Bonding standard enthalpy

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. 

J 16 Use average bond enthalpies to estimate the standard enthalpy of a reaction, (Example 6.14). 

Phenol and hexamethylphoshoramide (HMPA) were selected, respectively, as the standard H-bond donor and H-bond acceptor with their values fixed on free energy and enthalpy H-bond scales for phenol, -2.50 for the H-bond donor enthalpy factor (TJ and also for the H-bond donor free energy factor (Ca) for HMPA, 2.50 for the H-bond acceptor enthalpy factor ( J and 4.00 for the H-bond acceptor free energy factor (CJ. 

Simple amides of this type are the bis(trimethylsilyl)amides M[N(SiMe3)2]2 (M = Cd and Hg) the essential thermodynamic data of which have been determined in calorimetric measurements of the heats of hydrolysis in dilute H2S04.146 Evaluation of the measured data yielded the standard enthalpies of formation AH° = —854(21)kJmoU1 and —834(9)kJmol-1 for M =Cd and Hg, respectively. Using subsidiary data, the average thermochemical bond energies E—(Cd—N) 144 and E(Hg—N) 108 kJ mol-1 were also obtained, i.e., the Cd—N bonds are considerably stronger than the Hg—N bonds. 

The standard enthalpy of formation, A fH, of a compound at 0 K reflects the strength of the chemical bonds in the compound relative to those in the constituent elements in their standard state. The standard enthalpy of formation of a binary oxide such as CaO is thus the enthalpy change of the reaction... 

The bond-dissociation energy is defined as the standard enthalpy change of the reaction in which the bond is broken R X - R —X. 

The standard enthalpy of formation of monomeric HF is a hypothetical state that must be related to that of the real associated liquid, gas, or aqueous solution met in calorimetiy. Considerable difficulty has been encountered in allowing for the heat of association, which varies with temperature and pressure. For example, the presence of traces of water can affect the polymerization by entering into the hydrogen bonding (30) the treatment of results will depend on the association model adopted. The magnitude of corrections for gas imperfections has... 

Table 1. Standard enthalpy of formation of metal carbonyls [Mm(CO) ] in the gas phase. Bond description and bond enthalpy contributions (T, M and B) to the enthalpy of disruption, AHq...

Table 9. Valence m.o. electron configuration Ofg e"g e%g, enthalpy of sublimation, AHSU, and standard enthalpy of formation, (AHf, g), of metallocenes. Mean bond dissociation enthalpy, D (M-Cp) (Refs.68 69 ) and (Metal-cyclopentadienyl ring) bond length, r(M-Cp). (Ref.72 ).

Table 12a. Standard enthalpy of formation, A/ff(g), enthalpy of disruption, AHjy, and metal-halogen bond enthalpy contribution, (M-X), in metal carbonyl halides (kJ mol-1)...

Table 14. Standard enthalpies of sublimation, formation and disruption and bond enthalpy contributions, iT(W-N) kJ mol-1, for N-donor complexes of tungsten W(CO)6 nLn]...

Adiabatic detachment energy [7]. Abbreviations used rcoy (Em) = covalent radius of element E in a trivalent compound BE(E-E) = bond enthalpy of a single E-E bond D°298(E2) = dissociation enthalpy of the E2 molecule at standard conditions IE = ionization enthalpy EA = electron affinity AHf°(E2 g) = standard enthalpy of formation of the gaseous E2 molecule. 

No symbol has been approved by the IUPAC for dissociation energy in the chemical thermodynamics section [13]. Under Atoms and Molecules, either El or D is indicated. The latter is more common, and IUPAC recommends Do and De for the dissociation energy from the ground state and from the potential minimum, respectively. Because the bond energy concept will be omnipresent in this book and can be explored in a variety of ways, some extra names and symbols are required. This matter will be handled whenever needed, but for now we agree to use DUP for a standard bond dissociation internal energy and DHj for a standard bond dissociation enthalpy, both at a temperature T. In cases where it is clear that the temperature refers to 298.15 K, a subscript T will be omitted. 

Although standard enthalpies of formation provide information about the net stability of molecules and their transformations, they do not always indicate stability of individual bonds. This analysis normally involves parameters, loosely called bond energies, that reflect the amount of energy required to cleave chemical bonds. 

Consider a molecule AB, where A and B can be atoms or groups of atoms. The A-B bond dissociation enthalpy, represented by DH (A-B), is defined as the standard enthalpy of the gas-phase reaction where the only event is the cleavage of that bond at a given temperature ... 

Bearing in mind that a bond dissociation enthalpy is simply the standard enthalpy of a particular reaction, the relationship between two values of )//) ( A B) at different temperatures is easily obtained as... 

According to the definition of the A-B bond dissociation enthalpy, reactants and products in reaction 5.1 must be in the gas phase under standard conditions. That is to say that those species are in the ideal gas phase, implying that inter-molecular interactions do not exist. DH (A-B) refers, therefore, to the isolated molecule AB, and it does not contain any contribution from intermolecular forces. Though this is obviously the correct way of defining the energetics of any bond, there are many literature examples where bond dissociation enthalpies have been reported in solution. 

Figure 5.1 Thermochemical cycle (f = 298.15 K), showing how solution and gas-phase bond dissociation enthalpies are related. AS0 VH° are standard enthalpies of solvation.

There is an additional advantage in using relative solution phase bond dissociation enthalpies. In most cases, solution phase bond dissociation enthalpies do not refer to standard states (see section 2.3), and the required corrections are hard to predict. When we consider relative bond dissociation enthalpies in a series of similar molecules in solution it is likely that the unknown corrections to standard states are nearly constant. 

Experimental values of bond dissociation enthalpies are scarce compared with the data available for standard enthalpies of formation. This is not surprising because most chemical reactions that have been studied thermochemically involve the cleavage and the formation of several bonds. The measured standard reaction enthalpies are thus enthalpy balances of various bond dissociation enthalpies, whose individual values are often unknown. Consider, for example, reaction 5.10, where the arene ring in (q6-bcnzene)chromium tricarbonyl is replaced by three carbonyl ligands. The enthalpy of this reaction at 298.15 K,... 

Although we ignore the values for some of the six stepwise Cr-CO bond dissociation enthalpies in chromium hexacarbonyl, their sum is equal (equation 5.16) to the enthalpy of reaction 5.15, which is calculated as 641.7 4.8 kJ mol-1 (at 298.15 K) from the well-known standard enthalpies of formation of all the species involved [16,17,31],... 

Let us concentrate on the thermochemical cycle of figure 5.6 that involves the disruption of the complex Cr(CO)3(C6H6). The enthalpy of this reaction, previously calculated as 497.9 10.3 kJ mol-1 from standard enthalpy of formation data, can be related (equation 5.24) to the bond enthalpy contributions EsfCr-CO ) andE s(Cr (V.He) through the reorganization energies ER(C() ) and ER(C(tHf )- Two asterisks indicate that the fragment has the same structure as... 

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... 

If we make the assumption that the reverse of reaction 15.5 is diffusion-controlled and assume that the activation enthalpy for the acyl radicals recombination is 8 kJ mol-1, the enthalpy of reaction 15.5 will be equal to (121 - 8) = 113 kJ mol-1. This conclusion helps us derive other useful data. Assuming that the thermal correction to 298.15 K is small and that the solvation enthalpies of the peroxide and the acyl radicals approximately cancel, we can accept that the enthalpy of reaction 15.5 in the gas phase is equal to 113 kJ mol-1 with an estimated uncertainty of, say, 15 kJ mol-1. Therefore, as the standard enthalpy of formation of gaseous PhC(0)00(0)CPh is available (-271.7 5.2 kJ mol-1 [59]), we can derive the standard enthalpy of formation of the acyl radical Af//°[PhC(0)0, g] -79 8 kJ mol-1. This value can finally be used, together with the standard enthalpy of formation of benzoic acid in the gas phase (-294.0 2.2 kJ mol-1 [59]), to obtain the O-H bond dissociation enthalpy in PhC(0)0H DH° [PhC(0)0-H] = 433 8 kJ mol-1. 

The types of values reported in the database standard enthalpies of formation at 298.15 K and 0 K, bond dissociation energies or enthalpies (D) at any temperature, standard enthalpy of phase transition—fusion, vaporization, or sublimation—at 298.15 K, standard entropy at 298.15 K, standard heat capacity at 298.15 K, standard enthalpy differences between T and 298.15 K, proton affinity, ionization energy, appearance energy, and electron affinity. The absence of a check mark indicates that the data are not provided. However, that does not necessarily mean that they cannot be calculated from other quantities tabulated in the database. 

The acidity of a substrate AH in the gas phase is measured using the standard enthalpy change A//°cid for the heterolytic bond dissociation ... 

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