Comment
Figure shows only the first and second heating processes. Sulfidation reaction proceeds gradually above 500℃, but does not finish during first run. A large amount of nonreacted Zn is left.（See the events of melting of S and Zn in the 2nd run！）.
The enthalpy of formation
　Product is the cubic zincblend ZnS.
The enthalpy of formation could not be calculated owing to the incompleteness of the reaction.｛Ref.：ΔHf=48.2kcal/mol（Kubaschwski）｝