Standard molar enthalpies of reaction and formation

A standard molar reaction enthalpy, ArH°, is the same as the molar integral reaction enthalpy A//ni(rxn) for the reaction taking place under standard state conditions (each reactant and product at unit activity) at constant temperature (page 318). 

At constant temperature, partial molar enthalpies depend only mildly on pressure. It is therefore usually safe to assume that unless the experimental pressure is much greater than p°, the reaction is exothermic if ArH° is negative and endothermic if A H° is positive. 

The formation reaction of a substance is the reaction in which the substance, at a given temperature and in a given physical state, is formed from the constituent elements in their reference states at the same temperature. The reference state of an element is usually chosen to be the standard state of the element in the allotropic form and physical state that is stable at the given temperature and the standard pressure. For instance, at 298.15 K and 1 bar the stable allotrope of carbon is crystalline graphite rather than diamond. 

Phosphorus is an exception to the rule regarding reference states of elements. Although red phosphorus is the stable allotrope at 298.15 K, it is not well characterized. Instead, the reference state is white phosphorus (crystalline P4) at 1 bar. 

The standard molar enthalpy of formation (or standard molar heat of formation), 

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