# Why is the heat of formation defined?

by Woopydalan
Tags: defined, formation, heat
 P: 380 Enthalpy of formation describes a special reaction where you form some compound from "scratch." Of course we can't violate energy/matter conservation, so we simply assign a value of zero to the most stable elemental form of the elements which make our compound at some temperature and 1 bar pressure. Its a way for us to set an arbitrary benchmark from which to start measuring other things, like heats of reaction which can be calculated by summing the heats of formation of each term in the chemical equation weighted by their stoichiometric coefficients and where the coefficients are positive for products and negative for reactants ($\sum _{i} \Delta H ^{°} _{f,i} \ast \nu _{i}$). It helps to keep in mind the path independent nature of the state functions in thermodynamics, you imagine that any compound you are considering will have ultimately come from the constituents in there elemental form which have a defined enthalpy of zero and became transformed into your compound. Think Standard Hydrogen Electrode for defining E°. These kinds of definitions allow us to set up scales and make useful measurements.