Which of the following have a standard enthalpy of formation (ΔHf°) equal to zero?

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

  • F2(g) and Hg(l) have a standard enthalpy of formation (ΔHf°) equal to zero.

    They are elemental and in their standard state.

    ~~~~

    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.)

  • All elements in in standard state (298 kelvin and 1atm) have the standard enthalpy of formation equal to zero. Therefore diamond is the answer.

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