Question

enthalpy of formation vs standard enthalpy of formation

Answer 1

When 1 mole of a substance is formed from its constituents in pure form,the heat released or absobed(rarely) is called enthalpy of formation.

When 1 mole of a substance is formed from its constituents in pure form at 1 atm pressure and 298K temp,the heat released or absobed is called standard enthalpy of formation.

Enthalpy of formation depends on conditions of temperature whereas standard enthalpy of formation of a substance is fixed.

For example when 1 mol H2O is formed from H2(g) and O2(g) then standard enthalpy of formation is -285kJ.That means -285kJ/mol while the enthalpy of formation of H2O depends on conditions of temperature and pressure.

P.S. enthalpy of formation can be positive for unstable substances like N2O4.