Thermochemistry standard-state enthalpy changes

The enthalpy change for a chemical reaction in which all reactants and products are in their standard states and at a specified temperature is called the standard enthalpy (written AFf°) for that reaction. The standard enthalpy is the central tool in thermochemistry because it provides a systematic means for comparing the energy changes due to bond rearrangements in different reactions. Standard enthalpies can be calculated from tables of reference data. For this purpose, we need one additional concept. The standard enthalpy of formation AH° of a compound is defined to be the enthalpy change for the reaction that produces 1 mol of the compound from its elements in their stable states, all at 25°C and 1 atm pressure. For example, the standard enthalpy of formation of liquid water is the enthalpy change for the reaction... 

Note that arbitrarily assigning zero AHf for each element in its most stable form at the standard state does not affect our calculations in any way. Remember, in thermochemistry we are interested only in enthalpy changes, because they can be determined experimentally whereas the absolute enthalpy values cannot. The choice of a zero reference level for enthalpy makes calculations easier to handle. Again referring to the terrestrial altitude analogy, we find that Mt. Everest is 8708 ft higher than Mt. McKinley. This difference in altitude is unaffected by the decision to set sea level at 0 ft or at 1000 ft. 

Step 1 is the decomposition of reactants into elements in their standard states. But this is just the opposite of the formation reaction of the reactants, so the enthalpy change of the process is -AHf°(reactants). Similarly, Step 2, the formation of the products from elements in their standard states, has an enthalpy change of AHf°(products). Remember, however, that the formation reaction is defined for the generation of one mole of the compound. Consequently, to use tabulated heats of formation we must multiply by the stoichiometric coefficients from the balanced equation to account for the number of moles of reactants consumed or products generated. Taking these factors into account leads to one of the more useful equations in thermochemistry. 

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