Standard Reaction Enthalpy and Standard State

[Michelle_Nguyen_3F]

I know that when reactants and products are in their standard state at 1 atm, the reaction enthalpy is the standard reaction enthalpy. However, how do we know that a reactant/product is in its standard state for any element? For example, I know that is the standard state for N, but why is that? Thank you!

[Danny Nguyen 2H]

Isn't it because it's in its natural state in the world so it's formula is that way. For your example N, nitrogen, exists in the real world as a gas, N2. Similar to the example given in lecture, oxygen exists in the real world as O2.

[Vincent Tse 2B]

There exists diatomic molecules that naturally occur in the world because forming a bond with another of the same element could increase stability; e.g. H2, N2, F2, O2, I2, Cl2, Br2.

For a substance that contains different elements, I think that you would have to draw lewis structures and consider formal charges to determine if the substance is at its most stable form. In this class, the standard enthalpy of formations for substances are usually provided.

Hopefully I did okay in answering your question!