Heat of formation and other properties

The importance of the heat of formation arises from the fact that — AH for any reaction can be evaluated in terms of the AH values for the reactant and product molecules. This follows from the additive property of heats of reaction, often referred to as Hess s law. If any given process is broken into a number of parts, then from the fact that if is a state function it is clear that AH for the given process is the sum of the AH values for each part. Since an arbitrary reaction [for example, equation (17)] may be broken into reactions in which each reactant (in succession) is decomposed into its elements (in their standard states) followed by reactions in which the elements are recombined to form the products, it follows that 

AHy values are known for a set of compounds, — AH for any reaction among these compounds may easily be computed. It is therefore conventional to tabulate AHf for all compounds and to use equation (43) to compute heats of reaction from the tables. 

Thestandard heat of formation of a material is its value of AH at one atmosphere and 25°C. Values of standard heats of formation may be found in [13], [15]-[18], and [26]-[29], for example [13] and [29] are particularly extensive. Estimates of standard heats of formation for materials not appearing in tables may be derived by the bond-energy procedure described in [17]. 

In order to obtain —AH at conditions other than 1 atm and 25 C from equation (43) and tables of standard heats of formation, it is necessary to compute the enthalpy change of the reactant mixture and of the product mixture in going from 1 atm and 25°C to the given p and T additional tables are available to facilitate these computations for a number of materials [13], [15] [17], [26]-[28]. Usually the pressure dependence of —AH is negligible, and from equation (38) it follows that for ideal-gas reactions, 

A thermodynamic relationship between the differential heat of reaction, —dH/de, and the temperature dependence of the equilibrium condition for the reaction can be derived in the following way. The relation 

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