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Sulphur dioxide - Preparation, Properties, Structure, Uses | Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail |

Chapter: 12th Chemistry : p-Block Elements-II

Sulphur dioxide

From sulphur: A large-scale production of sulphur dioxide is done by burning sulphur in air.

Sulphur dioxide


Preparation

From sulphur: A large-scale production of sulphur dioxide is done by burning sulphur in air.

About 6-8% of sulphur is oxidised to SO3.

S + O2 → SO2

2S + 3O2 → 2SO3

From sulphides: When sulphide ores such as galena (PbS), zinc blende (ZnS) are roasted in air, sulphur dioxide is liberated. Large amounts of sulphur dioxide required for manufacturing of sulphuric acid and other industrial purpose is prepared by this method.


Laboratory preparation: Sulphur dioxide is prepared in the laboratory treating a metal or metal sulphite with sulphuric acid

Cu + 2H2SO4 → CuSO4 + SO2 + 2H2O

SO3- + 2H+ → H2O + SO2


Properties:

Sulphur dioxide gas is found in volcanic eruptions. A large amount of sulphur dioxide gas is released into atmosphere from power plants using coal and oil and copper melting plants. It is a colourless gas with a suffocating odour. It is highly soluble in water and it is 2.2 times heavier than air. Sulphur dioxide can be liquefied (boiling point 263 K) at 2.5 atmospheric pressure and 288 K.


Chemical properties

Sulphur dioxide is an acidic oxide. It dissolves in water to give sulphurous acid.


SO2 + H2O ↔ H2SO3 (Sulphurous acid)

H2SO3 ↔ 2H+ + SO32

Reaction with sodium hydroxide and sodium carbonate: Sulphur dioxide reacts with sodium hydroxide and sodium carbonate to form sodium bisulphite and sodium sulphite respectively.


SO2 + NaOH NaHSO3 (Sodium bisulphite)

2SO2 + Na2CO3 + H2O 2NaHSO3 + CO2

2 NaHSO3 Na2SO3(Sodium sulphite) + H2O + SO2

Oxidising property: Sulphur dioxide, oxidises hydrogen sulphide to sulphur and magnesium to magnesium oxide.

2H2S + SO2 → 3S + 2H2O

2Mg + SO2 → 2MgO + S

Reducing property: As it can readily be oxidised, it acts as a reducing agent. It reduces chlorine into hydrochloric acid.

SO2 + 2H2O + Cl2 → H2SO4 + 2HCl

It also reduces potassium permanganate and dichromate to Mn2+ and Cr3+ respectively.

2KMnO4 + 5SO2 + 2H2O K2SO4 + 2MnSO4 + 2H2SO4

K2Cr2O7 + 3SO2 + H2SO4 K2SO4 + Cr2 (SO4 )3 + H2O

Reaction with oxygen: Sulphur dioxide is oxidised to sulphur trioxide upon heating with oxygen at high temperature. This reaction is used for the manufacture of sulphuric acid by contact process.


Bleaching action of sulphur dioxide: In presence of water, sulphur dioxide bleaches coloured wool, silk, sponges and straw into colourless due to its reducing property.

SO2 + 2H2O → 2 H2SO4 + 2(H)

(Coloured) X + 2(H) → XH2(Colourless)


However, the bleached product (colourless) is allowed to stand in air, it is reoxidised by atmospheric oxygen to its original colour. Hence bleaching action of sulphur dioxide is temporary.


Uses:

·           Sulphur dioxide is used in bleaching hair, silk, wool etc...

·           It can be used for disinfecting crops and plants in agriculture.


Structure of sulphur dioxide:

In sulphur dioxide, sulphur atom undergoes sp2 hybridisation. A double bond arises between S and O is due to pπ- dπ overlapping.


 

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Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail


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