Bond enthalpies atomization

Derivation of bond enthalpies from themioehemieal data involves a system of simultaneous equations in which the sum of unknown bond enthalpies, each multiplied by the number of times the bond appears in a given moleeule, is set equal to the enthalpy of atomization of that moleeule (Atkins, 1998). Taking a number of moleeules equal to the number of bond enthalpies to be determined, one ean generate an n x n set of equations in whieh the matrix of eoeffieients is populated by the (integral) number of bonds in the moleeule and the set of n atomization enthalpies in the b veetor. (Obviously, eaeh bond must appear at least onee in the set.)... 

What is the average enthalpy of atomization of the four C—H bonds in methane Compare this value with the accepted value of the C—H bond enthalpy. 

Calculate the bond enthalpy of the C—C bond in ethane using only the enthalpies of atomization of methane and ethane. Compare this result with the accepted result. 

Table 6.3 Some enthalpies of atomization (Af/j, 298 K) and comparative bond-enthalpy contributions, E...

Bond enthalpies for a variety of single and multiple bonds are listed in Table 8.4. Note that bond enthalpy is always a positive quantity heat is always absorbed when chemical bonds are broken. Conversely, heat is given off when bonds are formed from gaseous atoms. Thus... 

You will note from Table 8.4 that the bond enthalpy is larger for a multiple bond than for a single bond between the same two atoms. Thus... 

We should point out a serious limitation of the bond enthalpies listed in Table 8.4. Whenever the bond involves two different atoms (e.g., O—H) the value listed is approximate rather than exact, because it represents an average taken over two or more different species. Consider, for example, the O—H bond where we find... 

Draw the Lewis structure for the hypothetical molecule N6, consisting of a six-membered ring of nitrogen atoms. Using bond enthalpies, calculate the enthalpy of reaction for the decomposition of N6 to N2(g). Do you expect N6 to be a stable molecule ... 

Carbon monoxide is a colorless, odorless, flammable, almost insoluble, very toxic gas that condenses to a colorless liquid at — 90°C. It is not very reactive, largely because its bond enthalpy (1074 kj-mol-1) is higher than that of any other molecule. However, it is a Lewis base, and the lone pair on the carbon atom forms covalent bonds with J-block atoms and ions. Carbon monoxide is also a Lewis acid, because its empty antibonding Tr-orbitals can accept electron density from a... 

When there are bonds of two or more kinds in a molecule, the determination of the bond enthalpies is slightly more complicated and is based on the assumption that bond enthalpies can be transferred from one molecule to another, at least to a reasonable approximation. For example, the enthalpy of atomization of ethane, which is 2826 kJ mol-1, is the sum of six C—H bond enthalpies and one C—C bond enthalpy. The C—C bond enthalpy can be determined only if we make an assumption about the C—H bond enthalpy. If we make the rea-... 

The group 14 M—M bond enthalpy may also be derived as one-half the enthalpy of the atomization process6... 

The chemical environment of a given atom in a molecule will influence the magnitude of the bond enthalpy, so tabulated data such as that in Table 3.3 represent average values. 

Table 3.3 Table of mean bond enthalpies A H- - as a function of bond order and atoms. All... 

On the assumption that the bond enthalpies M M and M C0 increase systematically down the triad Fe, Ru, and Os, it may be taken that the tendency to form M(CO)s remains more or less constant. Part of the kinetic stability will then be due to the availability of a bridged form of Os3(CO)12. This does not necessarily mean that the bridged form has to be of the same structure as Fe3(CO)12, but merely one in which there are five carbonyl groups in association with the osmium atom. Thus, an alternative might be... 

Fig. 1. Variation of the terminal metal-CO bond enthalpy contribution, T (M-CO) kj mol as a function of the enthalpy of atomization of the metal, AHf (M, g) kJ mol-1...

In summary, the previous example shows that bond dissociation enthalpies should not be correlated with bond lengths unless the relaxation energies of the fragments are comparable. On the other hand, when two bonds between the same pairs of atoms have identical bond lengths, it is sensible to assume that they have similar bond enthalpy contributions. Hence, in this case, a bond enthalpy contribution can be transferred from one molecule to another. 

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 ... 

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