Question

Ozone, o3(g), is a form of elemental oxygen produced during electrical discharge. is δh∘f for o3(g) necessarily zero?

Answer 1

The answer is negative.

The entalphy of formation, ΔH°f, is defined as the change of enthalpy enthalpy that accompanies the formation of one mole of the compound in its standard state, from its consitututive elements in its standard state.

Hence, by this definition, the change of enthalpy of the elements in its standard state is zero.

Ozone, O₃ (g), is not the standard state of oxygen - the standard state of oxygen is O₂ (g), so you cannot expect that its enthalpy of formation be zero.

Ozone can be seen as the product of a series of reactions in the atmosphere which involve a net enthalpy change, so ithe enthalpy of formation is, indeed, not zero.

The net reaction for the formation of ozone is O₂(g) + O → O₃, but, as said, the actual mechanism involve a series of reaction, whose total change of enthalpy is greater than zero.

Answer 2

I believe that the delta h formation or enthalpy of formation is only zero for pure elements. They are zero because they are the most basic substances, that is you cannot form an element from an element. Since Ozone is already a compound, a mixture of 2 oxygen O2 atoms, therefore the δh∘f is not necessarily zero.