The elements of a delta Hf of ___ kj/mole at standard temp and pressure

One mole of a substance forms Instead of standard atmospheric pressure and temperature from its pure elements{ conditions being the same} then this change and enthalpy is known as enthalpy of formation.

Explanation:

The equation of this formation can let us know what actually happens

ΔH = ΔHf (products) - ΔHf (reactants)

Hence we can say this change in enthalpy formation is the difference between the sum of product and sum of reactants.