Reaction of Iron and Sulfur

A stoichiometric mixture of iron and sulfur combines in a sparkling, glowing reaction initiated by a red hot steel rod. 

Curriculum Notes 

This demo cam be used at several places in the curriculum. The most common topics would include types of reactions, redox reactions, and enthalpy. It is an example of a combination reaction and a formation reaction. allow about four or five minutes for this demo. One day of lead time is required for this project. 

Lead Time 
One day of lead time is required for this project.
Discussion 

It is assumed that the main reaction that occurs is: Fe(s) + S(s) --> FeS(s) but the system is probably much more complex than that. The standard enthalpy of formation of FeS is -100 kJ/mol. The iron and sulfur are present in the reaction mixture in roughly equimolar quantities.1 Shakashiri states that this reaction should not be used to demonstrate the difference between physical and chemical change, but I disagree. Though the color of the product is not very different from the reaction mixture, the change in physical properties can be demonstrated by allowing the product to cool, removing it from the ring stand base and bringing a magnet close to demonstrate that the product is not magnetic, whereas one of the reactants (iron) is. (Be careful though, sometimes unreacted bits of iron remain in the product, especially around the edges.) 1Shakashiri, Bassam Z.Chemical Demonstrations. vol.1. Madison, WI: The University of Wisconsin Press, 1983. p.56. 

Materials 
  • 10g of iron in a vial
  • 5g of sulfur in a vial
  • glass stir rod
  • ring stand base
  • long steel rod
  • propane torch
  • striker
Procedure 

Pour the contents of one vial into the other vial and stir to mix. Pour the mixture onto the center of the ring stand base to make a small pile. Darken the room lights. Light the propane torch. Heat the end of the steel rod until it glows. Holding the steel rod horizontally, insert the glowing tip of the steel rod into the base of the mixture pile. Hold the rod still as the reaction initiates and proceeds through the pile. Carefully lift the rod after the reaction is complete. The glowing lump of product should adhere to the rod. Place the lump back on the ring stand base and twist the rod to remove it from the lump of product. It is difficult to remove the product from the rod if it is allowed to cool before removing the rod. 

Safety Precautions 

Wear safety goggles. The tip of the steel rod remains hot for several minutes after the demonstration, so let it rest on the ring stand base for several minutes after the demo is performed. The "cool" end of the rod is marked with yellow tape. Remove all flammable materials from the vicinity of the demo. Have a fire extinguisher at hand. A considerable amount of sulfur dioxide gas is generated, so perform this demo in a well ventilated room and avoid breathing the fumes. 

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group