CTEVT Chemistry Note : Inorganic Chemistry(III) : Sulphur and Halogens.

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CHAPTER- 6 : SULPHUR

Symbol        : S

Atomic number : 16

Atomic weight : 32

Electronic configuration : 1s22s22p63s23p4

Sulphur dioxide (SO2) :

Laboratory preparation of sulphur dioxide (SO2) :

Sulphur dioxide gas is prepared in laboratory by heating copper turnings with conc. H2SO4.

lab preparation of Sulphur dioxide gas

Procedure: Copper turnings are taken in a round bottom flask fitted with thistle funnel and delivery tube as shown in figure. When conc. H2SO4 is poured through thistle funnel and mixture is heated sulphur dioxide gas is evolved, which is collected in the gas jar by upward displacement of air.

Drying of SO2:

If sulphur dioxide gas is to be dried, the gas obtained is passed through conc. Sulphuric acid . The moisture is absorbed by conc. H2SO4.

Chemical Properties of Sulphur Dioxide :

1. Reaction with lime water: When SO2 gas is passed into lime water, it turns milky due to formation of white ppt. of calcium sulphite.

2. Reaction with sodium carbonate: When SO2 gas is passed into Na2CO3 solution, carbondioxide gas is evolved.

3. SO2 as bleaching agent:

[Formation of colourless product from coloured substance is called bleaching. Chemical substances such as SO2, H2O2, CaOCl2, Cl2, etc. show bleaching action.]

SO2 can act a a bleaching agent in presence of moisture. It can bleach coloured wool, silk, flower, hair, etc. The bleaching action of sulphur dioxide is due to the formation of nascent hydrogen in presence of moisture, which reduces colouring substance to colourless reduced product.

In some cases, bleaching action of SO2 is due to formation of colourless addition product.

→ The bleaching action of SO2 is temporary. The bleached colourless compound will regain its original colour slowly on standing in air due to oxidation by air.

Comparison of bleaching action of SO2 and Cl2:

Sulphur dioxide (SO2) Chlorine (Cl2)
  1. Sulphur dioxide reacts with water to yield nascent hydrogen which bleaches coloured substance to colourless product by reduction.

  1. Bleaching by SO2 may be sometimes due to formation of colourless addition products.
  2. Bleaching by SO2 is reversible i.e. a temporary process.
  3. SO2 is mild bleaching agent.
  4. It is used to bleach delicate articles such as wool, silk, cane sugar, etc.
  1. Chlorine reacts with water to yield nascent oxygen which bleaches coloured substance to colourless product by oxidation.

  1. No such colourless addition product is formed with chlorine.
  2. Bleaching by Cl2 is irreversible ie. a permanent process.
  3. Cl2 is strong bleaching agent.
  4. It is used to bleach wood, textiles, paper, etc.

4. SO2 as reducing agent :

i) It reduces acidified potassium permanganate solution and pink colour of KMnO4 is discharged.

ii) It reduces acidified K2Cr2O7 solution and orange color of K2Cr2O7 changes to light green.

5. SO2 as an oxidizing agent : SO2 oxidizes powerful reducing agents like H2S, HI, Mg, Fe, etc and itself get reduced to sulphur or sulphide.

Hydrogen sulphide (H2S) :

Laboratory preparation of Hydrogen sulphide (H2S) :

Hydrogen sulphide (H2S) gas is prepared in the laboratory by treating iron sulphide with dil. H2SO4.

Procedure :

Iron sulphide (FeS) pieces are kept in woulf’s bottle. Then a thistle funnel and delivery tube is fitted as shown in fig. When dilute H2SO4 is poured in woulf’s bottle, reaction between FeS and dil. H2SO4 takes place and H2S gas is evolved which is collected in the gas jar by upward displacement of air.

Test of H2S : When a lead acetate paper is dipped in the jar containing H2S gas, it turns to black due to formation of lead sulphide.

Chemical Properties of Hydrogen Sulphide :

1. H2S as reducing agent :- Sulphur in H2S bears -2 oxidation state which in minimum oxidation state of sulphur. In chemical reaction, H2S tends to get oxidized . So H2S acts as reducing agent.

  •  It reduces halogens into halogen acids.

  • It reduces sulphur dioxide to sulphur.

  • It reduces acidified potassium permanganate solution and pink colour of KMnO4 is discharged.

  • It reduces acidified K2Cr2O7 solution and orange color of K2Cr2O7 changes to green.

CHAPTER- 7 : HALOGENS

Q) What do you mean by halogen and halogen acids?

  • The elements present in group VIIA or 17 of modern periodic table i.e. F, Cl, Br and I are called halogens. Electronic configuration of halogens is ns2np5 and oxidation number is -1 to +7.
  • The compounds of hydrogen with halogens are called halogen acids. HCl, HBr and HI are halogen acids.
Laboratory preparation of halogens :

Chlorine, bromine and iodine are prepared in the laboratory by heating respective halide salts with manganese dioxide and concentrated sulphuric acid.

Eg. Laboratory preparation of chlorine gas: Cl2 gas is prepared in laboratory by heating NaCl with MnO2 and conc. H2SO4.

Lab preparation of Chlorine gas

Procedure : NaCl and MnO2 is taken in a round bottom flask and all the apparatus are fitted as shown in figure. When conc. H2SO4 is added from dropping funnel and the mixture is heated, Cl2 gas is evolved, which is collected in a gas jar by upward displacement of air.

{Note : Same for Br2 and I2 }

Chemical properties of halogens ( eg. Chlorine) :

1. Reaction with hydrogen: Chlorine reacts with hydrogen in presence of sunlight to give HCl.

2. Reaction with slaked lime: Chlorine reacts with slakewd lime to give bleaching powder.

3. Reaction with organic compounds : Alkanes react with halogen in presence of sunlight to give haloalkane.

4. Oxidizing nature : Halogens are strong oxidizing agents.

Eg. Cl2 oxidizes H2S to S.

5. Bleaching action of Cl2 :

Chlorine reacts with water to yield nascent oxygen which bleaches coloured substance to colourless product by oxidation.

Bleaching by Cl2 is irreversible ie. a permanent process.

Cl2 is strong bleaching agent. It is used to bleach wood, textiles, paper, etc.

Uses of chlorine :
  1. It is used in sterilization of drinking water.
  2. It is used to bleach textiles, papers, wood, etc.
  3. It is used in the manufacture of insecticides like DDT, polymer like PVC, dyes, drugs, etc.

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