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I was taught that double displacement reactions occur when reactants are aqueous. However, I came across this reaction

$$\ce{ZnO(s) + H2S (g) -> ZnS(s) + H2O (g)}$$

This is the reaction that is used to remove hydrogen sulfide that is naturally found in methane gas. This reaction indeed looks like a double replacement reaction. However, the reactants are not aqueous.

Additionally, this reaction happens at the temperature of $\pu{700 K}$. Does that mean if enough energy is given, double replacement reactions can occur without an aqueous medium? How so?


Source 1, Source 2

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  • $\begingroup$ At high temperatures, it is likely that the outer layer of the solid reactants melts and the mobile ions in these molten outer layers react with one another. And if the reaction is exothermic, more of the solid reactants would melt and react as the outer layer reacts. The rate of reaction would increase quickly and soon, the double displacement would be complete before you know it. $\endgroup$ – Tan Yong Boon Mar 14 '18 at 16:52
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Generally, reactions occur with liquids and gases (which encompasses solutions). This is because there is the fundamental problem of mass transport. When reactants react, they leave product in the wake of the reaction, which isolates the reactants from each other. For fully solid reactants, it means that more reactants have trouble finding each other. When reactions take place in liquid or a gas, there is the prospect of diffusion, which allows more reactant to find each other.

In the reaction you point out, hydrogen sulfide is a gas so it can easily diffuse to find more zinc oxide. The product zinc oxide might not form a perfect coating (i.e., it has holes in it) that allow the hydrogen sulfide to diffuse through this product to find more zinc oxide.

Other examples: when you burn a piece of wood, this is a solid reacting with a gas. But the combustion reaction is actually between vaporized wood and oxygen. The presence of a gaseous fuel is what allows good mixing between it and oxygen to produce a flame.

Finally, an example from a chemistry demonstration: solid barium hydroxide and solid ammonium chloride are mixed together and shaken. They reaction is quite endothermic and can freeze water on the outside of the reaction vessel. The trick here is that the barium hydroxide is usually the octahydrate. The hydrate provides enough water to create small amounts of solution for the reaction.

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  • $\begingroup$ The barium Hydroxide/ammonium chloride system is a eutectic, not a double displacement. $\endgroup$ – A.K. Mar 14 '18 at 18:18
  • $\begingroup$ I am referring to reactions in general, not just double displacements. @A.K. $\endgroup$ – Zhe Mar 14 '18 at 18:56
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First of all most of the literature will refer to double displacement as metathesis instead so that should help you search. But yes your reactant can be in any phase and perform a double displacement. However, mobility is a factor that is not considered in liquid or gas phases, but does impact solid phases. To make solid phases react at an observable rate, heat is typically required to increase mobility and thus effect a reaction. One example is the synthesis of gallium nitride from gallium iodide and lithium nitride: $$\ce{GaI3_{(s)} + Li3N_{(s)} \rightarrow GaN_{(s)} + 3 LiI_{( s)}}$$

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  • $\begingroup$ This is a very interesting reaction. If you're able to, please provide a procedure so that we can see we how the problem of mass transport is overcome. $\endgroup$ – Zhe Mar 14 '18 at 18:58
  • $\begingroup$ pubs.acs.org/doi/pdfplus/10.1021/cm9901764 $\endgroup$ – A.K. Mar 14 '18 at 20:10

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