Iron and sulfur reaction

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.

Equipment

Apparatus

• Eye protection
• Balance (1 or 2 decimal places)
• For the demonstration the teacher will need:
  • Test tube made from borosilicate glass (eg Pyrex)
  • Bunsen burner
  • Heat resistant mat
  • Clamp stand and clamp
  • Spatulas x2
  • Small bar magnet
  • Watch glass
  • Filter paper (2 peices or use 2 weighing boats)
  • Mineral wool (or mineral fibre)
• For the class practical each group of students will need:
  • Prepared ignition tube (note 1)
  • Bunsen burner
  • Heat resistant mat
  • Test tube tongs

Apparatus note

1. 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 first step of the demonstration procedure). Using 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.

Chemicals

• Iron powder (potential IRRITANT)
• Sulfur – finely powdered roll or flowers

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout and ensure that the lab is well ventilated.
• Iron powder, Fe(s), (potential IRRITANT) – this can cause severe irritation in eyes because the iron oxidises rapidly in the saline environment – see CLEAPSS Hazcard HC055a. 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.
• Sulfur, S(s) – see CLEAPSS Hazcard HC096a. Roll sulfur or flowers of sulfur should be finely powdered using a pestle and mortar.
• Sulfur dioxide, SO₂(g), (TOXIC) is formed if the sulfur catches fire – see CLEAPSS Hazcard HC097.

Procedure

Demonstration

1. 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.
2. 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.
3. Place about 2 g of the mixture into a borosilicate test tube.
4. Insert a plug of mineral wool (mineral fibre) into the mouth of the test tube. Clamp the test tube as shown in the diagram.
5. Heat the powder mixture at the base of the test tube – 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.
6. Allow the test tube to cool down. At this point the students could carry out their own small scale version of the reaction.
7. 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.
8. 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.

 Class practical

1. Students should be provided with pre-prepared ignition tubes containing the iron–sulfur mixture and a mineral wool plug.
2. Using suitable tongs or test tube holders, the iron-sulfur mixture in the tube should be heated until it just starts to glow. Bunsen burners should then be turned off.
3. The ignition tubes should be left to cool on the heat resistant mat. It may be sensible to get the students to place all their used ignition tubes onto one heat resistant mat set aside for this purpose (eg 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 (see step 8 the demonstration procedure).

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’.