Oleum, also known as fuming sulfuric acid, is a corrosive and highly concentrated solution of sulfur trioxide and sulfuric acid in water. It is a powerful oxidizing agent and is used in a variety of industrial applications.

It is used in the production of certain chemicals, such as dyes and pharmaceuticals, and can be used as a catalyst in the manufacture of plastics, rubber, and other products. In addition, oleum is used in the manufacture of sulfonates, which are important detergents and surfactants.

structure of H2S2O7

Oleum is produced by the oxidation of sulfur dioxide in the presence of oxygen and water. The sulfuric acid in the oleum is highly corrosive and can cause severe burns and irritation to the skin and eyes. The fumes produced by oleum are toxic and can cause respiratory irritation and damage. Therefore, proper protective measures should be taken when working with oleum.

Properties of Oleum

Oleum, also known as fuming sulfuric acid, is a solution of sulfur trioxide (SO3) in sulfuric acid (H2SO4). It is a dense, viscous liquid that is colorless to yellowish-brown in appearance.

It has a strong, pungent odor and is highly corrosive to metals and tissues. Oleum is a powerful oxidizing agent and reacts exothermically with organic materials and reducing agents. It is also a strong dehydrating agent and can remove water from compounds.
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Properties Table of Oleum

Properties of Oleum

There is no sharp boiling point of the oleum. This is because when it is heated above 200°C, it starts fuming so extensively that may cause self-ignition. However, in a controlled environment, oleum decomposes into H2SO4 and SO3 and exhibits a boiling point similar to that of 100% sulfuric acid, which is 290°C.

Production of Oleum by Contact Process

The contact process is a method for the industrial production of sulfuric acid (H2SO4) and it is also used for the production of oleum (H2S2O7). The process involves the oxidation of sulfur dioxide (SO2) to sulfur trioxide (SO3) using a platinum catalyst.

The basic steps in the contact process are:

  • The SO2 is first scrubbed with water to remove impurities and then heated to a high temperature in the presence of a platinum catalyst.
  • The SO2 reacts with oxygen (O2) to form SO3.

SO+ O→ SO3

  • The SO3 is then cooled and dissolved in concentrated sulfuric acid (H2SO4) to form H2S2O7.

SO3 + H2SO4 → H2S2O7

  • The H2S2O7 is then distilled to separate it from any remaining sulfuric acid, and then stored or transported in a cool, dry place.

The process is highly corrosive and requires careful handling to prevent damage to equipment.

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Applications of Oleum 

Per-oxy-disulfuric acid, also known as per-sulfuric acid or disulfuric acid, is a strong oxidizing agent that is used in a variety of industrial and laboratory applications. Some of the common uses of oleum include:
  1. Oxidation reactions: It is used as an oxidizing agent in synthesizing organic compounds and in the bleaching of pulp and paper.
  2. Analytical chemistry: It is used to determine sulfur and sulfur compounds in various samples.
  3. Bleaching: is used as a bleaching agent in the paper, textile, and pulp industries. It is also used to bleach hair and teeth.
  4. Disinfection: It is used as a disinfectant in water treatment and as a sanitizer in the food industry.
  5. Water treatment: It is used in the treatment of wastewater and the purification of drinking water.
  6. Environmental: It is used to treat industrial wastewater and to remove sulfur from natural gas.

Handling of Oleum

It is highly reactive, corrosive, and toxic and should be handled with extreme caution. If ingested, can cause severe burns and can be fatal. Proper handling and storage of the oleum are essential to ensure safety.

Protective clothing, including eyewear and gloves, should be worn when handling the chemical. It should be stored in a cool, dry place away from other chemicals, and should be kept in a closed container with a secure lid or seal. It should be disposed of according to local regulations.

First Aid – In Case of Contact

Oleum is a strong oxidizing agent and can cause severe burns and tissue damage if it comes into contact with skin or eyes, or if it is ingested.

If someone comes into contact with the oleum, they should immediately flush the affected area with water for at least 15-20 minutes. If the person has ingested oleum, they should drink water or milk to dilute it.

In addition, medical attention should be sought immediately and the affected person should be taken to a hospital as soon as possible.

Concepts Berg

What is the formula of oleum? 

H2S2Oor H2SO4.SO3

What is the most powerful acid on earth?

The most powerful acid on earth is floro-antimonic acid, which is a mixture of hydrofluoric acid and antimony pentafluoride that has superacid properties.

Are oleum and sulfuric acid the same?

No, oleum and sulfuric acid are not the same. Oleum (fuming sulfuric acid) is a mixture of sulfuric acid with up to 80% sulfur trioxide.

Is oleum a strong acid? 

Yes, oleum is a strong acid. It tends to react strongly with bases and can erode metals, making it a highly corrosive substance.

What is the percentage labeling of oleum?

The total mass of H2SO4 obtained by diluting 100 g of sample of oleum with the desired amount of water, is equal to the percentage labeling of oleum.
% labeling of oleum = Total mass of H2SO4 present in oleum after dilution.

What is 100% oleum? 

Sulfan is 100% oleum. Sulfan and Oleum are interchangeable terms that refer to a mixture of highly concentrated sulfuric acid combined with an excess of sulfur trioxide.

Which one is stronger H2S2Oor H2SO4 and why?

Oleum, represented by the chemical formula H2S2O7 is stronger than sulfuric acid (H2SO4) because the conjugate base of oleum is more stable than that of sulfuric acid.
H2S2O7 + H2SO4 → HS2O7+ H3SO4+

What is the oxidation state of sulfur in oleum? 

The oxidation state of H2S2O7 =?
2(+1)+2S+7(-2) = 0
2S = 14-2
2S = 12
S = +6

Why is sulfuric acid produced by adding water to oleum rather than directly adding in sulfur trioxide? 

Dissolving sulfur trioxide directly in water is highly exothermic, leading to the formation of acidic fumes. To obtain concentrated sulfuric acid, oleum is reacted with water instead.