Sulphur And Its CompoundsSulphur is the chemical element with atomic number 16,

represented by the symbol S. It is an abundant, multivalent non-metal. At normal conditions, sulphur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulphur is a bright yellow crystalline solid.

Sulphur has been known for its medicinal value and its germicidal effect since 1000 B.C. However, its chemical nature remained unknown until 1787 when Lavoisier recognized it as an element.

Extraction Of Sulphur (The Frasch Process): Superheated steam at 170oC under 10 atm pressure is pumped down the outer most pipe . Since melting point of sulphur is 113oC , it is melted and collects at the bottom. Hot compressed air at 15 atm is blown down the inner most pipe to produce foam of molten sulphur . This foam rises through the annular space between the inner most pipe and the next. The foam of sulphur is collected in wooden tubs. After few hours, sulphur is converted into solid blocks. The sulphur obtained is about 99.5% pure.

Natural gas contains a small amount of hydrogen sulphide. The H2S is first separated out from the natural gas and then oxidized to sulphur by mixing it with air and passing it over heated aluminium oxide (Al2O3) catalyst.

Allotropes Of SulphurThere are crystalline (rhombic and monoclinic) and amorphous sulphur. There is also another known as plastic sulphur.(i) Rhombic (α-sulphur) is the free existing sulphur in nature, being only stable at temperature below 96oC.its crystals are bright yellow and octahedral, made up of S8 molecules. Each S8 molecules consists of a ring of eight atoms.Rhombic sulphur is prepared by allowing a saturated solution of sulphur in carbon (IV) sulphide to evaporate slowly. Octahedral crystals will gradually be deposited.NOTE: Great caution should be taken to carry it out in fume cupboard due to the poisonous and highly inflammability nature of carbon (IV) sulphide.

(ii) Monoclinic sulphur (β-sulphur or Prismatic sulphur: is the only stable allotrope at temperatures between 96oC and 119oC. The crystals are long, thin and needle-shaped, and consists of S8 molecules. At room temperature, they slowly revert to rhombic sulphur crystals.

Monoclinic is obtained by cooling molten sulphur.Comparing Physical Properties Of Rhombic & Monoclinic

96oC is known as the transition temperature at which the two allotropes change from one form to another.

Property Rhombic Sulphur Monoclinic sulphurColour Bright yellow AmberShape Octahedral Needle-shapedDensity (g cm-3) 2.08 1.98Melting point 113oC 119oCStability Below 96oC Between 96oC and 119oC

The Physical Properties Of Rhombic And MonoclinicRhombic sulphur Monoclinic sulphur

3. Amorphous carbon: has no definite crystal shape. It is prepared as a pale yellow, almost white deposit when hydrogen sulphide is bubbled through water for a long time and the saturated solution is exposed to air.. It is also deposited in chemical reactions, e.g. by action of dilute hydrochloric acid on trioxothiosulphate (VI) solution.2H2S(g) + O2(g) → 2H2O(l) + 2S(s).

S2O32-

(aq) + H+(aq) → H2O(l) + SO2(g) + S(s).

Between 96oC and 119oC

Below 96oCRhombic sulphur Monolinic sulphur

Colour Bright yellow Amber Shape Octahedral Needle-shapedDensity (g/cm3) 2.08 1.98Melting point 113oC 119oCStability Stable below 96oC Stable between 96oC and 119oC)

4. Plastic sulphur: is a supercooled form of sulphur. If the yellow sulphur is heated and poured into cold water, it will roll up into yellow ribbons which looks as if they are made of plastic material. It is soft and elastic and will not dissolve in carbon (IV) sulphide. Plastic sulphur is generally not considered to be true allotrope of sulphur because it is unstable and reverts to rhombic sulphur on standing.

Physical Properties Of Sulphur(i) Sulphur is a yellow solid. It exists in two forms –

amorphous and crystalline.(ii) It is insoluble in water but soluble in carbon (IV) sulphide.(iii) Being a non-metal, sulphur is a bad conductor of heat and

electricity.(iv) Density depends on allotropic form(v) Its melting point is 119oC and its boiling point is 444oC.

When the vapour of sulphur is condensed on a cold surface, a fine powder which forms a floral pattern is obtained called Flowers of sulphur. And when molten sulphur is often poured into a tubular moulds to form Roll sulphur. At 160oC zig-zag chains form is formed and at 200oC it reaches its maximum liquid viscosity.

Chemical Properties Of SulphurSulphur has oxidation states ranging from -2 to +6 as shown below:

(i) Direct Combination with other elements: nearly with all metals and most non-metals.

Name of compound Formula Oxidation stateTetraoxosulphate (VI) acid H2SO4 +6Sulphur (IV) oxide SO2 +4Sulphur S8 0Hydrogen sulphide H2S -2

(a) With metals: to form sulphides when heated in the absence of air. Fe(s) + S(s) → FeS(s). Cu(s) + S(s) → Cu2S(s).

(b) With oxygen: burn in plentiful air with bright blue flame to form sulphur (IV) oxide. O2(g) + S(s) → SO2(g).

(c) With hydrogen H2(g) + S(s) → H2S(s). With carbon C(s) + S(s) → CS2(l)

Uses of sulphur(i) Sulphur is used to produce sulphur (IV) oxide for the

manufacture of tetraoxosulphate (VI) acid.(ii) Sulphur is used in vulcanization of rubber.(iii) Sulphur and other products of sulphur are used as

fungicides and insecticides for spraying crops.(iv) sulphur is also used to manufacture the bleaching agents

used in pulp and paper industries, also for skin ointments, CS2, dyes, matches, fireworks and gunpowder etc.

Sulphur (IV) OxideIs the chemical compound with the formula SO2. It is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulphur compounds, their combustion generates sulphur (IV) oxide unless the sulphur compounds are removed before burning the fuel. Further oxidation of SO2, usually in the presence of a catalyst such as NO2, forms H2SO4, and thus acid rain. Sulphur (IV) oxide emissions are also a precursor to particulates in the atmosphere. Both of these impacts are cause for concern over the environmental impact of these fuels.

Preparation of Sulphur• The action of hot tetraoxosulphate (VI) acid on copper turnings

produces sulphur (IV) oxide. Cu + 2H2SO4 → CuSO4 + SO2 + 2H2O• Sulfur dioxide is a by-product in the manufacture of calcium

trioxosilicate (IV) cement: CaSO4 is heated with coke and sand in this process: 2 CaSO4 + 2 SiO2 + C → 2 CaSiO3 + 2 SO2 + CO2

• The combustion of hydrogen sulphide and organosulfur compounds proceeds similarly. 2 H2S + 3 O2 → 2 H2O + 2 SO2

Physical Properties Of Sulphur (Iv) Oxide(i) Sulphur (IV) oxide is a colourless and poisonous gas with a

very irritating sell like that of burning matches.(ii) It is very soluble in water, one volume of water dissolves

about 70 volumes of the gas to form trioxosulphate (IV) acid.

(iii) It is about 2.5 times denser than air.(iv) It is easily liquefied under pressure (about 3 atm.) at room

temperature.Chemical properties of sulphur(v) As an acid – with alkalis to form normal salt. Treatment of

basic solutions with sulphur (IV) oxide affords sulphite salts: SO2 + 2NaOH → Na2SO3 + H2O. In the presence of excess of the gas. SO2(g) + 2NaOH(aq) → NaHSO3(aq).

(ii) As a reducing agent: Sulphur dioxide is also a good reducing agent. In the presence of water, sulphur (IV) oxide is able to decolorize substances. Specifically it is a useful reducing bleach for papers and delicate materials such as clothes. This bleaching effect normally does not last very long. Oxygen in the atmosphere reoxidizes the reduced dyes, restoring the colour. In municipal wastewater treatment sulphur dioxide is used to treat chlorinated wastewater prior to release. Sulphur (IV) oxide reduces free and combined chlorine to chloride.(a) Purple Acidified KMnO4(aq) is decolourizes by SO2(g).2KMnO4(aq) +5SO2(g)+2H2O(l)→K2SO4(aq)+MnSO4(aq) + 2H2SO4(aq).

2MnO4-(aq) +5SO2(g)+2H2O(l)→SO4

2-(aq)+Mn2+

(aq) + 4H+(aq).

(b) Orange Acidified K2Cr2O7(aq) turns to green K2Cr2O7(aq) +3SO2(g)+2H2SO4(l)→K2SO4(aq)+Cr2(SO4)3(aq) +H2O(aq).

Cr2O72-

(aq) +3SO2(g)+2H+(aq)→ 3SO4

2-(aq)+2Cr3+

(aq) +H2O(l).

(iii) As a bleaching agent: A solution of SO2 in water bleaches both natural and artificial dyes. It dissolves in water to form trioxosulphate (IV) acid which donates the electron to dye and then oxydized to tetraoxosulphate (VI) acid, hence the dye is reduced .H2SO3(aq) + Dye(aq) → H2SO4(aq) + Bleached dye(aq).

coloured colourless(iv) Direct combination reactions:(a) With certain metallic oxide: PbO2(s) + SO2(aq) → PbSO4(s).

Na2O2(s) + SO2(aq) → Na2SO4(s).(b) With oxygen: 2SO2(g) + O2(g) 2SO3(g). It is

reversible with the forward reaction being exothermic.(v) As an Oxidizing agent:It acts as oxidizing agent in the presence of a stronger reducing agent like H2S. 2H2S(g) + SO2(g) → 2H2O(l) + 3S(s).

Test for sulphur (IV) OxideSO2 may be recognized by its irritating smell and confirmed by these test:(i) Bleaching action: it bleaches moist flower. This test makes it

suspected which can be confirmed with its reducing effect on oxidizing agent.

(ii) Action of oxidizing agents: it decolourizes purple acidified KMnO4(aq) and turns orange acidified K2Cr2O7(aq) green.

Both SO2(g) and H2S(g) produce similar changes with acidified KMnO4(aq) and K2Cr2O7(aq). But H2S(g) produces yellow ppt of sulphur while SO2(g) does not.

Uses of Sulphur (IV) Oxide(iii) It is mostly used for making tetraoxosulphate (VI) acid.(iv) It is used as a germicide and a fumigant mostly to destroy

termites.(v) It is used as a bleaching agent for straw, sponges, silk, wool

and other fabrics which could be damaged by chlorine.

iv. Liquid SO2 is sometimes used as a refrigerant.v. The gas is used as a preservative in some liquids e.g.

orange juice.TRIOXOSULPHATE (IV) ACID

Trioxosulphate (IV) acid, H2SO3, is an unstable acid formed when SO2 dissolves on water. The reaction is reversible.

SO2(g) + H2O(l) H2SO3(aq).The acid is dibasic and ionizes as follows:

H2SO3(aq). HSO3-(aq). H+

(aq).H2SO3(aq). SO3

2-(aq). + H+

(aq).PREPARATION OF TRIOXOSULPHATE (IV) ACIDTrioxosulphate (IV) acid, H2SO3, is prepared by adding conc. HCl to sodium trioxosulphate (IV) and heating the mixture gently. The SO2(g) liberated is dissolved in water to form Trioxosulphate (IV) acid.

Physical Properties Of Trioxosulphate (IV) Acid(i)Trioxosulphate (IV) acid is a colourless and unstable acid.(ii)It smells strongly of sulphur (IV) oxide, which is a product of its decomposition.(iii)It turns blue litmus red.

Chemical properties Of Trioxosulphate (IV) Acid(iv)Action of air: on exposure to the air for some time, it becomes oxidized to tetraoxosulphate (VI) acid.

H2SO3(aq) + O2(g) →H2SO4(aq).(v)As a reducing and bleaching agent: is responsible for the reducing and bleaching actions of sulphur (IV) oxide.(vi)As an acid.

Uses Of Trioxosulphate (IV) Acid(vii)It is used for bleaching straw and other fabrics.(viii)It is used as a germicide.

TRIOXOSULPHATE (IV) – SO32-

The salts are normal salts formed when sulphur (IV) oxide or trioxosulphate (IV) acid acid react with an excess of alkalis.

Preparation:(i)Neutralization: (a) SO2(g) + 2NaOH(aq) → Na2SO3(aq) + H2O(l).

(b) H2SO3(aq) + NaOH(aq) → Na2SO3(aq) + 2H2O(l).(ii)Precipitation: insoluble SO3

2- are precipitated out of solution when SO2(g) is bubbled through a solution of a corresponding metallic salt.

Properties:(iii)Solubility: only trioxosulphate (IV) of Na, K, Ca and NH4

+ are soluble while other metallic forms are insoluble.(iv)Reaction with acids: reacts with conc. or dilute acids, SO(g) is evolved. CaSO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + SO2(g).(v)With the air: oxidizes to tetraoxosulphate (VI) acid(vi)As a reducing agent: same reducing power as H2SO3(aq).

TEST FOR TRIOXOSULPHATE (IV)(vii)When SO2(g) when an unknown substance is warmed with dilute acid, the SO3

2- is present.2H+(aq)+SO3

2-(aq) → SO2(g)+H2O(l).

(viii)On add Ba(NO3)2(aq), a white ppt is formed which dissolve on adding dilute HNO3(aq) to evolve SO2(g). HNO3(aq) and BaCl2(aq) can also be used respectively. Ba2+

(aq) + SO32-

(aq) → BaSO3(s) BaSO3(s) + HNO3(aq) → SO2(g) + Ba(NO3)2(aq) + H2O(l).

TETRAOXOSULPHATE (VI) ACID – H2SO4Tetraoxosulphate (VI) acid is a strong mineral acid with the molecular formula H2SO4. Its historical name is vitriol. The salts of tetraoxosulphate (VI) acid are called tetraoxosulphate (VI). Sulphuric acid is soluble in water at all concentrations.Sulphuric acid has many applications, and is a central substance in the chemical industry. Principal uses include lead-acid batteries for cars and other vehicles, ore processing, fertilizer manufacturing, oil refining, wastewater processing, and chemical synthesis.

Industrial PreparationTetraoxosulphate (VI) acid is produced from sulphur, oxygen and water via the conventional contact process.

CONTACT PROCESSStage 1: In the first step, sulphur is burned in air to produce sulphur dioxide. S (s) + O2(g) SO2 (g).Stage 2: In presence of V2O5 as catalyst sulphur (IV) oxide is burnt in air at 450oC – 500oC and atmospheric pressure.

2SO2(g) + O2(g) 2SO3(g) + heatThis is a reversible reaction.

Stage 3: The sulphur (VI) oxide is absorbed into 97–98% H2SO4 to form oleum (H2S2O7), also known as fuming tetraoxosulphate (VI) acid. H2SO4 (aq) + SO3(g) → H2S2O7 (l).

Stage 4: The oleum is then diluted with water to form concentrated tetraoxosulphate (VI) acid.

H2S2O7 (aq) + H2O (l) → 2H2SO4 (aq)

Note that directly dissolving SO3 in water is not practical due to the highly exothermic nature of the reaction between sulphur trioxide and water. The reaction forms a corrosive aerosol that is very difficult to separate, instead of a liquid.SO3 (g) + H2O (l) → H2S2O7(aq).

Physical Properties Of Tetraoxosulphate (VI) Acid(i)Conc. H2SO4, is colourless, viscous liquid.(ii)Its density is 1.84 gcm3.(iii)It is corrosive and causes severe burns if it comes into contact with the skin.

(i) Conc. H2SO4, has a great affinity for water, evolving a large amount of heat as it dissolves. The heat evolved is a result of the hydration of the tetraoxosulphate (VI) ions. The conc. Acid is also hygroscopic, absorbing water vapour from the surroundings and becoming more dilute in the process.

Chemical Properties Of Tetraoxosulphate (VI) Acid.(ii) As an Acid: Dilute H2SO4(aq) is a strong acid. Hence, reacts

with metals, trioxocarbonate (IV) and bases to produce salts along side with hydrogen gas, carbon (IV) oxide and water, and water respectively.

(iii) As an oxidizing agent – with metals, non-metals and hydrogen sulphide. (a) SO4

2-(aq) + 4H+

(aq) + 2e- → 2H2O(l) + SO2(g). (b) Zn(s) + 2H2SO4(aq) → ZnSO4(aq) + 2H2O(l) + SO2(g). (c) ) C(s) + 2H2SO4(aq) → CO2(aq) + 2H2O(l) + SO2(g) (d) H2S(g) + H2SO4(aq) → SO2(g) + 2H2O(l) + S(s).

ii. As a dehydrating agent – Conc. H2SO4 is able to remove the elements hydrogen and oxygen in the form of water from compounds like sugar, ethanol, methanoic acid, ethanedioic acid etc. - 11H2O(l) C12H22O11(s) 12C(s).

Sugar Conc. H2SO4 Sugar charcoal Dehydration effects of Conc. H2SO4 account for its corrosion of cloth, paper, wood, even skin etc and removal of water of crystallization in hydrated salts

iii. Displacement of other acids from their salts: Conc. H2SO4 is able to displace volatile acids from their corresponding salts.

2KCl(s) + H2SO4(aq) → K2SO4(aq) + 2HCl(g). Uses of tetraoxosulphate (VI) Acid

(i) For fertilizer making e.g. ammonium tetraoxosulphate (VI) and calcium hydrogentetraoxophosphate (V) etc.

(ii) Manufacture of pigments e.g. titanium (IV) oxide and barium tetraoxosulphate (VI) for used in paints and dyes.

(iii) It is used for making cellulose film, fabrics and plastics etc.(iv) Purification of crude oil and manufacture of artificial silk.

(i) Pickling metals before electroplating and enamelling.(ii) Dehydrating agent & nitration of compounds for explosives.(iii) As an electrolyte in lead accumulators and batteries.(iv) In refining metals by electrolysis.(v) Preparation of chemicals like HCl, HNO3, metallic sulphates.

(vi) Industrial processes as a cheap raw material.(vii)As drying agent except alkaline gases like ammonia.(viii)For removing gums, waxes etc in petroleum refining.

Tetraoxosulphate (VI) – SO42-

Test for Tetraoxosulphate (VI)(ix)(i) On add Ba(NO3)2(aq), a white ppt is formed which is insoluble on adding dilute HNO3(aq) to evolve SO2(g). HNO3(aq) and BaCl2(aq) can also be used respectively. Ba2+

(aq) + SO42-

(aq) → BaSO4(s) The acidification process is necessary to prevent the precipitation of other insoluble compounds like Barium trioxocarbonate (IV) and Barium trioxosulphate (IV).

Methods of preparing tetraoxosulphate (VI)Preparation

Methods of preparing ionic tetraoxosulphate (VI) include:i. dissolving a metal in tetraoxosulphate (VI)acidii. reacting tetraoxosulphate (VI) acid with a metal hydroxide or oxideiii. oxidizing metal sulphides or sulphites

Usesiv. Sulphates are important in both the chemical industry and biological

systems:v. The lead-acid battery typically uses tetraoxosulphate (VI) acid.vi. Some anaerobic microorganisms, such as those living near deep sea

thermal vents use sulphates as an energy source for chemosynthesis.vii. Copper tetraoxosulphate (VI) is a common algaecide.viii. Magnesium tetraoxosulphate (VI), commonly known as Epsom salts, is

used in therapeutic baths.ix. Gypsum, the natural mineral form of hydrated calcium tetraoxosulphate

(VI), is used to produce plaster.x. The tetraoxosulphate (VI) ion is used as counter ion for some cationic

drugs.