Thermodynamic aspects of hydrogen insertion

Standard enthalpies of formation of pure phases H ,MO and enthalpy changes for the insertion reaction (I) 

The insertion process, I, is invariably exothermic but — A// decreases with increasing degree of reduction, x. For different systems, the magnitude of — Aff, parallels the oxidizing capacity of the parent oxide MO , as illustrated by the sequence V2O5 M0O3 WO3, for example. 

These qualitative trends can be rationalized and put on a quantitative basis by using a simple bond model for the topotactic insertion of hydrogen into It is assumed that the product is an oxy-hydroxide 

From these assumptions of bond additivity, it follows that qh be identified as the enthalpy change for the reaction 

A plot of —AHi (for systems of different parent oxides and products which have comparable x values in against the term 

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