Which of the following have a standard enthalpy of formation (ΔHf°) equal to zero?
1) F2(g)
2) Hg(l)
3) NH3(g)
4)C(diamond)
2 Answers
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F2(g) and Hg(l) have a standard enthalpy of formation (ΔHf°) equal to zero.
They are elemental and in their standard state.
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NH3 is not elemental and it has a nonzero enthalpy of formation.
C(s, diamond) is not the standard state of carbon. Diamond has a nonzero enthalpy of formation.
(Solid graphite is the standard state of carbon, and hence has a standard enthalpy of formation equal to zero.)
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All elements in in standard state (298 kelvin and 1atm) have the standard enthalpy of formation equal to zero. Therefore diamond is the answer.