Enthalpy of physical transformations and chemical reactions

The enthalpy that accompanies a change of physical state at standard conditions is called the standard enthalpy of transition and is denoted A trsH°. Enthalpy changes accompanying chemical reactions at standard conditions are in general termed standard enthalpies of reaction and denoted A r H0. Two simple examples are given in Table 1.3. In general, from the first law, the standard enthalpy of a reaction is given by 

Of particular importance is the standard molar enthalpy of formation,,  

These are derived by subtraction of the standard molar enthalpy of formation of the binary oxides, since standard enthalpies of individual reactions can be combined to obtain the standard enthalpy of another reaction. Thus, 

Whereas the enthalpy of formation of A12Si( 5 from the elements is large and negative, the enthalpy of formation from the binary oxides is much less so. Af ox m is furthermore comparable to the enthalpy of transition between the different polymorphs, as shown for A SiOs in Table 1.5 [3], The enthalpy of fusion is also of similar magnitude. 

The temperature dependence of reaction enthalpies can be determined from the heat capacity of the reactants and products. When a substance is heated from T to T2 at a particular pressurep, assuming no phase transition is taking place, its molar enthalpy change from AHm (T]) to AHm (T2) is 

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