16 Sulfur 

Sulfur sources and uses

• Sulfur is found in its elemental state underground in the USA, Mexico and Poland
• It is also a by-product from the removal of sulfur from petroleum and natural gas
• Sulfur can also be obtained from sulfide ores
• The main use of sulfur is in making sulphuric acid which is a very important chemical used in many industries
• It is also used extensively in making rubber tyres more flexible (vulcanising), where the rubber is heated with sulfur

Sulfur dioxide

• Sulfur dioxide can be made by burning sulphur in air

• This is the method used in the first stage of the manufacture of sulfuric acid

• Sulfur dioxide is used as a bleach in the manufacture of wood pulp for paper, and as a preservative for foods and drinks by killing bacteria

• Sulfites are often added to foods and these release sulfur dioxide in acidic conditions

  

  

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Sulfuric Acid : Properties, Manufacture and uses

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• Sulfuric acid is synthesised by the Contact process which use sulfur and oxygen from air and is done in three distinct stages

  The Contact Process

  Stage 1

• The first stage is the oxidation of sulfur:

  • S + O2 → SO2

Stage 2

• The main stage is the oxidation of sulfur dioxide to sulfur trioxide using a V2O5 catalyst:

  

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• The conditions for the main stage of production of sulfur trioxide need to be considered

Conditions during Stage 2

Temperature 450°

• The reaction is exothermic, so increasing the temperature shifts the position of equilibrium to the left in the direction of the reactants
• Therefore the higher the temperature, the lower the yield of sulfur trioxide
• The optimum temperature is a compromise between a higher rate of reaction at a higher temperature and a lower equilibrium yield at a higher temperature

Pressure: 2 atm

• An increase in pressure shifts the position of equilibrium to the right in the direction of a smaller number of gaseous molecules
• However the position of equilibrium lies far to the right (the equilibrium mixture contains about 96% sulfur trioxide)
• So the reaction is carried out at just above atmospheric pressure because:
  • it is not worth spending the extra energy or money required to produce high pressures
  • a higher pressure causes the sulfur dioxide to liquefy

Stage 3

• Once stage 2 is completed, the sulfur trioxide is absorbed into a solution of 98% sulphuric acid to produce a thick liquid called oleum:

  • SO3 + H2SO4 → H2SO7

• It is not absorbed into water because a fine mist of sulfuric acid would be produced and this would be difficult to condense and is also highly dangerous

• Oleum is added to water to form concentrated sulfuric acid:

  

Properties of sulfuric acid

• Sulfuric acid is a strong dibasic acid as two of its hydrogen atoms can be replaced by a metal

  • Mg + H2SO4  → MgSO4 + H2

• It reacts in a similar way to other acids with metal carbonates, oxides, hydroxides (and ammonia) and metals, e.g:

  • Na2CO3 + H2SO4  → Na2SO4 + CO2+ H2O
  • ZnO + H2SO4  → ZnSO4 + H2O

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• Concentrated sulphuric acid is corrosive and a powerful oxidising agent

• Concentrated sulphuric acid is also a very powerful dehydrating agent and is very good at removing water from other substances

• For example, if mixed with sugar (C6H12O6), concentrated H2SO4 will remove water molecules and leave behind carbon in a spectacular looking black tower

  

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Uses of Sulfuric Acid

In dilute solution it is used as a catalyst in many organic reactions and to clean the surface of metals

• Concentrated sulfuric acid is used in car batteries, making fertilisers, soaps and detergents

• It is also used to make acid drain cleaners and in the production of paints and dyes

  

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