The synthesis of iron (II) sulfide involves reacting 1 equivalent of sulfur with 1 equivalent of iron. The reaction is initiated by a hot glass stirring rod into it. It is an extremely exothermic reaction. This is an interesting reaction to use as a demonstration, but the product is useful. The Iron (II) sulfide, or FeS, can be reacted with hydrochloric acid to produce hydrogen sulfide gas. This is a very useful gas, but it is extremely toxic.