I got an exercise and I have to calculate de reaction enthalpy for
acetylsalicylic acid formation considering the reaction between salicylic acid and acetic anhydride in presence of an acid.
In the exercise I have a table with values in kJ/mol. I don't know if I'm doing something wrong but I calculated ΔH and it is equal to zero. In another question they say that the experimental ΔH is equal to -5,2 kJ/mol, asking for an explanation for the diference between calculated ΔH and experimental one.
Am I calculating ΔH correctly? And, how this difference could be explained?
In total, one O-H and one C-O bond are both cleaved and formed, thus, the overall enthalpy change calculated by this method is zero (so-called isodesmic reaction).
The discrepancy comes from the fact that the bond dissociation energies in the table are average values taken from many different compounds. For example, in your reaction, a phenolic O-H is cleaved and a "carboxylic" O-H is formed. They certainly have different enthalpies. This method does not take this into account.
Tibor Dubaj your answer makes sense! I never heard about of isodesmic reactions before. Thank you, I'll research more abou this reactions.
- Badges
- Science topic
- Similar topics
- Human-Computer Interaction
- Collaboration