SULPHUR & ITS COMPOUNDS
Formation of atmospheric SO₂
-natural source: -volvanic eruptions [natural S burns in air > SO₂; S(s) + O₂(g) > SO₂(g)], -rotting of planktons & vegetation
-human activities (50% of atmospheric SO₂): -combustion of sulphur-containing fossil fuels, -industrial emission [SO₂ formed from roasting metal sulphides to obtain metals: 2ZnS(s) + 3O₂(g) > 2ZnO(s) + 2SO₂(g)]

fossil fuel	sulphur content (% by mass)
coal	bet 0.5% & 6%
gasoline	0.04%
diesel	0.22%
fuel oil bet	0.75% & 3%

combust fossil fuels in car-engines, furnaces & generators > SO₂, CO₂, CO & NO_x
sulphides: iron pyrites- FeS₂ (in Fe ore), zinc blende- ZnS, galena- PbS, copper pyrites- CuFeS₂, on roasting > SO₂
Acid rain
CO₂, oxides of N & S occur naturally in unpolluted air > rain w/ pH 5.0-5.6
acid rain- rain w/ pH < 5
acidic air pollutants (SO₂ from burning fossil fuels, NO_x from vehicle exhausts) dissolve in rain water > pH as low as pH 4 (10x more acidic than unpolluted value)
Formation of acid rain
acid rain formed when SO₂, SO₃, & NO₂ dissolves in rainwater > mixture of acids
SO₂(g) + H₂O(l) > H₂SO₃(aq)
SO₃(g) + H₂O(l) > H₂SO₄(aq)
2NO₂(g) + H₂O(l) > HNO₂(aq) + HNO₃(aq)
SO₂ in atmosphere oxidised to SO₃:
-by nitric oxide (NO catalyses oxidation):
        2NO(g) + O₂(g) > 2NO₂(g)
        SO₂(g) + NO₂(g) > SO₃(g) + NO(g)
-by ozone in air: SO₂(g) + O₃(g) > SO₃(g) + O₂(g)
-by metal ions present in atmosphere: 2SO₂(g) + O₂(g) >(metal ions)> 2SO₃(g)

Effects of acid rain
-aquatic insects affected by increase acidity of water > die > no food for fish > fish pop decrease
-more acidic water > more Al cpds & heavy metal cpds in soil dissolve > metal ions (Al³⁺, Zn²⁺, Cd²⁺) thicken protective mucous layer of fish gills > clog gills > fish suffocate
-heavy metal ions (dissolve in acidic rain) absorbed by trees > weaken tree's resistance to disease & extreme weather
-acidic rain can damage trees > leaves shed > growth restricted
-buildings, structures made of basic materials (limestone, marble) & metallic structures > corroded by acidic rain: CO₃^2-(s) + 2H⁺(aq) > CO₂(g) + H₂O(l), O^2-(s) + 2H⁺(aq) > H₂O(l), M(s) + 2H⁺(aq) > M²⁺(aq) + H₂O(l) (M = metal ions- Fe)

Reducing sulphur emission
by -removing S before combustion (prevent SO₂ formation) -by removing SO₂ (after S combustion)
-Coal gasification: S removed from coal before combustion > coal combusted w/o SO₂ emission
-Pulverised fluidised bed combustion, PFBC: S removed from coal during burning of coal
(when gas forced through suitable powdered solid > solid is 'fluidised' > surface area of soil in contact w/ gas greatly increases), burn coal in a bed of fluidised limestone > 80% SO₂ emission reduced
-Flue gas desulphurisation, FGD: remove SO₂ in waste gases by treating w/ powdered limestone > calcium sulphite produced & oxidised > calcium sulphate (used in palter-board & cement manufacture):
SO₂(g) + CaCO₃(s) > CaSO₃(s) + CO₂(g), 2CaSO₃(s) + O₂(g) > CaSO₄(s)

Uses of SO₂
-mild bleaching agent: bleach wool, straw & wood pulp
-food preservative > prevent growth of fungi & bact (preserves fruit jams & juice) (SO₂ can cause bronchoconstriction - narrowing of windpipe, esp among asthmatics)
-manufacture of sulphuric acid by Contact Process
-reducing agent to remove Cl from fabrics after bleaching

Contact Process
(1)Formation of SO₂
-burn S in air: S(s) +O₂(g) > SO₂(g)
-burn hydrogen sulphide (from crude oil) in air: 2H₂S(g) + 3O₂(g) > 2H₂O(l) + 2SO₂(g)
-roasting sulphide ores in air: 2ZnS(s) + 3O₂(g) > 2ZnO(s) + 2SO₂(g)
-from flue gas desulphurisation process
SO₂ then purified by electrostatic precipitate to remove impurities & dust
(2)Catalytic oxidation of SO₂ to SO₃
SO₂ passed through catalytic converter at 450°C, 2 atm, w/ V₂O₅(vanadium(V) oxide / vanadium pentoxide)
2SO₂(g) + O₂(g) 2SO₃(g) [95% conversion under these conditions]
(3)Formation of sulphuric acid
SO₃ absorbed in 98% conc H₂SO₄(l) > oleum, H₂S₂O₇: SO₃(g) + H₂SO₄(l) > H₂S₂O₇(l)
diff conc of sulphuric acid obtained by diluting oleum: H₂S₂O₇(l) + H₂O(l) > 2H₂SO₄(l)
[SO₃(g) + H₂O(l) > H₂SO₄(l): exothermic > mist of H₂SO₄ vapour form > dangerous & damages buildings]

Uses of H₂SO₄(an industry acid used extensively)
-manufacture fertilisers: large quantity used manufacture of super phosphate fertilisers (mixture of calcium hydrogen phosphate & calcium sulpahte) & ammonium sulphate
-manufacture of artificial fibres: converts viscose to cellulose in making silk
-manufacture of detergents: sulphonates long chained hydrocarbons w/ benzene ring > detergents
-cleaning metal surface ('prickling' of metals): removes metallic oxide before electroplating / galvanising
-electrolyte in car battery (rechargeable car battery / lead accumulator)
-manufacture of explosives, pigments & dyestuffs

Structure and bonding of sulphur cpds
sulphur dioxide, SO₂; v-shaped covalent molecule

sulphur trioxide, SO₃; trigonal planar covalent molecule

sulphate ion,SO₄^2-; tetrahedral ion w/ delocalised electrons

	sulphite, SO32-	sulphate, SO42-
+ dil HNO3	choking gas smell of burning sulphur; turns Cr2O7-/H+ green > gas is SO2 SO32-(aq) + 2H+(aq) > SO2(g) + H2O(l)	no reaction
+ Ba(NO3)2(aq), + dil HNO3	white ppt, soluble in acid SO32-(aq) + Ba2+(aq) > BaSO3(s) (white ppt)	white ppt, insoluble in acid SO42-(aq) + Ba2+(aq) > BaSO4(s) (white ppt)

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