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Standard enthalpy of formation of an element in its standard state:
AZERO (by definition; reference state)
BVariable
CNegative
DAlways positive
Answer & Solution
Correct answer: A. ZERO (by definition; reference state)
ΔH_f° for elements in standard state (most stable form at 298 K, 1 atm) = 0 by definition. e.g., ΔH_f°(O₂, g) = 0, ΔH_f°(C, graphite) = 0, ΔH_f°(H₂, g) = 0.
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