Question

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

O2(g)

N(g)

H2O(l)

Br2(l)

Fe(s)

He(g)

Answer 1

ΔHf° = 0 kJ/mol for elements in their standard state. From the list that is:

- O2(g) — yes (oxygen gas is the standard state)
- Br2(l) — yes (bromine liquid is the standard state)
- Fe(s) — yes (iron solid is the standard state)
- He(g) — yes (helium gas is the standard state)

N(g) is not (the standard state is N2(g)), and H2O(l) is a compound so its ΔHf° ≠ 0.