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Oxides of Sulphur

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OXIDES OF SULPHUR

Sulphur forms a number of oxides, these are

OXIDE

FORMULA

CHARECTERISTIC PROPERTY

Sulphur suboxide

S2O

Colourless gas

Sulphur monoxide

SO

Colourless gas

Sulphur sesquioxide

S2O3

Green crystalline solid

Sulphur dioxide

SO2

Colourless gas

Sulphur trioxide

SO3

Volatile liquid

Sulphur heptoxide

S2O7

Liquid

Sulphur tetroxide

SO4

White solid

Of all these oxides sulphur dioxide and sulphur trioxide are most important

SULPHUR DIOXIDE

Sulphur dioxide is a colourless and a pungent smelling gas, charecterised by the following

  1. Relative density 1.43

  2. It is heavier than air.

  3. Melting point -72.7OC.

  4. Boiling point -10OC.

  5. It is highly soluble in water.

PREPARATION

  1. When sulphur is burned in a supply of air ( oxygen)

    2. S + O2 è SO2

  2. By the action of heat on sulphur with concentrated sulphuric acid

    3. heat

    S + 2H2SO4 è 2H2O + 3SO2

  3. By roasting sulphides

2ZnS + 3O2 è 2ZnO + 2SO2

CHEMICAL PROPERTIES

  1. Although it is not a supporter of combustion nor itself combustible, burning magnesium and potassium continue burning in its presence.

    2. 3Mg + SO2 è 2MgO + MgS

    4K + 3SO2 è 2SO3 + S

  2. Sulphur dioxide decomposes at 1200OC yielding Sulphur trioxide and sulphur

    3. 3SO2 è 2SO3 + S

  3. Sulphur dioxide dissolves in water forming sulphuric acid.

    4. SO2 + H2O è H2SO3

  4. Reducing action

    5. Cl2 + SO2 + 2H2O è H2SO4 + 2HCl

    The reducing action of sulphur dioxide in its aqueous solution is because of sulphurous acid changing into sulphuric acid giving a nascent hydrogen.

  5. As a bleaching agent

Sulphur dioxide in the presence of moisture acts as a bleaching agent.

SO2 + 2H2O è H2SO4 + 2H

Coloured organic matter + H è colourless organic matter

USES

  • One of the main uses of sulphur dioxide is in the manufacture of sulphuric acid.

  • It is used as a mild bleaching agent for bleaching delicate articles.

  • It is used as a disinfectant.

  • Used in refining petroleum and sugar.

SULPHUR TRIOXIDE

Sulphur trioxide is a colourless fuming solid. It has three crystalline forms. In the decreasing order of stability these are

  1. ALPHA (

    2. a)

    • Relative density 1.97

    • Melting point 16.83OC

    • Boiling point 44.8OC

b. BETA (b)

    • Relative density 2.29

    • Melting point 32.5OC

    • Sublimes at 50OC into the alpha variety

c. GAMMA (g)

    • Melting point 62.2OC at 2 atm pressure

All there forms are polymeric with linked SO4 tetrahedra.

PREPARATION

  1. When ferric sulphate crystals are heated strongly

    2. Fe2(SO4)3 è Fe2O3 + 3SO3

  2. by dehydrating sulphuric acid with an excess of phosphorous pentoxide.

    3. 6H2SO4 + P4O10 è 4H3PO4 + 6SO3

CHEMICAL PROPERTIES

  1. It is an exceptionally strong Lewis acid which fumes in moist air and reacts explosively with water to form H2SO4.

    2. H2O + SO3 è H2SO4

  2. With conc. H2SO4 it reacts to form oleum or fuming sulphuric acid, H2S3OÕ

    3. H2SO4 + SO3 è H2S2O7

     

  3. With basic oxides, it combines to form sulphates ( this is an exothermic reaction)

    4. BaO + SO3 è BaSO4

     

  4. It also functions as an oxidising agent and will liberate Br2 from hydrogen bromide

2HBr + SO3 è H2O + Br2 + SO2

USES

  • In the manufacture of oleum
  • As a drying agent for gases

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