Bond enthalpies values

Polymers such as PVC and PTFE ( Teflon ) are very stable. Use the bond enthalpy values E(C-C1) = +346kJmor and E(C-F) = + 480kjmor to explain this fact. (2)... 

The Mi° values do not agree exactly because average bond enthalpies are nsed, and nitroglycerin is a liquid (strictly, the bond enthalpy values are for gases). 

Bond enthalpy values derived from the aforementioned calorimetric approaches and all other D(M-R) quantities determined in solution (vide infra) are not strictly gas phase quantities as specified in the formal definitions (Equations 1,3,4). However,... 

In this section we will apply some of the concepts mentioned above to discuss the energetics of a number of relevant elementary reactions. The required bond enthalpy values will be taken from Table 1 and the entropy values from the literature or estimated. It is worth pointing out that 7A,.y terms are often disregarded in similar discussions, mainly because most enthalpy data are obtained by calorimetric methods, rather than by van t Hoff plots. However, they may determine the thermodynamics of organometallic reactions and therefore should always be considered. As mentioned above, there is a very simple method to estimate entropy terms.This method will be used in the following discussion. The temperature will always be referred to 298 K, unless stated otherwise. 

The reality is, however, different the photochemically generated W(Cp)2 prefers to react with itself and forms a dimer where each tungsten is bonded to a hydrogen atom and to a Cp ring of the other tungstenocene moiety.The efficiency of this competitive reaction is likely to be mainly kinetic rather than thermodynamic. Although the estimate for ArG° (66) could be questioned (recall that it relies on anchored bond enthalpy values), it seems to be in line with the fact that the compounds W(Cp)2(Me)H and W(Cp")2(Me)H are stable at room temperature. 

We observe that bond strengths vary with the number of shared electron pairs as well as other factors. We use average bond enthalpy values to estimate the enthalpies of reactions in cases where thermodynamic data are unavailable. 

Employing the bond enthalpy values hsted in Table 8.4, estimate the molar enthalpy change occurring upon (a) polymerization of ethylene, (b) formation of nylon 6,6, (c) formation of polyethylene terephthalate (PET). 

Strategy We basically follow the same procedure as that in Example 9.13. Note, however, that H2O is a polyatomic molecule, and so we need to use the average bond enthalpy value for the O—H bond. 

Use the bond enthalpy values in Table 7.6 to calculate the enthalpy of combustion for ethane (C2H5) ... 

See Figiue 7.28. Strictly speaking the average bond enthalpy values apply to species in the gas phase. The heat of formation of Sg(g) is 102.3 kj/mol. Apply the needed correction in order to estimate the enthalpy change for the reaction as shown.)... 

Employing the bond enthalpy values listed in Table 8.4, estimate the molar enthalpy change occurring upon... 

Calculating enthalpy changes from known bond enthalpy values and comparing these to experimentally measured values... 

We have used the mean bond enthalpy value from Table 1.4 for the O-H bond and the exact bond enthalpy value for the 0-0 bond in HO-OH from Table 1.3. In the second step, four O-H bonds and one 0=0 bond are formed. The standard enthalpy change for bond formation (the reverse of dissociation) is the negative of the bond enthalpy. We can use exact values for the enthalpy of the O-H bond in H20(g) and for the 0=0 bond in Oj(g) ... 

The controversy as to how best to write Lewis structures will no doubt continue in the chemical literature, but you should not be too dismayed by this situation. Our approach to depicting the electronic structure of a molecule is based on the simplest Lewis structure and its concomitant use in determining the shape of a molecule through VSEPR theory. In order to probe more deeply into the nature of a chemical bond—for example, to understand experiment results, such as bond enthalpy values— we must analyze a computed electron density map for that molecule rather than rely just on the Lewis structure. 

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