Sulfur

Properties of Sulfur

• Yellow solid at room temperature
• Second only to carbon, sulfur is found in large number of allotropes (various forms). For examples: disulfur (S2), trisulfur (S3), tetrasulfur (S4), cyclo-pentagon (cyclo-S5), and so-on. 

Here are the common sulfur allotrope:

Rhombic sulfur

Monoclinic sulfur

Powder

• Sulfur reacts with metal to form metal sulfide (sulphide)

             mg(s)          +     S(s)    --->      MgS(s)

             magnesium  +    sulfur   --->     magnesium sulfide

             Zn(s)           +     S(s)    --->      ZnS(s)

             zinc             +     sulfur  --->     zinc sulfide

• Sulfur reacts with oxygen to produce sulfur dioxide 

             S(s)           +     O2 (g)     --->    SO2 (g)

             sulfur         +     oxygen    --->   sulfur dioxide

• Sulfur is a non-metallic element, does not conduct electricity and insoluble in water
• Sulfur atomic number = 16

Sulfur atom

Sources of Sulfur

Sulfur may be obtained from the following sources:

1. Copper pyrite (CuFeS2)
2. Zinc blend (ZnS) is heated in air (to produce zinc oxide and sulfur dioxide)
3. Fossil fuel/ natural gas and oil/petroleum
4. Sulfur ore (sulfide/sulphide) from volcanic activity
5. Elemental sulfur from sulfur beds in Poland, Russia and US (Louisiana)  

Recovery sulfur from refinery of petroleum/natural gas  using Claus process

Mining sulfur ore from active volcano area

Sulfur minor carrying sulfur ore

Frasch process, named after Hermann Frasch. 

Highly heated  water and compressed air is forced into the sulfur beds. 

Sulfur becomes molten and is forced out together with water. 

Molten sulfur can be easily separated from water. 

99.5% purity may be obtained.

Uses of Sulfur

Sulfur is very important in the chemical industry as the following:

1. to produce sulfuric acid

• millions of tonnes of sulfuric acid are produced worldwide every year. 
• sulfuric acid is used to make chemicals, paints & pigments, detergents, soaps, synthetic fibres, dyes, metal salts, fertilisers, and so-on.

1. to vulcanise rubber, a process which makes the rubber harder and increases its elasticity.
2. in manufacture of matches, fireworks and fungicides
3. as a sterilising agent in medicines
4. to produce bleaching agent (sulfur dioxide gas) for paper manufacture 
5. to produce food preservative (sulfur dioxide gas) by killing bacteria ( also called fumigation; to exterminate pests or disinfect)

For knowledge only:
The standard method to control postharvest decay of grapes is to fumigate with sulphur dioxide gas the fruit immediately after harvest and during storage.  However, the concentration of sulphur dioxide necessary to inhibit fungal infections may induce injuries in both rachis and berries, in addition sulphur dioxide application has been restricted in many Countries for the human health risks due to the sulphite residues. Several studies have shown the efficacy of heat treatments on harvested fruits to control biological agents, delay ripening and preserve quality of fresh-cut table grapes.
(source:http://www.freshplaza.com/article/97631/How-to-preserve-the-quality-of-ready-to-eat-grapes#SlideFrame_1)

Industrial Manufacture of Sulfuric Acid by Contact Process

A contact process plant

Procedure of making sulfuric acid

Step 1

Sulfur dioxide is produced by burning sulfur in the air.
    S(s)     +     O2 (g)    ---> SO2 (g)

  sulfur      +   oxygen    ---> sulfur dioxide

Step 2

Sulfur dioxide, heated to a temperature of about 450^o is fed into a converter passing over  a catalyst, vanadium (V) oxide (V2O5) at a atmosheric pressure (atm) between 2 to 5. Sulfur dioxide is catalysed to  react with oxygen to become sulfur trioxide.

 

This reaction is reversible. Forward reaction is exothermic while back reaction is endothermic.

Condition to obtain maximum production of sulfur trioxide in the equilibrium reaction of contact process

1. Low temperature

• Low the temperature favours forward reaction because forward reaction is exothermic. 
• The optimum temperature is 450^o. 
• If too low a temperature is used, the reaction of sulfur dioxide with oxygen would be too slow and would not be economically feasible.

2. High pressure

• High pressure favours forward reaction, decreasing the number of gas molecules
• 1 to 2 atmosheric pressure (atm) 

3. Catalyst of vanadium (V) oxide (V2O5)

Step 3

Sulfur trioxide is dissolved in concentrated sulfuric acid to produced oleum

H2SO4 (aq)      +     SO3 (g)              --->    H2S2O7 (l)

sulfuric acid    +   sulfuric trioxide --->    oleum

Step 4

Oleum added to the correct amount of water to form sulfuric acid with the required concentration.

H2S2O7 (l)    +     H2O (l)        --->    2H2SO4 (l)

oleum          +      water       --->    sulfuric acid

!! Warning !!

Sulfur trioxide can be added to water to form sulfuric acid.

Why sulphur trioxide is not directly dissolved in water to produce sulphuric acid?

This reaction is very exothermic, too violently to be used in large-scale manufacturing. A lot of heat is released, forming thick acid mist which is difficult to deal with. 

SO3 (g)              +    H2O (l)      --->    H2SO4 (l)  

sulfur trioxide +    water       --->    sulfuric acid

Thick mist formed
sulfur trioxide + water

Sulfur dioxide, sulfur trioxide and water vapour emission
Kilauea vocano , Hawaii



 

For revisions: Sulphur - IGCSE past year paper 3 & 6