Oxyacid of Sulfur | Manufacturing of H2SO4


Oxyacid of Sulfur

Sulfur forms a huge number of oxyacid. These acids are classified into five groups:

Sulphoxylic acid:

The chemical formula of sulphoxylic acid is H2SO2, which is not known in the free state.

Sulfurous acids:

Examples of sulfurous acids are:

  • Sulfurous acid, H2SO3 is not known in the free state.
  • Hyposulphurous acid or hydrosulphurous acid or dithionous acid, H2S2O4 is not known in a free state.
  • Thiosulphurous acid, H2S2O is not known in a free state.
  • Disulphurous acid or pyrosulphurous acid, H2S2O5 is not known in a free state.

Sulphuric acids:

Examples of sulphuric acids are:

  • Sulphuric acid, H2SO4
  • Thiosulphuric acid, H2S2O3
  • Pyrosulphuric acid or disulphuric acid, H2S2O7

Thionic acids:

Examples of thionic acids are:

  • Dithionic acid, H2S2O6
  • Polythionic acids, H2SnO6 (where n=3, 4, 5, 6)

Peroxy (peroxo or per) sulphuric acids:

Examples of peroxysulphuric acids are:

  • Peroxomonosulphuric acid or permono sulphuric acid, H2SO5. It is also called Caro’s acid.
  • Peroxodisulphuric acid or perdisulphuric acid, H8S2O8. It is also called Marshall’s acid.

Here, we shall discuss only sulfurous acid and sulfuric acid.

1. The Oxyacid of Sulfur, Sulphurous Acid, H2SO3

Sulfurous acid is known only in solution. It is formed by dissolving SO2 in H2O.

SO2 + H2O → H2SO3

Sulfurous acid acts as a strong oxidizing agent toward strong reducing agents. It is dibasic acid and ionizes as:

H2SO3 → 2H+ + SO32-

Two structures have been suggested for the acid. One of these is unsymmetrical and has a pyramidal shape while the other is symmetrical and has a tetrahedral shape.


The preparation of sulfurous acid by the action of water on thionyl chloride suggests the symmetrical formula.

Sulfurous acid

H2SO3 readily takes up one oxygen atom to give sulphuric acid, H2SO4.

Sulphurous acid to sulphuric acid

H2SO3 readily takes up one sulfur atom to give thiosulphuric acid, H2S2O3.

Sulphurous acid to thiosulphuric acid

The lone pair of electrons on the sulfur atom in H2SO3 can easily be shared by another oxygen or sulfur atom to produce sulphuric acid or thiosulphuric acid.

The unsymmetrical structure shown above explains the reducing property of H2SO3 due to the S-H bond. Now it is believed that the two forms of the acid exist in equilibrium with each other.

2. The Oxyacid of Sulfur, Sulfuric Acid, H2SO4

Sulfuric acid is an oxyacid of sulfur. It is a colorless, oily, dense, corrosive liquid. It is produced by the reaction of sulfur trioxide with water. It is used in accumulators and in the manufacturing of dyes, fertilizers, and explosives.

SO3 + H2O → H2SO4

Discovery of Sulfuric Acid

In the 9th century, Islamic physician and chemist Ibn Zakariya-al-Razi discovered sulfuric acid by dry distillation of alcohol (ethanol) vitriol (al-zajat).

In the 17th century, by burning sulfur with saltpeter (Potassium Nitrate-KNO3), German-Dutch chemist Johann Glauber discovered sulfuric acid.

In the 18th century, Joseph Gay-Lussac, and John Glover discovered sulfuric acid with the help of the lead chamber process.

In the 19th century, Peregrine Phillips discovered sulfuric acid by contact process.

Structure of Sulfuric Acid

  1. Geometry: Tetrahedral
  2. Bond angle: 109.39 or 109.5 degree
Sulfuric acid, oxyacid of sulfur

Different Names of Sulfuric Acid

  1. King of compounds
  2. Battery acid
  3. Mattling acid
  4. Dipping acid
  5. Oil of vitriol
  6. Electrolyte acid
  7. Dihydrogen sulfate

Physical Properties of Sulfuric Acid

  • H2SO4 is soluble in water.
  • It is an oily liquid.
  • It is highly corrosive.
  • H2SO4 is non-volatile acid.
  • It is a dense, viscous colorless liquid.
  • It is a diprotic acid. Diprotic mean sulfuric acid has a tendency to lose two protons.
  • Molar mass: 98 g/mol
  • Appearance: Clear, colorless, corrosive, odorless
  • Boiling point: 337℃
  • Melting point: 10℃
  • Density: 1.84 g/cm3 at 20℃
  • Viscosity: 26.7cp at 20℃
  • Special form: oleum (when a high concentration of SO3 is added)
  • Solubility in water: fully miscible (exothermic process)
  • As a dehydrating agent: Sulfuric acid acts as a catalyst in the following reactions:

C12H22O11 + H2SO4 → C12 + 11H2O

Sulfuric acid acts as a dehydrating agent in laboratories to dry gas mixtures that are being analyzed or prepared.

Chemical Properties of Sulfuric acid

1. Reaction with Water

The hydration energy of sulfuric acid is highly exothermic. The dilution should always be performed by adding the acid to water (not water to acid). It is because, in equilibrium, the reaction favors the rapid protonation of water. The addition of acid to water confirms that the acid is a limiting reagent. This reaction is good for the formation of hydronium ions:

H2SO4 + H2O → H3O+ + HSO4

Because the hydration of sulphuric acid is thermodynamically favorable and its contact with water is enough strong. As we discussed above, Sulphuric acid is an excellent dehydrating agent. Conc. H2SO4 is a very powerful dehydrating property, removing water from other compounds including carbohydrates, sugars, and producing carbon, stream, and a more dilute acid containing an increased amount of hydronium and bisulfate ions.

C12H22O11 + H2SO4 → C12 + 11H2O + H2SO4 (unreacted remaining H2SO4)

2. Acid-base Properties

Sulfuric acid reacts with most bases to give the corresponding sulfate.

CuO + H2SO4 → CuSO4 + H2O

Sulphuric acid can also be used to displace weaker acids from their salts.


3. Reactions with Metals and Strong Oxidizing Property

Dilute sulphuric acid reacts with metal via a single displacement reaction, producing hydrogen gas and salts.

3Fe(s) + H2SO4(aq) → H2(g) + FeSO4(aq)

In the reactant. the oxidation state of iron is 0 but in the product, the oxidation state of iron is +2. So, sulfuric acid is the oxidizing agent.

4. Reaction with Sodium Chloride

H2SO4 reacts with NaCl and gives sodium bisulfate and HCl gas.

NaCl + H2SO4 → NaHSO4 + HCl

5. Electrophilic Aromatic Substitution

Benzene undergoes electrophilic aromatic substitution with sulfuric acid to give the corresponding sulfonic acid.

Electrophilic Aromatic Substitution

Process for Manufacturing of Sulfuric acid

Sulfuric acid is manufactured by the following three industrial processes:

  1. Contact process
  2. Lead chamber process
  3. Wet sulfuric acid process (WSA)

1. Contact Process

Raw Material

  • Sulfur (source sulfide ore)
  • Water
  • Air

Sources of SO2

  • Sulfur burning
  • Metal sulfate roasting
  • Pyrites roasting
  • Combustion of H2S or other sulfur-containing gas
  • Metal sulfide roasting and smelting

Steps Involve in the Formation of H2SO4

Step 1: Burning of Sulfur

S + O2 → SO2

It is an exothermic process.

Burning of sulfur


  • Firstly, 93% of sulfuric acid is added to the air drying tower. The main purpose of adding the sulfuric acid
  • Then, sulfur is sprayed into the burner from the storage
  • The reaction temperature is 2000F.
  • The exothermic reaction must be cooled
  • In the end, the stream is recovered.

Treatment of Burner gas

During the burning of sulfur, other gases are also produced.

  • Sulfur dioxide, burner gas contains impurities like N2, As, F, Cl2, CO2, and dust.
  • It also contains moisture which can cause corrosion to equipment.
  • The burner gas is passed through a dust filter chamber, washing Tower, and then drying tower to remove all these impurities.
Treatment of Burner gas

Step 2: Catalytic Oxidation of Sulfur Dioxide

Sulfur dioxide is converted to sulfur trioxide with the help of V2O5 as a catalyst in a converter or contact chamber.

2SO2 + O2 → 2SO2 △H = -197 KJ/mol

SO2 is mixed with air and passed through trays containing loosely packed porous pellets of catalysts.

Catalytic Oxidation of Sulfur Dioxide

Why Heat Exchanger is Used in Contact Chamber?

1) To Maintain Heat:

The conversion of sulfur dioxide into sulfur trioxide is an exothermic process. It releases -197 KJ/mol of energy. To remove this extra heat, heat exchangers or coolers are used on the outlets of the reaction bed. The temperature and the pressure in the converter are maintained between 673K to 773K (400℃ to 500℃) and close to 1 atm respectively.

2) Equilibrium Yield

By using Le-Chatelier’s principle, the concentration of SO3 will be increased.

  1. By Decreasing Temperature:

According to Le-Chatelier’s principle, if the reaction is exothermic, so by decreasing the temperature reaction is moved in the forward direction.

2SO2 + O → 2SO3 △H = -197 KJ/mol

It is an exothermic process. So, if we decrease the temperature reaction will move in a forward direction.

2. By Increasing Pressure:

According to Le-Chatelier’s principle, by increasing pressure, the reaction is moved in that direction where a lesser number of moles are.

2SO2 + O → 2SO3 △H = -197 KJ/mol

As we see in the above particular reaction, 3 and 2 moles are present in the reactant and in the product respectively. So, if we increase the pressure, the reaction moves forward.

3. Effect of Concentration:

If we increase the concentration of SO2 and O2, the reaction is moved in the forward direction.


  • By decreasing temperature
  • By increasing pressure
  • By increasing the concentration of SO2 and O2, the reaction is moved forward and more and more concentration of SO3 is produced.

Step 3: Absorption of SO3

The direct reaction of sulfur trioxide with water is highly exothermic so as a result, steam is produced. H2SO4 as gas is very difficult to collect.

SO3 + H2O → H2SO4 △H = -103 KJ/mol

Due to this sulfur trioxide is absorbed in sulfuric acid to produce oleum. The chemical formula of the oleum is H2S2O7. Oleum is a very concentrated form of sulfuric acid.

SO3 + H2SO4 → H2S2O7

Step 4: Dilution Tower

Oleum is react with water to form sulfuric acid.

H2S2O7 + H2O → 2H2SO4

Dilution tower

Advantages & Disadvantages

1. Contact Process


  • A large amount of sulfuric acid is manufactured by the contact process.
  • A high concentration of sulfuric acid is obtained as compared WSA and lead chamber process.
  • It is the widely used process over the world.
Manufacturing of sulfuric acid by contact process
Manufacturing of sulfuric acid by contact process


  • The main problem is that the catalyst vanadium pentaoxide can be poisoned.

2. Wet Sulfuric Acid Process


  • By using this process, there are no or little waste by-products.
  • WSA process is the economic way to get rid of sulfurous waste gases.


  • A low concentration of sulfuric acid is obtained.

3. Lead Contact Process


  • The original acid to be used can be obtained at any concentration.


  • 62% and 68% sulfuric acid is in the chamber.
  • This process is not widely used as a contact process. This is because this process produces more dilute acid than the contact process. The contact process always produces much more sulfuric acid than the lead chamber process.

Uses of Sulfuric Acid

Sulfuric acid is one of the most important industrial chemicals used in our daily life. A huge amount of sulfuric acid is used in fertilizers. About 75% sulfuric acid is used in fertilizer, 5% in petroleum refining, 5% in metal production, and 15% sulfuric acid is used in the manufacturing of other chemicals.

Sulfuric acid is also used in daily routines such as:

  1. Chemical manufacturing
  2. Metal processing
  3. oil refining
  4. The manufacturing of rayon
  5. Potato harvesting
  6. The manufacturing of medicines
  7. The manufacturing of lead-acid type batteries

The formula of Oxyacid of Halogens

Oxidation State of HalogenOxyacid of ChlorineOxyacid of BromineOxyacid of IodineGeneral names of Oxyacid
+1HClOHBrOHIOHypohalous acid
+3HClO2—–—–Halous acid
+5HClO3HBrO3HIO3Halic acid
+7HClO4—–HIO4, H5IO6Perhalic acid

Related Posts

Sulfur | Properties, Allotropes, Compounds & Uses

Hydrosulfuric Acid | Basic information

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