Delta H*f is 0 except for?

The enthalpy change of formation (ΔHf) of the substance is the enthalpy change when the substance is formed from its constituent elements IN THEIR STANDAND STATE.

A. Ca(s) → Ca(s) …. ΔHf[Ca(s)] = 0

B. Br₂(l) → Br₂(l) …. ΔHf[Br₂(l)] = 0

C. Si(s) → Si(s) ….. ΔHf[Si(s)] = 0

D. O₂(g) → O₂(g) …. ΔHf[O₂(g)] = 0

E. Cu(s) → Cu(g) …. ΔHf[Cu(g)] ≠ 0

For each thermochemical equation from A to D, the reactant and the product are the same because the product is the element in state state. Therefore, ΔHf = 0

However, in E, the product is copper vapor while the standard state of copper is the solid form. Therefore, ΔHf ≠ 0 as the reactant and product are different.

The answer: E. Cu(g)