## Reaction Enthalpies

Sonia Perez 1G
Posts: 29
Joined: Fri Sep 25, 2015 3:00 am

### Reaction Enthalpies

I am a bit confused of when the enthalpy of an element in its natural state is zero. If we are looking for the standard enthalpy of formation, would the enthalpy of 02 be zero? What if we are given the bond enthalpies, would O2 also have a bond enthalpy of zero?

Estefania Lima 2G
Posts: 14
Joined: Fri Sep 25, 2015 3:00 am

### Re: Reaction Enthalpies

The enthalpy of formation of a substance is the amount of heat at constant pressure that went into the chemical reaction that produced that substance. Since oxygen is most stable as a diatomic molecule, and therefore occurs naturally as $O_{2}$, the enthalpy of formation is 0. There is no change in the reaction $O_{2}\rightarrow O_{2}$, so there is no heat required for oxygen to become oxygen. The bond enthalpy of $O_{2}$ is not zero, however, because bond enthalpies many kJ/mol are needed to break a bond (remember that breaking a bond is an endothermic process).

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