Sulphur and it’s compounds – Basic note

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Allotropes of sulphur

The two or more forms of same element having similar chemical properties but different physical properties are called allotropes and the phenomenon is called allotropy.

Sulphur exists on following allotropic forms:

Uses of sulphur

  • It is used for the manufacture of sulphuric acid, sulphur dioxide and other sulphur compounds.
  • It is used in medicine to manufacture sulpha drugs and ointments for skin disease.
  • It is used for the vulcanization of rubber.

Note: Vulcanization is a process of heating raw rubber with sulphur or its compounds to destroy its sticky nature and to increase the durability of rubber products.

  • It is used for the manufacture of black gun powder which is a mixture of carbon (charcoal), sulphur and nitre (KNO3).
  • It is used in the manufacture of fungicides and insecticides.

Compounds of sulphur

sulphur compounds

Hydrogen sulphide (H2S)

Working principle of Kipp’s apparatus : Intermittent supply of H2S gas by Kipp’s apparatus

Intermittent = Stopping and starting at irregular intervals

H2S gas is frequently required for salt analysis in the laboratory. The special apparatus which is used for the preparation of H2S is called Kipp’s apparatus.

Kipp’s apparatus consist of three bulbs A, B and C. The bulbs are interconnected with each other. The upper bulb C contains a long stem which reaches upto bulb A. When dil. H2SO4 is poured from bulb C, liquid level rises in bulb A and reaches upto bulb B just to cover iron sulphide. Then the reaction between iron sulphide and H2SO4 takes place to form H2S gas. As H2S gas is formed, the pressure inside bulb B increases and forces the dil. H2SO4 up into the upper bulb C. Then the contact between FeS and dil. H2SO4 breaks and the formation of gas is stopped (ceased). When the tap is opened H2S comes out and the pressure decreases. As a result liquid level again rises at bulb B and H2S gas start to evolve again. In this way H2S gas is prepared intermittently by Kipp’s apparatus.

Phsical Properties of hydrogen sulphide

1. It is colourless gas with rotten egg smell.

2. It is soluble in water.

3. It is poisonous gas.

{In its low concentration it causes nausea, dizziness and headache. At high concentration it is fatal (i.e. deadly poisonous). Conc. of H2S in air 1 part in 1000 parts of air may be fatal.}

4. It is heavier than air{with vapour density 17}.

Chemical Properties of hydrogen sulphide

1. Acidic character : It acts as a weak diprotic(dibasic) acid. It ionizes in two steps to give two protons.

H2S reacts with base to form two series of salts. Eg.

H2S reacts with basic oxides to give salt. eg.

2. 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 ferric salt to ferrous salt.

* It reduces conc. HNO3 to NO2

* It reduces conc. H2SO4 to SO2.

* 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.

3. H2S as analytical reagent in salt analysis ( Precipitation of metal sulphide) :

H2S gas is used as analytical agent to detect group II basic radicals in acidic medium i.e. HCl and group IIIB basic radicals in in alkaline medium i.e. NH4Cl and NH4OH.

Group II basic radicals :

Group II basic radicals consists of Cu ++ , Hg ++, Pb++, Cd++, Sn++, As+++, Sb+++, Bi+++. In qualitative salt analysis this group II radicals are precipitated in the form of their sulphide in acidic medium. By noting the colour of ppt., we can identify the metal cations.

Group IIIB basic radicals:

Group IIIB basic radicals consist of Zn++, Co++, Ni++, Mn++. These are precipitated by passing H2S in alkaline medium ( i.e. NH4Cl+ NH4OH).

Structure of H2S:

Uses of H2S:

  • It is used as analytical reagent in laboratory.
  • It is used as reducing agent.
  • It is used to prepare metallic sulphide, which can be used as pigment.

Sulphur dioxide (SO2)

Chemical Properties of sulphur dioxide:

1. Weak acidic nature: It dissolves in water to give sulphurous acid, a weak diprotic (dibasic) acid.

Sulphurous acid ionizes in two steps:

It produces two series of salts if reacted with base. Eg.

2. SO2 as a reducing agent: Aqueous sulphurdioxide shows reducing character and SO2 itself gets oxidized to H2SO4.

*  It reduces halogens (Cl2, Br2, I2) in aq. Solution to respective halides. Eg.

* 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 light green.

* It reduces ferric salt to ferrous salt and colour changes from yellow to light green.

* It reduces potassium iodate (KIO3) solution to iodine.

3. SO2 as an oxidizing agent:

SO2 oxidizes powerful reducing agents like H2S, HI, Mg, Fe, etc. and itself get reduced to sulphur or sulphide.

4. 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 as 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.

SO2 + 2H2O –> H2SO4 + 2 [H]

2. Bleaching by SO2 may be sometimes due to formation of colourless addition products.

3. Bleaching by SO2 is reversible i.e. a temporary process.

4. SO2 is mild bleaching agent.

5. 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.

Cl2 + H2O –> 2HCl + 2[O]

2. No such colourless addition product is formed with chlorine.

3. Bleaching by Cl2 is irreversible ie. a permanent process.

4. Cl2 is strong bleaching agent.

5. It is used to bleach wood, textiles, paper, etc.

Test for SO2 :

Potassium dichromate paper test : When SO2 comes in contact with acidified potassium dichromate paper, the orange coloured paper turns to green due to formation of chromium sulphate.

Uses of SO2:

  • It is used as bleaching agent.
  • It is used in the manufacture of sulphuric acid.
  • It can be used as disinfectant, fungicide and food preservative.

Sulphuric acid (H2SO4)

Sulphuric acid is widely used chemical in industries. It is used in the manufacture of fertilizers, drugs, dyes, polymers, etc. thousands of tons of sulphuric acid is manufactured every year worldwide. So it is called king of chemicals.

Manufacture of sulphuric acid by contact process: 

  • In early days, sulphuric acid used to be manufactured by lead chamber process.
  • Contact process is modern method. The acid obtained is pure ( free from impurities) and is quite concentrated (96-99%). The contact process, it’s name is mainly from the fact that the conversion of sulphur dioxide into sulphur trioxide is carried out in contact with surface of catalyst.

Principle :

1. Production of sulphur dioxide: Sulphur dioxide gas can be prepared either by burning sulphur or roasting of iron pyrites.

2. Catalytic oxidation of sulphur dioxide: Sulphur dioxide is oxidized to sulphur trioxide in presence of catalyst vanadium pentoxide at about 4500C temperature and 2 atm pressure.

3. Conversion of SO3 into H2SO4: sulphur trioxide obtained is absorbed in 98% H2SO4 to produce oleum or fuming sulphuric acid. The oleum is diluted with calculated amount of water to get desired concentration of H2SO4.

Conditions for optimum yield of H2SO4:

Formation of SO2 to SO3 is one of the must important steps in the manufacture of sulphuric acid. The production of H2SO4 entirely depends on the amount of SO3 formed. As reaction is reversible, exothermic and proceeds with decrease in volume, Le- Chatelier’s principle can be applied for the maximum yield of sulphur trioxide.

  • Low temperature : This reaction is exothermic . So, low temperature is required for maximum yield. But too low temperature is too slow to attain equilibrium. So an optimum temperature of about 4500C is supplied.
  • High concentration of reactants : High concentration of SO2 and O2 is used for more production of SO3 .
  • High pressure : High pressure favours the reaction because the product formed has less volume than reactant . But the acid resistant tower which are able to withstand high pressure are difficult to build. Hence, an optimum pressure of about 2 atm is applied.
  • Use of catalyst: Rate of reaction is increased by the use of positive catalyst. So, vanadium pentoxide is used as catalyst for higher yield of H2SO4.

Details of the plant or process:

1. Sulphur or pyrite burner: SO2 gas is obtained by burning sulphur or iron pyrite with air in sulphur or pyrite burner.

2. Purification unit: The impure SO2 gas obtained is purified by the purification unit.

a. Electrical dust precipitator: Dust particles present in sulphur dioxide gas is precipitated in electrical dust precipitator by the influence of high potential difference applied between the metallic conductors fitted in the chamber.

b. Steam chamber: The lighter dust particles are settled down by using steam in steam chamber.

c. Cooler: The gases coming out from the steam chamber are passed through cooler. The gases get cooled down to about 1000C.

d. Washing tower(scrubber): The cooled SO2 gas is then passed into a tower called scrubber which is packed with quartz and water is sprayed from the top. The water soluble impurities are washed away.

e. Drying tower: The moisture present in sulphur dioxide is absorbed by the spray of conc.H2SO4 in drying tower.

f. Arsenic purifier: Arsenous oxide ( As2O3) present in gas is absorbed by ferric hydroxide( because it may causes catalytic poisoning).

3. Testing box: The purity of sulphur dioxide is checked in testing box by passing through a darkened box.

4. Preheater: The purified mixture of SO2 and O2 is heated upto 4500C.

5. Contact chamber: The SO2 gas is catalytically oxidized to sulphur trioxide (SO3) in the presence of V2O5 as catalyst at 4500C temperature and 2 atm pressure.

6. Absorption tower: This tower is packed with quartz (or acid proof stone) in which 98% H2SO4 is sprayed from the top of the tower. Concentrated H2SO4 absorbs sulphur trioxide to form oleum (or pyrosulphuric acid). Oleum is then treated with calculated amount of water to form sulphuric acid of desired concentration.

Physical Properties of H2SO4:

i. It is colourless, hygroscopic, syrupy liquid.

ii. It is highly soluble in water (due to formation of intermolecular hydrogen bond). It dissolves in water with the liberation of large quantity of heat(19Kcal/mol) which may causes the explosion or spurt the acid out of the container.

H2SO4 + nH2O H2SO4.nH2O + Heat

Therefore, sulphuric acid is diluted by adding the acid slowly to water with constant stirring and not by adding water to the acid.

iii. It’s melting point is 100C and boiling point is 3380C.

iv. Pure H2SO4 is covalent compound and bad conductor of electricity. However aqueous H2SO4 conducts electricity.

v. Litmus test : Sulphuric acid is covalent compound. In pure state it does not change the colour of the blue litmus paper. However, aqueous acid gets ionized and produces hydrogen(or hydronium) ion into solution. Due to which it can change the colour of blue litmus paper to red.

Chemical properties of H2SO4:

1. Decomposition: On heating H2SO4 decomposes into SO2, H2O and O2.

2. Acidic nature: H2SO4 is a strong dibasic (diprotic) acid and ionizes in two steps.

It gives two series of salts like bisulphate and sulphate when reacted with base.Eg.

3. H2SO4 as an oxidizing agent:

i. Action with metals: More electropositive metals ( i.e. metals lying above hydrogen in electrochemical series) like Zn, Fe, Mg, Al, etc. react with dil H2SO4 to produce hydrogen gas. Here H2SO4 reduces to H2 and metals oxidize to metal sulphates.

Conc. H2SO4 oxidizes Zn, Cu, Ag, Hg, etc. to respective metal sulphates and H2SO4 gets reduced to SO2.

ii. Action with non metals:

* Carbon is oxidized to carbondioxide.

* Sulphur is oxidized to sulphurdioxide.

* Phosphorus is oxidized to phosphoric acid.

iii. Action with some other reducing agents:

* H2SO4 oxidizes H2S to S and itself reduces to SO2.

* HBr and HI are oxidized to Br2 and I2 respectively.

{Note : HCl can not be oxidized to Cl2 because of strong bond.}

4. Sulphuric acid as dehydrating agent:

H2SO4 is a good dehydrating agent. It absorbs water molecules from sugar, cellulose, copper sulphate crystals, etc.

* With sugar (charring action):

When conc. H2SO4 is treated with sugar, wood, paper, etc. it absorbs water and a black mass of carbon is formed. This process is called charring.

* With copper sulphate crystals:

It removes water of crystallization from hydrated salts.

* With oxalic acid crystals and formic acid:

Lewis structure of H2SO4

Uses of sulphuric acid:

  • It is used for the manufacture of fertilizers.
  • It is used in manufacture of HCl, HNO3, H3PO4, etc.
  • It is used as an oxidizing agent.
  • It is used as a drying and dehydrating agent.

Test of H2SO4:

Sulphuric acid gives H+ ions and SO4– – ions in aqueous solution. The presence of H+ ion can be detected by litmus paper. The presence of SO4– – can be detected by BaCl2 solution which gives white precipitate.

Sodium thiosulphate

Molecular formula: Na2S2O3. 5H2O

It is commonly called hypo.

Uses of sodium thiosulphate:

  • It is used as a fixer in photography. { i.e. for fixing silver bromide in photographic plate}
  • It is used in (iodometric) titration for the estimation of iodine.
  • It is used as an antichlor agent to remove excess of chlorine from bleached textiles/fibers.

References