Which of the following substances would be expected to have an enthalpy of formation value D H f equal to zero?
(Pay careful attention to the phase indicators.)
I. O₂(g)
II. N₂( )
III. H₂ O ( )

Question

Which of the following substances would be expected to have an enthalpy of formation value D H f equal to zero?
(Pay careful attention to the phase indicators.)
I. O₂(g)
II. N₂(  )
III. H₂ O (  )

Quizplus · Accepted Answer

Enthalpy of formation (ΔHf) is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. Standard state elements are those in their most stable physical form at 1 atm and the specified temperature. Entropy of formation of an element in its standard state is zero since it is already in its most stable state. Therefore, O₂(g) which is in its standard state, should have zero enthalpy of formation. N₂(g) and H₂O(l) are not in their standard states, so they should have non-zero enthalpies of formation.

Which of the Following Substances Would Be Expected to Have