Conventional values of molar enthalpies

The molar enthalpy H ofany substance is a function of T and p,H = H(T, p). Choosing p = 1 atm as the standard pressure, we define the standard molar enthalpy H° of a substance by 

From this it is clear that H° is a function only of temperature. The degree superscript on any thermodynamic quantity indicates the value of that quantity at the standard pressure. (Because the dependence of enthalpy on pressure is very slight—compare with Section 7.15—we will often use standard enthalpies at pressures other than one atm the error will not be serious unless the pressure is very large, for example, 1000 atm.) 

As we showed in Section 7.19, the enthalpy change in any chemical reaction does not depend on the numerical values of the enthalpies of the elements that compose the compound. Because this is so we may assign any arbitrary, convenient values to the molar enthalpies of the elements in their stable states of aggregation at a selected temperature and pressure. Clearly, if we chose the required values randomly from the numbers in a telephone directory this could introduce a good deal of unnecessary numerical clutter into our work. Since the numbers do not matter, they can all be the same if they can all be the same, they all might as well be zero and eliminate the clutter entirely. 

The enthalpy of every element in its stable state of aggregation at 1 atm pressure and at 298.15 K is assigned the value zero. For example, at 1 atm and 298.15 K the stable state of aggregation of bromine is the liquid state. Hence, liquid bromine, gaseous hydrogen, solid zinc, solid (rhombic) sulfur, and solid (graphite) carbon all have = 0. 

Given the definition of the formation reaction, if we introduce the conventional assignment, 77°(elements) = 0, into the expression for the heat of formation, Eq. (7.63) or Eq. (7.64), we find that for any compound 

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