Mean bond enthalpy

Bond Bond order Bond length /pm Mean bond enthalpy /(k J moi" )... 

Reaction enthalpies can be estimated by using mean bond enthalpies to determine the total energy required to break the reactant bonds and form the product bonds. In practice, only the bonds that change are treated. Because bond enthalpies refer to gaseous substances, to use the tabulated values, all substances must be gases or converted into the gas phase. 

EXAMPLE 6.14 Using mean bond enthalpies to estimate the enthalpy of a reaction... 

STRATEGY Decide which bonds are broken and which bonds are formed. Use the mean bond enthalpies in Table 6.8 to estimate the change in enthalpy when the reactant bonds break and the change in enthalpy when the new product bonds form. For diatomic molecules, use the information in Table 6.7 for the specific molecule. Finally, add the enthalpy change required to break the reactant bonds (a positive value) to the enthalpy change that occurs when the product bonds form (a negative value). 

TABLE 6.8 Bond Mean Bond Enthalpies (kl mol 1 Mean bond enthalpy ) Bond Mean bond enthalpy... 

A note on good practice. The use of mean bond enthalpies is hazardous because actual bond enthalpies often differ considerably from mean values. The modem procedure for estimating a reaction enthalpy is to use commercial software to calculate the enthalpies of formation of the reactants and products and then to take the difference, as in Section 6.18. 

A mean bond enthalpy is the average molar enthalpy change accompanying the dissociation of a given type of bond. 

The bond enthalpy in NO is 632 kj-mol 1 and that of each N—O bond in N02 is 469 kj-mol. Using Lewis structures and the mean bond enthalpies given in Table 6.8, explain (a) the difference in bond enthalpies between the two molecules (b) the fact that the bond enthalpies of the two bonds in N02 are the same. 

Benzene is more stable and less reactive than would be predicted from its Kekule structures. Use the mean bond enthalpies in Table 6.8 to calculate the lowering in molar energy when resonance is allowed between the Kekule structures of benzene. 

Maxwell, J. C., 164 Maxwell distribution of speeds, 164, 560 mean bond enthalpy, 254 mean relative speed, 559 mechanical equilibrium, 290 mechanics, 1... 

Table 2.7 Mean Bond Enthalpies (kJ mol ) Homonuclear Bonds ...

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

These data for D (M-L) offer some basis for making predictions about the enthalpies of metal-carbon bonds involving other metals in these groups. It is important to bear in mind that all of the data in Table 6 concern the metals in their highest formal oxidation state. It is usually true that the mean bond enthalpy increases as the formal oxidation state of the metal decreases. This is exemplified by the values of >(M-Cl) (M = Nb, Ta, Mo, W) in various oxidation states of M (Table 7a), and... 

Table 7a. Variation of mean bond enthalpy, D (M-Cl) kJ mol 1 with formal oxidation state, (rf-electron configuration) of the metal. (Ref.23))...

The standard enthalpies of formation of BH3(g) and diborane are +100 kj-mol 1 and +36 kj-mol, respectively, and the enthalpies of formation of B(g) and H(g) are +563 kj-mol-1 and +218 kj-mol"1, respectively, (a) Use these values to calculate the mean bond enthalpies of the B—H bonds in each case (b) Assume that terminal B—H bonds have the same strengths in each compound and estimate the bond enthalpy of the three-center B—H—B bonds in diborane. (c) Which bonds would you expect to be longer, the terminal B—H bonds or the three-center bonds Explain your answer. 

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