Answer:
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.
Step-by-step 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.