Question

Which substances have /_\Hf = 0 kJ/mol by definition? Select all that apply.

Answer 1

Answer: O₂(g), Br₂(l) Fe(s), He(g)


Justification:

1) The state variable given, ΔHf°, is the standard enthalpy of formation. By definition it is the change in the enthalpy that accompanies the formation of a mole of compound in the standard state, from its constituent elements in the standard state.


2) Since the standard state of O₂(g), Br₂(l) Fe(s), He(g) is the very same shown, they do not evolve any heat for its formation. This is, that is the same state of the constituents, and no change is involved.


That is why, ΔHf°, of those elements is 0.

Answer 2

The answers are O2(g), Br2(I), Fe(s), He (g).