Question

_______ has a ΔHf of zero.

A. Br–
B. Na+
C. H2O
D. CO2
E. None of the Above

Answer 1

The right answer is E. None of the Above.

(In case of standard conditions (T = 298.15 K, P = 1 bar))

Standard enthalpy of formation is equal to 0 only in the most stable allotropes such as O2, H2, Cl2, Na or He (examples).

All answers (except E) contains compounds or ions, which isn’t the most stable allotropes of elements. So, the right answer is E.