Explain why the calculated value for the (1) enthalpy of combustion for propane using bond energies (bond breaking and bond formation)might not be in close agreement to the values calculated using (2) Hess Law and the values calculated using the (3) summation of the change in enthalpies of products minus the summation the change of enthaplpies of reactions?
(1) Bond Energies Value:
(2) Hess Law Value:
(3) Summation of Enthalpies Value:
Explanation:
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.
The calculated value for the enthalpy of combustion for propane using bond energies may not be in close agreement with the values calculated using Hess Law or the summation of enthalpies for a few reasons.
(1) Bond Energies Value: The bond energy approach utilizes the average bond energies of the various bonds broken and formed during the combustion reaction. However, these average bond energies are not exact values and can vary depending on the specific molecules involved. In addition, this method assumes that all bonds in the reactants and products are broken or formed simultaneously, which may not be the case in reality. These approximations can lead to some discrepancies in the calculated enthalpy value.
(2) Hess Law Value: Hess's Law states that the enthalpy change of a reaction does not depend on the pathway taken, but only on the initial and final states. This method involves adding individual enthalpy changes of known reactions to obtain the overall enthalpy change. However, the accuracy of this approach relies on the accurate experimental determination of the individual enthalpies of those reactions. Small errors in these individual enthalpy values can accumulate and result in discrepancies in the calculated enthalpy of combustion for propane.
(3) Summation of Enthalpies Value: This method involves calculating the change in enthalpy for each component of the reaction (reactants and products) and then subtracting the sum of the enthalpies of the reactants from the sum of the enthalpies of the products. However, accurately determining and measuring the enthalpies of individual species can be challenging and may introduce errors. In addition, this method assumes that there are no significant changes in temperature or pressure during the reaction, which may not always be the case.
Overall, the discrepancies between the calculated values using these different approaches can arise due to approximations, experimental errors, and assumptions made in each method. To obtain more accurate results, one can use experimental data or more sophisticated computational methods.