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TeacherExpt:Iron and sulfur reaction

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This demonstration or class experiment shows the exothermic reaction of two elements, iron and sulfur, to form the compound, iron sulfide. The two solids are mixed and heated in a test-tube (or ignition tube). The reaction can be used to illustrate elements, mixtures and compounds.

Contents

Lesson organisation

This reaction can be carried out as a demonstration or class experiment in a well-ventilated laboratory provided that the instructions provided here are strictly adhered to.

The reaction can be carried out in borosilicate glass test-tubes as a demonstration or in smaller (ignition) tubes by students. The reaction provides an opportunity to show that the properties of a compound are different from its constituent elements.

The reaction must not be carried out on tin lids in the open laboratory as is suggested in some sources. The sulfur may boil or burn releasing sulfur dioxide which is a Toxic and Corrosive gas and may trigger an asthmatic attack.

Apparatus and chemicals

  • Eye protection
  • Balance (1 or 2 d.p.)

For the demonstration the teacher will need:

  • Test-tube made from borosilicate glass (e.g. Pyrex)
  • Bunsen burner
  • Heat resistant mat
  • Clamp stand and clamp
  • Spatulas (2)
  • Small bar magnet
  • Watch glass
  • Filter paper, 2 pieces (or use 2 weighing boats)
  • Mineral wool (or mineral fibre)
  • Iron powder (potential Irritant) (see note 1)
  • Sulfur – finely powdered roll or flowers (Low hazard) (see note 2)

For the class practical each group of students will need:

  • Prepared ignition tube (see note 3)
  • Bunsen burner
  • Heat resistant mat
  • Test-tube tongs

Technical notes

Iron powder – this can cause severe irritation in eyes because the iron oxidises rapidly in the saline environment. Refer to CLEAPSS® Hazcard 55A. Sulfur is Low hazard. Refer to CLEAPSS® Hazcard 96A. Sulfur dioxide is formed if the sulfur catches fire. This is Toxic by inhalation and Corrosive. Refer to CLEAPSS® Hazcard 97.

  1. Iron powder is preferred to iron filings. If fine sulfur powder is mixed with iron filings, it is difficult to obtain a consistent mix, because the two solids can separate.
  2. Roll sulfur or flowers of sulfur should be finely powdered using a pestle and mortar.
  3. Ignition tubes (75 mm x 10 mm test-tubes) should be filled to no more than one-quarter full with the iron – sulfur powder mix (see Procedure note a). 0.2 g of the mixture is sufficient for the effect to be seen. Place a small plug of mineral wool in the mouth of each ignition tube. After the experiment, the iron(II) sulfide is low hazard and can be discarded into the refuse.

Procedure

Health & Safety

Wear eye protection throughout and ensure that the lab is well ventilated.

Procedure

Demonstration

A. Prepare a mixture containing iron powder and sulfur powder in the ratio 7:4 by mass. Do this by weighing out 7 g of iron powder and 4 g of finely powdered sulfur onto separate pieces of filter paper (or use weighing boats). Mix the two powders by pouring repeatedly from one piece of paper to the other until a homogeneous mixture (by appearance) is obtained.

B. Note the appearance of the pure elements and the mixture. Demonstrate that iron can be separated from the mixture by physical means. Do this by wrapping the end of a small bar magnet in a paper tissue or cling film, and dipping it into a teaspoon-sized heap of the mixture on a watch glass. The iron will be attracted, but the sulfur remains on the watch glass.

C. Place about 2 g of the mixture into a borosilicate test-tube.

D. Insert a plug of mineral wool (mineral fibre) into the mouth of the test-tube. Clamp the test-tube as shown in the diagram.

E. Heat the powder mixture at the base of the testtube – gently at first and then more strongly (use a blue flame throughout). Heat until an orange glow is seen inside the test-tube. Immediately stop heating. Let the students see that the glow continues and moves steadily through the mixture.

F. Allow the tube to cool down. At this point the students could carry out their own small-scale version of the reaction.

G. Once cool, it is possible to break open the test-tube to show the appearance of the product, iron(II) sulfide. The test-tube can be broken open using a pestle and mortar. It is advisable to wear protective gloves.

H. It may be possible to show that the product, iron(II) sulfide is non-magnetic. However, this is not always successful. It has been suggested that using a very weak magnet is advisable.

Apparatus for the iron sulfur reaction

Class practical

A. Students should be provided with pre-prepared ignition tubes containing the iron–sulfur mixture and a mineral wool plug.

B. Using suitable tongs or test-tube holders, the iron-sulfur mix in the tube should be heated until the mixture just starts to glow. Bunsen burners should then be turned off.

C. The reaction tubes should be left to cool on the heat resistant mat. It may be sensible to get the students to place all their used reaction tubes onto one heat resistant mat set aside for this purpose (e.g. on the teacher’s desk or in a fume cupboard).


Teaching notes

On heating the reaction mixture, the sulfur melts and reacts with the iron exothermically to form iron(II) sulfide. The mineral wool plug in the mouth of the test-tube prevents sulfur vapour escaping and possibly catching fire. If, despite all precautions, the sulfur vapour does ignite, students must be trained to extinguish it by placing a damp rag firmly over the mouth of the tube.

The signs that a chemical reaction occurs are: the glow, and the fact that a new substance (black iron sulfide) is formed which cannot be separated by using a magnet. Although see note h in the Procedure section.

This may be an opportunity to introduce or reinforce the ‘rule’ that if only two elements are combined together, the name of the compound ends in ‘-ide’.

Reference

This experiment has been reproduced from Practical Chemistry: http://practicalchemistry.org/experiments/iron-and-sulfur-reaction-demonstration-and-class-experiment,88,EX.html

Useful resource

Clip on the Demonstrating Chemistry - Exciting Elements 1 video, free to download from http://www.chemsoc.org/networks/learnnet/videoclips.htm.