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1. OXYACIDS OF SULPHUR
INORGANIC CHEMISTRY
RABIA AZIZ (BS-III CHEMISTRY)
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2. INTRODUCTION
Oxyacids
of
Sulphur
Sulphur
OxygenHydrogen
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3. TABLE: OXYACIDS OF SULPHUR
S.NO OXYACIDS OF SULPHUR FORMULA
1. Sulphurous Acid H2SO3
2. Sulphuric Acid H2SO4
3. Peroxomonsulphuric Acid H2SO5
4. Peroxodisulphuric Acid H2S2O8
5. Thiosulphuric Acid H2S2O3
6. Dithionous Acid H2S2O4
7. Dithionic Acid H2S2O6
8. Polyphonic Acid H2SnO6 (n > 2)
9. Pyrosulphuric Acid H2S2O7
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4. Series of Oxyacid of Sulphur
• Oxyacids of sulphur are divided in four series according to the
their structural similarities which are following:
• Sulphurous acid series
• Sulphuric acid series
• Thionic acid series
• Peroxo acid series
– Sulphurous Acid Series:Under this series sulphurous acid, di or
pyrosulphurous acid and dithionous acid are grouped.
– Sulphuric Acid Series: Sulphuric acid, thiosulphuric acid and di
or pyrosulphuric acid.
– Thionic Acid Series: Dithionic and polythionic acid.
– Peroxo Acid Series: Peroxo monosulphuric acid and peroxo
disulphuric acid.
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5. 1. SULPHUROUS ACID (H2SO3):
• PREPARATION:
• Sulphurous acid is obtained by dissolving SO2
in water.
• SO2 + H2O → H2SO3
• A saturated solution at 3C gives crystal of
sulphurous acids having generally H2SO3.6H2O
formula.
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6. • Chemical Properties:
• Action of Heating: When heated, various products are formed.
• 3H2SO3 → H2SO4 + H2S2O4 + H2O
• H2S204 →H2SO4 + S
• Reaction with Halogen: Sulphuric acid is formed as a final product.
• H2SO4 + H2O + CL2 → H2SO4 + 2HCL
• H2SO3 + H2O + I2 → H2SO4 + 2HI
• Reducing Properties: It reduces ferric to ferrous and iodates to iodine.
• 2FeCL2 + H2SO3 + H2O → 2FeCL2 + H2SO4 + 2HCL
• 5H2SO3 + 2KIO3 → 2KHSO4 + 3H2SO4 + H20 +I
• Decolourization of KMnO4: It decolourises dilute mineral solution of
KMnO4.
• 2KMnO4 + 5H2SO3 → 2KHSO4 + 2MnSO4 + H2SO4 + 3H2O
• Reaction with Alkali: Sulphites and bisulphites are the important salts of
this acid which are obtained.
• NaOH + H2SO3 → NaHSO3 + H2O
• NaHSO3 + NaOH → Na2SO3 + H2O
• Sulphites are oxidised to sulphates in the presence of bromine water.
• Na2SO3 + Br2 + H2O →Na2SO4 + 2HBr
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7. Structure of Sulphurous Acid
• X-ray analysis of Na2SO3 crystals reveals that
SO3 ion has a pyramidal structure with three
oxygen atoms in a triangular. Lone pair distort
the tetrahedral structure to form pyramidal
structure.
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8. USES of Sulphurous Acid
• Sulphurous acid is a strong reducing agent.
• The solution has bleaching properties.
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9. 2. SULPHURIC ACID H2SO4 (Oil of
vitriol)
• Physical properties:
• Colourless,odourless, viscous oily liquid.
• Hygroscopic and corrosive in nature.
• Melting point = 10 °C, bioling point = 290 °C.
• Dilute sulphuric acid containing 65% H2SO4 and
density is 1.55g/cm3
• Concentrated sulphuric acid containing 98% H2SO4
and density is 1.839 g/cm3
• 100% H2SO4 having density is 1.838 g/cm3
• Fuming sulphuric acid containing dissolved SO3 having
density is 1.929 g/cm3
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10. • Chemical Properties
• Dibasic Acid: It is strongly dibasic acid. It ionises into two ions.
• H2SO4 → H+ + HSO4
-
• HSO4
- →H+ + SO4
-
• When acid reacts with alkali, two types of salts are produced.
• NaOH + H2SO4 → NaHSO4 + H2O
• NaHSO4 + NaOH → Na2SO4 + H2O
• Dehydrating Agent: It has great affinity towards water. Thus it removes water molecule when reacts with organic
compound.
• C12H22O11 + H2SO4 → 12C + 11H2O
• (COOH)2 + H2SO4 → H20 + CO + CO2
• As oxidizing Agent:
• It reacts with non-metals like C,S and oxides them into CO2 and SO2.
• C + 2H2SO4 → CO2 + 2SO2 + 2H2O
• S+ 2H2SO4 → 3SO2 + 2H2O
• Reaction with Metals: It reacts with metals to form salt.
• Cu + 2H2SO4 → CuSO4 + SO2 + 2H2O
• KI and KBr are oxidised to free iodine and free bromine.
• 2KI + 2H2SO4 → K2SO4 + 2H2O + I2 + SO2
• Dehydrating Of Alcohol: When alcohol is heated with conc. H2SO4 at 170°C, water is removed.
• C2H5OH → C2H4 + H2O
• Ethyl alcohol Ethylene
• As Sulphonating Agent: It reacts with organic compounds and replaces one or more hydrogen atoms from it by –
SO3H group the process is called sulphonation.
• CH4 + H2SO4 → CH3-SO3H + H2O
• Methane sulphonic acid
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11. PREPARATION OF SULPHURIC ACID
H2SO4
• Sulphuric acid is an important acid. It is
obtained by two process (i) lead chamber
process (ii) contact process. Sulphur dioxide is
obtained by chemical reactions which is
oxidised to SO3. SO3 is dissolved in water to
give H2SO4.
• SO3 + H2O → H2SO4
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12. LEAD –CHAMBER PROCESS
• In this process SO2 is oxidized by atmospheric
oxygen in the presence of catalyst of oxides of
nitrogen. The SO3 so obtained is absorbed by
water to get sulphuric acid.
• 2NO + O2 → 2NO2
• NO2 + SO2 + H2O → H2SO4 (99%) + NO
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13. LEAD –CHAMBER PROCESS
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• Source of SO2:
• Burning of sulphur: 2S + O2 → 2SO2
• Burning of pyrites: 4FeS2 + 11O2 → 2Fe2O3 +
8SO2
• Source of oxides of Nitrogen: By heating KNO3
with sulphuric acid in the nitre oven.
• KNO3 + H2SO4 → KHSO4 + HNO3
• 2HNO3 + 2SO2 → 2SO2 + H2O + NO + NO2

14. MANUFACTURE PROCESS
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LEAD-CHAMBER PROCESS
Pyrite Burner,
SO2production
Sulphide or Sulphur
is ignited at 600-
800°C.
Dust catcher’s:
Dust is settled down.
Glover Tower:
Lead Chambers :
SO2 + NO2+H2O →
H2SO4+NO
Nitre Pot:
KNO3 + H2SO4 is
heated and N oxides
poduces which use
as a catalyst.
Gay Lussac Tower:
2H2SO4+NO+O2→
2NOHSO4+H2O
Nitrous vitrol
Cooler:
65%acid

15. CONTACT PROCESS
• CONTACT PROCESS:
• By this process 100% pure sulphuric acid can be
obtained. The process involves the catalytic oxidation
of SO2 to SO3 by atmosphere oxygen.
• 2SO2 + O2 ↔ 2SO3 + 23Kcal
• SO3 + H2O → H2SO4
• Sulphuric acid absorbs more SO3 to give oleum. When
calculated amount of water added, sulphuric acid is
obtained.
• SO3 + H2SO4 → H2S2O7 oleum
• H2S2O7 + H2O → 2H2SO4
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16. CONTACT PROCESS
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17. 3. PEROXOMONOSULPHURIC
H2SO5Caro's acid
• Structure:
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18. Preparation of H2SO5Caro's acid
• For laboratory use, small quantities of Caro's acid may be
prepared by slowly adding H2O2 to concentrated sulfuric
acid contained in a inert container e.g., glass, surrounded
by an ice bath. The heat of dilution of the sulfuric acid
causes strong heating, which makes the acid more
unstable, so temperature control is essential.
• The process results in a mixture of Caro's acid, H2SO4, H2O2
and water, the ratios depending on the strength of the
reagents, and their proportion.
• H2O2 + H2SO4 → HOOSO2 + H2O
• From Hydrolysis of Peroxodisulphuric acid (Marshall‘s acid):
• H2S2O8 + H2O → H2SO5+ H2SO4
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19. Properties of H2SO5 Caro's acid
• Physical Properties:
• Anhydrous state, white crystalline solid,
hygroscopic.
• Chemical Properties:
• Action of H2O2 :
• H2SO5+ H2O2 → H2SO4+ H2O+O2
• Hydrolysis:
• H2SO5+ H2O→ H2SO4+ H2O2
• Oxidizing agent: H2SO5 + 2KI → K2SO4+ H2O+ I2
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20. 4. PEROXODISULPHURIC ACID H2S2O8
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• Structure:
• X-ray studies show that peroxodisulphate ion,
the structure –O3S-O-O-SO3
- with
approximately tetrahedral angles about each
sulphur atom. Also as the acid is prepared
from H2O2 and CISO3H reacting in the
molecular ratio 1:2, the structure is
represented as below:

21. Preparation of PEROXODISULPHURIC
ACID
• The acid is prepared by the reaction of
chlorosulfuric acid with hydrogen peroxide:
• 2ClSO3H + H2O2 → H2S2O8 + 2 HCl
• By electrolysis of 50% H2SO4 at low
temperature:
• At cathode:
• H+ + e- → H
• H + H → H2
• At anode:
• 2HSO4
- - 2e- → H2S2O8
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22. Properties of PEROXODISULPHURIC ACID
• It is a colourless crystalline solid having M.P 338K.
• It undergoes hydrolysis giving Caro’s acid.
• H2S2O8 + H2O → H2SO5 + H2SO4
• Many metals such as Zn etc. Dissolve forming their sulphates without evolution of
any gas.
• K2S2O8 + Zn → K2SO4 + ZnSO4
• Its salt potassium peroxosulphate, K2S2O8 on heating evolves sulphur trioxide and
oxygen whereas the solution of the salt gives only oxygen on heating.
• 2K2S2O8 → 2K2SO4 + 2SO3 + O2
• 2K2S2O8 + 2H2O → 2K2SO4 + 2H2SO4 + O2
• Amphoteric metals, Cr, As, etc., form their oxy-acids.
• 3K2S2O6 + Cr + 4H2O → 3K2SO4 + H2CrO4 + 3H2SO4
• Peroxodisulphate ion is one of the most powerful and useful oxidizing agents.
• S2O8
2- + 2e- ↔ 2SO4
2- (E° = 2.01 volt)
• It liberates iodine from potassium iodide, oxidizes Cr3+ to Cr2O7
2-, Fe2+ to Fe3+, Mn2+
to MnO4
- etc.
• K2S2O8 + 2KI → 2K2SO4 + I2
• K2S2O8 + 2FeSO4 → Fe2(SO4)3 + K2SO4
• 5K2S2O8 + 2MnSO4 + 8H2O → 2KMnO4 + 4K2SO4 + 8H2SO4
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23. 5. THIOSULPHURIC ACID H2S2O3
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• Structure:
• Preparation:
• By reaction of Sulphur trioxide with Hydrogen
sulphide in the presence of ether.
• SO3 + H2S → H2S2O3
• ClSO3H + H2S → H2S2O3 + HCl

24. Properties of THIOSULPHURIC ACID
• Free thiosulphuric acid is unstable and
decomposes into sulphur as soon as it is liberated
from its salts solution by adding an acid.
• H2S2O3 → SO2 + S + H2O
• Thus its reactions are manily due to S2O3
2-,
thiosulphate ion.
• The thiosulphate ion, S2O32- behaves as a
reducing agent of moderate strength.
• 2S2O3
2- → S4O6
2- + 2e-
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25. 6. DITHIONOUS ACID H2S2O4
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• Structure:
• Preparation:
• By reducing sulphurous acid containing sulphites with
amalgamated Zn or Zinc dust:
• 2S2O4
2- + H2O → S2O3
2- + 2HSO3
-
• Properties: Solution of dithionous acid and of its salts such
as dithionites are powerful reducing agents. For reducing
purpose. Sodium dithionite is ordinarily employed. Sodium
dithionite is prepared by reducing sodium bisulphite
solution with zinc dust and separated by removing excess
sulphite by precipitation with Ca(OH)2.

26. 7. DITHIONIC ACID H2S2O6
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• Structure:
• PROPERTIES:
– Diththionic acid is moderately a strong and stable
bibasic acid
– Dithionates are stable to oxidizing agents and mild
reducing agents, but on heating in acid solution they
give both H2SO4 and H2SO3.
– H2S2O6 + H2O → H2SO4 + H2SO3

27. PREPARATION OF DITHIONIC ACID
– Dithionic acid and its salts are best prepared by
oxidation of SO2 or sulphite solutions.
– Laboratory preparation involves treatment of aqueous
suspension of Mn(IV) dioxide with SO2 below 283K to
form Mn(II) sulphate and dithionate in solution.
Removal of manganese and sulphate ions is done by
adding Ba(OH); BaS2O6.2H2O is then crysatllized.
• 2MnO2 + 3SO2 → MnS2O6 + MnSO4
• MnO2 + 2SO3
2- +4H+ → Mn2+ + S2O6
2- +2H2O
• MnSO4 + Ba(OH)2 → BaSO4 + Mn(OH)2
• MnS2O6 + Ba(OH)2 → BaS2O6 + Mn(OH)2
• On treating the aqueous solutions of this salt with H2SO4,
free acid is obtained.
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28. 8. POLYTHIONIC ACID H2SnO6
STRUCTURE:
Polythionic acid is an oxoacid which has
a straight chain of sulfur atoms.l
It is observed that polythionates in
crater lakes are drastically decreased
before an eruption occurs. The
phenomenon may be useful to predict
volcanic activity
PREPARATION:
By Hydrolysis of Sulphur monochloride
S2Cl2 + 3H2O → H2SO3 + H2S + 2HCl
3H2SO3 + H2S → H2S4O6 + 3H2O
6H2SO3 + 2H2S → H2S3O6 + H2S5O6 + 6H2O
9H2SO3 + 3H2S → H2S2O6 + 2HS5O6 + 9H2O
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29. PROPERTIES OF POLYTHIONIC ACID
• 1. With a small number of sulfur atoms in the
chain (x = 3 - 6) are the most stable.
• 2. Polythionate ions are significantly more
stable than the corresponding acids.
• 3. Under the action of oxidants polythionic
acids and their salts are oxidized to sulfate,
and the interaction with strong reducing
agents converts them into sulfitesand
dithionites.
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30. 9. PYROSULPHYRIC ACID H2S2O7 (Oleum,
fuming sulphuric acid, disulfuric acid)
STRUCTURE:
Considered as an anhydride of sulfuric acid (H2SO4)
PREPARATION:
1. It is obtained is obtained by dissolving sulphuric trioxide in concentrated
sulphuric acid.
H2SO4 + SO3 → H2S2O7
2. Alkali metal pyrosulphate can be obtained by heating alkali bisulphite.
2NaHSO4 → Na2S2O7 + H2O
Uses:
•In the manufacture of explosives and dyes.
•As a sulfating agent and in petroleum refining.
•It can be diluted in water to readily give sulfuric acid of the
desired concentration.
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31. PROPERTIES OF PYROSULPHYRIC ACID
• It is a viscous brownish liquid, fumes in air and gradually
loses sulphuric trioxide. Thus it changes into ordinary
sulphuric acid on warming.
• H2S2O7 → H2SO4 + SO3
• Pyrosulphuric acid gives sulphuric acid when water is added
into it.
• H2S2O7 + H2O → 2H2SO4
• It is an excellent sulphonating agent used in organic
chemistry.
• It is a much weaker acid than sulfuric acid.
• It readily reacts with bases to form salts called pyrosulfates,
such as sodium and potassium pyrosulfates.
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32. CONCLUSION
• Oxyacids of sulphur are compounds of sulphur with
oxygen and hydrogen. Sulphurous acid does not exist
as free acid; it is diprotic and strong reducing agent.
Sulphuric acid is stable, diprotic and dehydrating agent.
Thiosulphuric acid does not exist as free acid but its
salts, all are stable reducing agent. Solution of
dithionous acid and of its salts such as dithionites is
powerful reducing agents. Peroxomonosulphuric acid is
stable and crystalline solid. Dithionic acid is moderately
stable but its salts are quite stable. Pyrosulphuric acid,
peroxomonosulphuric acid and peroxodisulphuric acid
are strong oxidizing agents.
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