Per NCERT §5.4(c), what is the CONVENTION for the standard enthalpy of formation Δ_f H° of an ELEMENT in its reference (most stable) state?
AΔ_f H° = 0 — e.g. H₂(g), O₂(g), C(graphite), Br₂(l) are all assigned zero
BΔ_f H° = 1 kJ/mol
CCannot be assigned
DEqual to atomic mass
Answer & Solution
Correct answer: A. Δ_f H° = 0 — e.g. H₂(g), O₂(g), C(graphite), Br₂(l) are all assigned zero
Per NCERT §5.4(c) + Table 5.2, by convention Δ_f H° = 0 for an element in its reference state. So C(graphite) = 0 but C(diamond) = +1.89.
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