shown in Scheme 1(A) clearly requires the breaking of a C-H bond at some point in the reaction sequence. This step is most difficult to achieve for R = alkyl as both the heterolytic and homolytic C-H bond dissociation energies are high. For example, the pKa of methane is estimated to be ca. 48 (6,7). Bond heterolysis, thus, hardly appears feasible. C-H bond homolysis also appears difficult, since the C-H bonds of alkanes are among the strongest single bonds in nature. This is particularly true for primary carbons and for methane, where the radicals which would result from homolysis are not stabilized. The bond energy (homolytic dissociation enthalpy at 25 °C) of methane is 105 kcal/mol (8). 

As with pure steam the properties of binary steam mixtures can be obtained from flow calorimetric measurements of the enthalpy of the mixture. With steam + n-alkane binaries, for which the enthalpies of both components are known, it is more sensible to measure the excess enthalpy directly rather than measure the large total enthalpy of the mixture to determine a small excess quantity. Extrapolation of the excess enthalpy Hp at pressure p to zero pressure yields = XjX2p(2< >j2 jj (f )... 

Table 3.7 Deviations (kcal/mol) in the enthalpies of formation of n-alkanes.a...

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