Thermochemistry of cluster compounds

9 Thermochemistry of cluster compounds. In this short summary of cluster structures and their bonding, a few remarks on their thermochemical behaviour are given, in view of a possible relationship with the intermetallic alloy properties. To this end we remember that for molecular compounds, as for several organic compounds, concepts such as bond energies and their relation to atomization energies and thermodynamic formation functions play an important role in the description of these compounds and their properties. A classical example is given by some binary hydrocarbon compounds. 

Before carrying out such an examination, however, it is necessary to underline that thermochemical quantities, even if in principle accurate and significant, in practice are affected by experimental errors, and often by inaccuracies, due to the need to obtain relatively small values as differences between large values. As a consequence, the reader may find in books and treatises differences in the values of the specific bond energies as large as 5% to 10%. 

Taking methane, CH4, as a reference compound, its standard enthalpy of formation is defined by the reaction  

Considering now the sublimation enthalpy of graphite (evaluated for the sublimation to monoatomic C gas) and the dissociation energy of hydrogen  

A similar calculation performed on the next hydrocarbon, C2H6, takes into account its formation enthalpy given by the reaction  

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