5 minutes reading time (1045 words)

    Allotropes of Sulfur

    Apparatus

    • Boiling tubes, 2
    • Test tube holders
    • Conical flask, 250mL
    • Cork, to fit conical flask
    • Beaker, 250mL
    • Beaker, 1L
    • Petri dish or watch glass.
    • Microscope - ideally a projection or video microscope.
    • Bunsen burner, tripod and gauze, or electric hotplate
    • Heat resistant mat.
    • Filter paper - about 18cm in diameter.
    • Access to a fume cupboard.

    Chemicals

    • Sulfur, powdered roll, 100g
    • Cooking oil, 700mL
    • Toluene (methylbenzene) or xylene (dimethylbenzene), 100mL

    Method

    Before the demonstration
    Two thirds fill a 1L beaker with cooking oil and heat to about 130°C. Half fill a 250mL beaker with cold water.

    The demonstration

     Two-thirds fill two boiling tubes with broken/powdered roll sulfur (about 20g in each tube) and place in the oil bath. The sulfur will melt to a clear, amber, transparent mobile liquid. This will take about 15 min. Some teachers may wish to pre-prepare at least one tube to save time. Remove one boiling tube and pour the sulfur into a filter paper cone held together by a paper clip (see Figure 4) and supported in a beaker. Allow the sulfur to cool and solidify. Break the crust with a spatula and pour off an remaining liquid. Needle-shaped crystals of monoclinic sulfur will be seen. When cool, the cone can be passed around the class.

    Take the second tube and, using a test-tube holder, heat it gently over a small Bunsen flame, keeping the contents of the tube moving to prevent local overheating. The liquid gets darker and, fairly suddenly, becomes a viscous gel-like substance. This occurs at about 200°C. The tube can be inverted and the sulfur will remain in it. Show that the mobile liquid re-forms on cooling.

    Further heating beyond the gel-like stage liquefies the sulfur again to a very dark red- brown liquid (the colour of bromine). Note that during heating it is probable that the sulfur will catch fire and sulfur dioxide will be produced. Have a heat-proof mat at hand to place over the mouth of the tube to extinguish the blue flames. When the sulfur begins to boil, pour the liquid sulfur in a slow stream into a beaker of cold water. A mass of brown plastic sulfur will form. Allow this to cool thoroughly, taking care because the inside of the plastic sulfur may remain molten alter the outside has solidified. Remove the plastic sulfur from the water and show that it is rubbery - it can be stretched and will return to its original shape. After about half an hour it will be noticeable that the shiny surface of the plastic sulfur is beginning to dull and some of the elasticity is lost as it begins to turn back to the more stable rhombic sulfur. Leave until the following lesson to monitor the progress of this change. This will be very noticeable after a week or so but complete change will take a long time.

    Working in a fume cupboard, put about 10g of powdered roll sulfur into a conical flask and add about 100mL of toluene or zylene. Leave the sulfur to dissolve. This will take some several minutes; warming to about 50°C will speed up dissolution. Some teachers may wish to prepare the solution before the demonstration to save time. pour a little of solution into a Petri dish, watch glass or microscope slide and leave it in the fume cupboard for the solvent to evaporate. This will take about 10 min. Small crystals of rhombic sulfur will form. These can be viewed under a microscope. The class will need to file past and view them in turn. It is worth the teacher selecting a well-formed crystal for viewing.

    Visual tips

    A projection microscope or video microscope can be used to show the shape of the rhombic crystals to the whole class.

    Teaching tips

    Some stages of this demonstration are time-consuming, eg. melting the sulfur in the oil bath, dissolving the sulfur in the toluene or zylene and evaporating the solvent. Some teachers may prefer to melt some sulfur before the lesson and to prepare rhombic crystals before the lesson to save time. In the latter case, slower evaporation (which be brought about by covering the Petri dish with filter paper with a few holes in) will produce larger crystals. Particularly large and/or well-formed crystals could be retained as examples for future use.

    Theory

     Powdered sulfur consists of puckered S8 rings in the shape of crowns. These can be packed together in two different ways - to form rhombic crystals and to form needle- shaped monoclinic crystals (see Figure 5). Below about 96°C, rhombic sulfur is the more stable allotrope. On melting at about 118°C, sulfur first forms a mobile, amber liquid containing S8 rings. If this is allowed to cool, monoclinic sulfur will turn slowly into the more stable rhombic form.

    Further heating of S8- containing liquid breads the rings into S8 chains which may join to form longer chains which tangle, causing an increase in viscosity. Further heating breads these chains into shorter ones, perhaps as short as S2, and the viscosity decreases again. Rapid cooling of this liquid traps the resulting solid sulfur in the tangled chain state - this is plastic sulfur. On stretching, the chains uncoil and on releasing the tension they return to the partly coiled state (see scheme). If solid sulfur is formed below 96°C, by evaporating the solvent from a solution for example, the stable rhombic form is produced (see Figure 6).

    Further details

    Carbon disulfide is a better solvent for making rhombic sulfur, but its smell, toxicity and flammability make it unsuitable for use in schools.Very slow heating is essential if all of the changes on heating sulfur are to be seen clearly. Sulfur is a poor thermal conductor, hence the changes can overlap one another if the heating is too fast. It is difficult to heat slowly enough using a Bunsen burner. Monoclinic crystals can be formed by allowing a hot solution of sulfur in boiling zylene to cool so that crystallisation starts at above 96°C. Full details can be found in, for example, Revised Nuffield chemistry teachers' guide II, p 138. London: Longman, 1978.

    Safety

    Wear eye protection. It is the responsibility of teachers doing this demonstration to carry out an appropriate risk assessment.

    Weblinks

    Wikipedia - Allotropes of Sulfur 

    Red, White and Blue
    The "Breathalyser" Reaction
     

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    Tuesday, 21 August 2018

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