I was working on my homework which was to write a description of what would occur if zinc and sulfur reacted and what steps would a scientist have to make in order to make them react. When I researched I bit, I saw that it would be a exothermic reaction, and I wanted to show this by using enthalpy. I haven't been taught this however, and as a result, I'm confused. I know that enthalpy equals the heat transfer and there is some sort relationship with bond energy, but I don't know how to apply this to the reaction.

Zinc Powder + Sulfur Powder = Zinc Sulfide + Energy

$\ce{Zn_{(s)} + S_{(s)} -> ZnS_{(s)}}$ + $205.98 \text{ kJ}$

Because $205>0$ this is an exothermic reaction. But where did this $205.98\text{ kJ}$ come from?


1 Answer 1


The free energies of formation of the elements in their standard states is zero, by definition. You can look up the (variously reported) standard enthalpy of formation of the product, −204.6 kJ/mol (exothermic!). It is roughly the binding energy of the crystal lattice less the ionization energies of the inputs. So, by the numbers, do you obtain spalerite or wurtzite? ZnS has crystal polymorphs and polytopes with differently populated unit cells

$\ce{Zn_2S_2}$ Wurtzite-2H
$\ce{Zn_4S_4}$ Wurtzite-4H and Sphalerite
$\ce{Zn_6S_6}$ Wurtzite-6H
$\ce{Zn_15S_15}$ Wurtzite-15R

  • $\begingroup$ If the reaction was Zn(s) + S_8(s) --> 8 ZnS(s), how would the enthalpy be affected? Also, you said that the free energies of the elements in their standard states is zero. So is the standard state of sulfur S_8 or S? $\endgroup$ Feb 11, 2014 at 20:41
  • $\begingroup$ Balance the equation. Enthalpy of formation is per mole product. 8x input giving 8x output is the same number. Sulfur has some 30 allotropes, plus crystal polymorphs. "Elemental sulfur" is an S_8 ring formula unit in a = 10.45, b = 12.845, c = 24.46 A; Z = 16 (128 atoms); V = 3283.27 A^3 D_{calc} = 2.08 g/cm^3; orthorhombic space group Fddd. gps.caltech.edu/~vijay/Papers/Chemistry/Meyer-76.pdf $\endgroup$
    – Uncle Al
    Feb 12, 2014 at 18:14

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