Bond enthalpy of formation

Enthalpies are often used to describe the energetics of bond formations. For example, when an amide forms through the condensation reaction between an ester and an amine, the new C-N bond, has an enthalpy of formation of -293 kj/mole. The higher the negative value for the bond enthalpy of formation, the stronger the bond. An even more useful concept is the enthalpy of a reaction. For any reaction, we can use the fact that enthalpy is a state function. A state function is one whose value is independent of the path traveled. So, no matter how we approach a chemical reaction, the enthalpy of the reaction is always the same. The enthalpy of... 

The thermochemical characteristics of l,3,5-trinitro-l,3,5-triazepane, such as energies of dissociation of N-NOz bonds, enthalpies of formation, vaporization, and combustion, as well as enthalpy of formation of amine radicals, have been determined <2004MI92>. The rate constants of initial monomolecular stages of thermal decomposition in the solid phase were measured for its furazano-fused analog 20 <1999RCB1250> and the ratio of the rate constants of decomposition in the melt and solid states, characterizing the reaction retardation in the crystal lattice, was determined. The kinetics of the thermal decomposition of 20 has also been studied <1995MI885>. 

A/14 the enthalpy of reaction, which is in this case twice the enthalpy of formation of hydrogen chloride. Clearly A/14 is the difference between the total bond energies of the products and the total bond energies ol the reactants, lhat is... 

Note the much larger enthalpy of formation of silieon dioxide as compared with carbon dioxide this arises in part because of greater strength in the Si—O bonds and also because the Si—Si bond in silieon is mueh weaker than the C—C bond (p. 162). 

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. 

Solve the same problem for propane and isobutane (2-methylpropane). The bond matrix is the same as it is for n-butane, but the enthalpy of formation is somewhat different (n-butane) = —127.1 kJ mol vs. (isobutane) = —134.2... 

Our results are in very good agreement with Benson s simpler bond additivity values (2.5 kcal mol and —3.75 kcal mol Benson and Cohen, 1998), as indeed they must be because they were obtained from the same set of experimental enthalpies of formation. Note that many applications in themiochemishy use energy units of kilocalories per mole, where 1.000 kcal mol =4.184 kJ mol . ... 

The difference between an MM calculation of the enthalpy of formation and a bond energy scheme comes in the steric energy, which was shown in Eile 4-3. The sum of compression, bending, etc. energies is the steric energy, E = 2.60 kcal mol in Eile 4-3. This is added to BE, as is the partition function energy contribution (see below), PCE = 2.40 kcal moP, to yield... 

A force field that can produce vibrational spectra has a second advantage in that the Ay// calculations can be put on a much more satisfactory theoretical base by calculating an enthalpy of formation at 0 K as in ab initio procedures and then adding various thermal energies by more r igorous means than simply lumping them in with empirical bond enthalpy contributions to Ay//-. The stronger the theoretical base, the less likely is an unwelcome surprise in the output. 

Write and run an MM3 input file for methane from scratch, that is, open an empty file and put in all the necessary infomiation to do the MM3 calculation on CH4. What is the enthalpy of formation of CH4 What are the C—H bond lengths and angles ... 

What is the MM3 enthalpy of formation at 298.15 K of styrene Use the option Mark all pi atoms to take into account the conjugated double bonds in styrene. Is the minimum-energy structure planar, or does the ethylene group move out of the plane of the benzene ring ... 

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. 

The heat of formation of 804X4 was determined to be AH°f = +163 kcal mor. " Thus 804X4 is even more endothermic than S4X4 (AH°f = +110 kcal mok ). The mean E-X bond energies in E4X4 were estimated to be 59 kcal mol (E = Se) " and 72 kcal mol (E = S) from the enthalpies of formation. 

The endothermic radical lO has also been studied in the gas phase the interatomic distance is 186.7 pm and the bond dissociation energy 175 20kJmol . It thus appears that, although the higher oxides of iodine are much more stable than any oxide of Cl or Br, nevertheless, lO is much less stable than CIO (p. 849) or BrO (p. 851). Its enthalpy of formation and other thermodynamic properties are A//f(298K) 175.1 kJmol", AGf(298 K) 149.8 kJmol-, 5°(298 K) 245.5 J K- mor . 

The atom and bond concepts dominate chemistry. Dalton postulated that atoms retained their identities even when in chemical combinations with other atoms. We know that their properties are sometimes transferable from one molecule to another for example, the incremental increase in the standard enthalpy of formation of a normal hydrocarbon per CHj group is —20.6 1.3 kJmol . We also know that more often there are subtle modifications to the electron density. 

Table 2 gives our calculated results for the equilibrium volume Vq, bulk modulus Bq, and enthalpy of formation AH. Theoretical results refer to T=0, uncorrected for zero point motion, whereas experimental values refer to room temperature. Note that the extensive quantities AH and Vq arc reported per atom in the present paper, i.e., divided by the total number of atoms. As well known the LDA underestimates the volume. Comparing the bulk modulus for T3 and D8s we see that the addition of Si to pure Ti has a large (26 %) effect on the bulk modulus, indicating that p electrons of Si have a strong effect on the bonding in this system. 

We have performed full-potential calculations on TisSia in its proposed stable crystal structure. The enthalpy of formation obtained from these calculations agrees well with the value deduced from experiment. Due to the low crystal symmetry, the possibility of a more complex bonding character arises. The charge density in this phase differs considerably from that in the hypothetical unstable structure, so two-electron bonds can be excluded in this phase. We have also showed that the opening of a quasigap in the Si DOS has its origin in the Ti-Si interaction. 

This limitation explains why, whenever possible, we use enthalpies of formation (AFff) rather than bond enthalpies to calculate the value of AH for a reaction. Calculations involving enthalpies of formation are expected to be accurate within 0.1 kj the use of bond enthalpies can result in an error of 10 kj or more. 

However, the fact that the derived S—O bond dissociation energy in sulfuric acid is identical to that found in the acid derivatives, strongly supports the estimated enthalpy of formation for gas-phase sulfurous acid given by Benson18. 

TABLE 9. Bond energies in sulfoxides and sulfones, and enthalpies of formation of sulfinyl and sulfonyl radicals... 

In the course of this study, the authors determined /Lvalues for dibenzyl, methyl phenyl, methyl p-nitrophenyl, di-p-tolyl, di-isopropyl and tetramethylene sulphoxides and for diethyl, dipropyl and dibutyl sulphites. The /Lscales are applied to the various reactions or the spectral measurements. The /Lscales have been divided into either family-dependent (FD) types, which means two or more compounds can share the same /Lscale, family-independent (FI) types. Consequently, a variety of /Lscales are now available for various families of the bases, including 29 aldehydes and ketones, 17 carboxylic amides and ureas, 14 carboxylic acids esters, 4 acyl halides, 5 nitriles, 10 ethers, 16 phosphine oxides, 12 sulphinyl compounds, 15 pyridines and pyrimidines, 16 sp3 hybridized amines and 10 alcohols. The enthalpies of formation of the hydrogen bond of 4-fluorophenol with both sulphoxides and phosphine oxides and related derivatives fit the empirical equation 18, where the standard deviation is y = 0.983. Several averaged scales are shown in Table 1588. 

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