JEE Advanced 2019 · Paper 1, Chemistry question 7

$\Delta_rH^\circ=\Delta_fH^\circ$ only if the reaction forms exactly one mole of one product from its elements in their standard states.

• (A) forms 2 mol of water, so $\Delta_rH^\circ=2\Delta_fH^\circ$.
• (B) uses C(g), which is not carbon's standard state (graphite).
• (C) $\frac32\ce{O2(g) -> O3(g)}$ qualifies.
• (D) $\frac18\ce{S8(s) + O2(g) -> SO2(g)}$ qualifies.

(C), (D)