There are several reasons why the calculated value for the enthalpy of combustion for propane using bond energies might not be in close agreement with the values calculated using Hess Law or the summation of enthalpies.
Firstly, the bond energy values used in the calculation are average values and may not accurately represent the specific bonds in propane. These average values can vary depending on the nature and environment of the molecules involved, leading to discrepancies in the calculated enthalpy values.
Secondly, the bond energies approach assumes that all bonds in the reactants are completely broken and all bonds in the products are completely formed. However, in reality, there might be partial bond breaking or formation occurring during the combustion process. This incomplete bond breaking or formation can affect the accuracy of the calculated enthalpy values.
On the other hand, Hess Law and the summation of enthalpies approach consider the overall energy changes in a reaction, taking into account all intermediate steps. These methods are based on experimental data and can provide a more accurate representation of the enthalpy changes during combustion.
Additionally, the bond energies approach does not consider any entropy changes that may occur during the reaction, which might also contribute to the discrepancy in the calculated values. The Hess Law and summation of enthalpies methods can account for any entropy changes and provide a more comprehensive analysis of the enthalpy changes involved.
In summary, the bond energies approach might not be in close agreement with Hess Law or the summation of enthalpies due to the use of average bond energy values, incomplete bond breaking or formation, neglecting entropy changes, and not considering the overall energy changes in a reaction.