Why isn't Hess's law helpful to calculate the heat of reaction involved in converting a diamond to graphite?
The free energy difference between graphite and diamond is rather small; graphite is a tad more thermodynamically stable. The activation energy required for the conversion would be monstrously large!
I don't know off hand the free energy difference between the 2 carbon allotropes; it is relatively small. The activation energy required for the conversion would be absolutely huge; so that the error in calculating or measuring the energy change is probably higher than (or at least comparable to) the value of the energy difference. Does this address your question?