Final answer:
The values from mean bond enthalpy calculations differ from those determined using Hess's law because they are based on average bond strengths, whereas Hess's law considers the actual bond energies of the specific bonds in the reaction.
Step-by-step explanation:
The values from mean bond enthalpy calculations differ from those determined using Hess's law because they are based on average bond strengths, whereas Hess's law considers the actual bond energies of the specific bonds in the reaction.
Mean bond enthalpies are calculated by taking the average bond energy values from a set of different compounds, whereas Hess's law uses the bond energy values of the specific bonds involved in the reaction.
Therefore, the values obtained using mean bond enthalpies may not accurately represent the actual bond strengths in the reaction, leading to differences from the values determined using Hess's law.