Question

How the formation enthalpy of a compound is related to its stability ?For eg- enthalpy of formation of graphite is zero whereas enthalpy of formation of fullerness and diamond is 38.1 and 1.90 kj mol ^-1 respectively. Here graphite is more stable than diamond and fullerness. How ?

Answer 1

The standard enthalpy of formation of a compound is the change of enthalpy from the formation of 1 mole of the compound from its constituent elements with all substances in their standard states at 101.3 kPa and 298 K. The enthalpy of formation is an exothermic reaction.
The reverse reaction is the decomposition of a compound into its constituent elements. More the energy required for this reaction the more stable the compound is. Hence, more the magnitude of enthalpy of formation, more stable the compound is.
The enthalpy of formation for any element in its standard state is taken as zero. Elements which exist in allotropic forms, the most stable allotrope has enthalpy as zero.