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Why is sulfur left over when there is not enough iron in an iron-sulfur reaction?
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chemical reaction
Log burning in a fire. Burning wood is an example of a chemical reaction in which wood in the presence of heat and oxygen is transformed into carbon dioxide, water vapor, and ash.
In the reaction between iron and sulfur, the two elements combine in a fixed ratio to form iron sulfide. Specifically, one atom of iron reacts with one atom of sulfur to produce one formula unit of the product.1
Because matter is neither created nor destroyed, the reaction is governed by stoichiometry, which dictates the precise mass relationships between reactants. When the amount of iron provided is insufficient to react with all the available sulfur, the iron acts as the limiting reagent. Once all the iron has been consumed, the reaction stops, leaving the excess sulfur behind as unreacted material.1