Hess's law a) makes it possible to calculate δh for complicated chemical reactions b) states that when you reverse a chemical equation, the sign of δh does not change. c) determines the way a calorimeter works d) describes the vaporization of solids

A.)   makes it possible to calculate ΔH for complicated chemical reactions 

Hess’s Law makes it possible to calculate ΔH for complicated chemical reactions. Moreover, the principle claims that the entire amount of “enthalpy” when the development or progression of a chemical reaction is complete whether the reactions are created in a single or numerous processes, it still changes. This law is now described as the principle of conservation of energy, and part of the thermodynamics law as the first.  

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Ithink is a because if say increased in electronegativity