Bond enthalpy terms

We now turn to the calculation of bond enthalpy terms. In a first approximation, the enthalpy required to break all the chemical bonds in, say, gaseous GeR4, the so-called... 

The calculation of other bond enthalpy terms, such as E (Ge—Ge), E (Ge—O), E (Ge—N) and E(Ge—S), can be made from data in Table 1. However, due to the above-mentioned controversy involving most of the data obtained with static-bomb combustion calorimeters, we refrain from tabulating those terms. 

Despite the previous remarks, organotin compounds are one of the few families of organometallic substances for which the thermochemical data justify the calculation of bond enthalpy terms. Some term values (consistent with the terms recommended by Pilcher and Skinner for hydrocarbons)27 are given in Table 4. A few words on this selection are, however, appropriate. 

The so-called Laidler scheme was developed as a tool to estimate standard enthalpies of formation of organic compounds [90], It relies on the bond-additivity concept, that is, it assumes that the standard enthalpy of atomization of a given molecule in the gas phase (Aat//°, defined as the standard enthalpy of the reaction where all the chemical bonds are cleaved, yielding the gaseous ground-state atoms) can be evaluated by adding the relevant bond enthalpy terms. For instance, in the case of phenol, its standard enthalpy of atomization, or simply its enthalpy of atomization, refers to reaction 5.28 at 298.15 K ... 

The most reliable values of Laidler terms that can applied to a wide variety of compounds are those recommended by Cox and Pilcher [89]. They were derived from a consistent database that includes experimentally determined standard enthalpies of formation for hundreds of organic compounds. This lengthy but simple exercise involves the choice of a set of bond enthalpy terms that affords the best agreement between experimental and calculated standard enthalpies of... 

The calculation of the bond enthalpy term for boron-fluorine bonds in BF3 is ... 

Ethyne, ethene and ethane contain 10. 12 and 14 valence electrons, respectively. They are isoelectronic with N2, 02 and F., respectively. Along both series of molecules the central link becomes progressively weaker and the bond lengths increase. The carbon-carbon bond length increases from 121 pm in ethyne to 133 pm in ethene and to 155 pm in ethane. The corresponding bond enthalpy terms are 837, 612 and 348 kJ mol Data for the diatomic molecules is contained in the text. Discuss these data in terms of the VSEPR and MO treatments of the molecules. 

Decomposition of NH3 (g) into its gaseous atoms breaks 3 N-H bonds and the enthalpy change of the reaction, Ar// can be equated to three (supposedly equal) bond enthalpy terms, thus ... 

Bond Dissociation Energies and Bond Enthalpy Terms... 

Before any discussion about the application of bond enthalpy terms, it is important to check the definitions of bond dissociation energies and bond enthalpy terms... 

Bond enthalpy terms, on the other hand, are quantities assigned to each bond in a molecule such that the sum over all bonds is equal to the enthalpy change associated with the conversion of the molecule into separate atoms. Bond enthalpy terms are assumed to be constant, and therefore transferable from molecule to molecule. So for methane ... 

Bond enthalpy terms can be used to explain differences in the reactivity and bond strengths of the main group elements and to explain general trends such as the inert pair effect and the tendency to form double or single bonds. 

Table 13.2 Some experimental covalent bond enthalpy terms (kJmoP ) the values for single bonds refer to the group 14 elements in tetrahedral environments.

Table 13.2 lists some experimentally determined values for covalent bond enthalpy terms. When we try to interpret the chemistry of the group 14 elements on the basis of such bond energies, caution is necessary for two reasons ... 

Reactions are often bimolecular processes in which bondmaking and bond-breaking occur simultaneously, and in such cases, the rate of reaction may bear no relationship to the difference between bond enthalpy terms of the reactants and products. 

To find the bond enthalpy term, start by writing an equation for the dissociation of gaseous Sip4 into gaseous atoms, and then set up an appropriate thermochemical cycle that incorporates Af ff°(SiF4,g). 

Analogies between groups 14 and 15 are seen if we consider certain bonding aspects. Table 14.3 lists some covalent bond enthalpy terms for group 15 elements. Data for most single bonds follow trends reminiscent of those in group 14... 

Given that AfH (298K) for PH3(g) is +5.4kJmol calculate a value for the P—H bond enthalpy term in PH3. [Other data see Appendix 10. ... 

Construct an appropriate Hess cycle, bearing in mind that the P—H bond enthalpy term can be determined from the standard enthalpy of atomization of PH3(g). 

Calculate a value for the Bi H bond enthalpy term in B1H3 using data from Table 14.4 and Appendix 10. 

Using bond enthalpy terms from Tables 13.2 and 14.3, estimate values of Aj// for the following reactions ... 

Some bond enthalpy terms for compounds of the group 16 elements are given in Table 15.2. In discussing groups 14 and 15, we emphasized the importance of ( j-p)7r-bonding for the... 

Table 15.2 Some covalent bond enthalpy terms (kJmol ) for bonds involving oxygen, sulfur, selenium and tellurium.

An ingenious method of preparing a salt of [113804] is to use reaction 15.107 which is thermodynamically driven by the high Si—F bond enthalpy term in Me3SiF (see Table... 

In a series XY in which the oxidation state of X increases, the X—Y bond enthalpy term decreases, e.g. for the Cl—F bonds in CIF, CIF3 and CIF5, they are 257, 172 and 153kJmor respectively. 

---