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Would the reaction between zinc and sulfur be

$$\ce{Zn_{(s)} + S_{(s)} -> ZnS_{(s)}}$$ or $$\ce{8 Zn_{(s)} + S_8\ _{(s)} -> 8ZnS_{(s)}}$$

I know that $\ce{S}$ and $\ce{S8}$ are allotropes of sulfur, but which one should I use for a reaction with sulfur at room temperature?

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Either reaction is valid for representing the reaction between zinc and sulfur. The important thing is that both equations stipulate the same stoichiometry (one atom of zinc for one atom of sulfur). The mathematics of chemical equations work just the same whether you add structural information about the reactants or not; the reaction equation is just shorthand for a system of mathematical equations involving the amounts and types of atoms.

In truth, neither equation is a correct description of the reaction at the microscopic scale (though I again restate that's not really the point of a reaction equation). Free zinc atoms do not interact with free sulfur atoms or $\ce{S8}$ rings. Rather, after a little bit of initial heating to get the reaction going *, liquid sulfur (a very complex mixture of sulfur chains ranging from $\ce{S6}$ to $\ce{S_{n}}$, where n is as large as 100,000) is in contact with a surface of zinc (or possibly the molten metal), and electrons are exchanged by some mechanism which probably hasn't even been elucidated, as electron transfer reactions are very quick and hard to monitor.

* You mention considering the reaction at room temperature. Even though thermodynamically, the reaction between zinc and sulfur is spontaneous at room temperature, it is kinetically very slow because both reactants are solid. Even if you mix them very very well, the reaction is diffusion-limited in the solid mixture; the atoms can't "reach" each other very well. Heating a little bit creates a small amount of liquid sulfur, which can then mix better with the zinc.

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  • $\begingroup$ So the reaction would occur like this: 8 Zn(s) + S_8(s) + Heat (activation energy?) --> 8 ZnS(s) + 205 kJ $\endgroup$
    – bandicoot12
    Feb 11, 2014 at 21:13
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    $\begingroup$ @bandicoot12 Usually any added heat is represented with a capital delta above the reaction arrow, like so: $\ce{Zn_{(s)} + S_{(s)} \overset{\Delta}{\rightarrow} ZnS_{(s)}}$ $\endgroup$
    – Nicolau Saker Neto
    Feb 11, 2014 at 21:20
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Crystalline sulfur, at room temperature, exists only as $\ce{S8}$.

So basically, that's the only option you have.

Breaking down $\ce{S8}$ chains is doable but over 160°C, followed by polymerization. At 200°C, polymer chains start to break.

Back to your question, $\ce{S8}$ is the most stable form for sulfur at room temperature.

From this paper.

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