Most stable form enthalpy

[505095793]

Why is it that an element in its lost stable form has ethalpy of formation equal to zero?

[Sarah Kiamanesh 1D]

Elements in their most stable form have standard enthalpy of formation equal to zero because that is how they exist naturally in nature. During reactions, these bonds aren't formed, they exist like that prior to the reaction. Because there is no formation, the value is zero.

[Philipp_V_Dis1K]

Let's look at the reaction of 2H2O2->2H2O + O2
Even though hydrogen peroxide is broken up into water an oxygen, the O2 is in it's natural or most occuring form and therefore doesn't take energy or release energy to be created

[Brandon Mo 4K]

The standard enthalpy of formation is the standard reaction enthalpy for one mole of a substance from its elements that are in their most stable form.
For example.
O2 --> O2
The standard enthalpy of formation for O2 is zero because it is already its most stable form so there is no change in enthalpy.