Standard Enthalpy Question

[Alexandra_Ivanova_1A]

I understand that the standard enthalpy of formation an element in its standard state is zero, however I'm confused as to the reactions used to signify them:

O₂ --> O₂
Br₂ --> Br₂

...etc.

What exactly do these reactions mean? What is the purpose of a reaction where the products and reactants are the same?

[Amir Patel 1G]

Hello, I believe they used those reactions (O2 --> O2, Br2 --> Br2) since they demonstrate that the standard enthalpy of formation is 0. Since the reactants and products are identical, as you mentioned, no additional energy is needed to break or form bonds, and therefore no heat is being absorbed or released during the reaction.

[Thomas Hui 2J]

In addition, the standard state of these diatomic gas molecules is 0, because the most stable form of any naturally occurring molecule of oxygen or bromine is in the diatomic gaseous state. As a result, to use the standard enthalpy of formation for other elements, such as Carbon, you must use the state in which it is the most stable. This means that carbon would be formed from graphite, as demonstrated in Dr. Lavelle's Wednesday lecture.

[Ashley_Thomas_3C]

Essentially, enthalpy of formation refers to the change in enthalpy that takes place when a substance is formed from the most stable form of its elements. But in the case of natural elements in their most stable state, no change in enthalpy takes place because the reactants and the product are the same. In other words, elements in their standard states (for example, oxygen in the form of O₂, Br₂, H₂, I₂, F₂) have a standard enthalpy of formation of zero because there is no change involved when they are formed from themselves. The element is already formed, so a formation reaction is not necessary. So, technically the purpose of showing the reactions O₂ --> O₂ and Br₂ --> Br₂ is to show that no reaction takes place at all and that both the reactant and product are the same.

[danae_blodgett_1H]

Is an element's standard enthalpy of formation always as it is presented on the periodic table?