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Hess' Law of Constant Heat Summation was developed by Germain Henri Hess who lived from 1802 - 1850. This law became important primarily for his thermochemical studies. This law states that the heat released or absorbed in the chemical reaction is unchanged or the same irregardless of how many steps the reactants take in order to convert to products.

This law is not applicable or not helpful in the conversion of diamond to graphite because both are just allotropes of carbon along with C60 (Buckminsterfullerene or buckyball), C540, C70, amorphous carbon, and carbon nanotube. 
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Hess's law is applicable when the reactants are different than the products. In a chemical reaction there are rearrangement of atoms such that the chemical property of the reactant is different than that of the product.

Diamond and graphite are purely made up of C atoms with different arrangements of C atoms thus making them allotropes. Hess's law is not applicable for allotropes.