why is the enthalpy of formation of an element zero
Standard enthalpy of formation All elements in their standard states (oxygen gas, solid carbon in the form of graphite, etc.) have a standard enthalpy of formation of zero, as there is no change involved in their formation.
First law of thermodynamics The first law of thermodynamics was developed empirically over about half a century. The first full statements of the law came in 1850 from Rudolf Clausius and from William Rankine; Rankine's statement is considered less distinct relative to Clausius'.[2] A main aspect of the struggle was to deal with the previously proposed caloric theory of heat.