O Level Notes : Chemistry - Sulphuric Acid

Hello students. In this topic, we learn about how Sulphuric acid is manufactured by a process called contact process. But before we begin, let’s learn a little about the oxides of sulphur: sulphur dioxide and sulphur trioxide.

Both oxides of sulphur are acidic in aqueous solution: sulphur dioxide dissolves to form sulphurous acid and sulphur trioxide dissolves to form Sulphuric acid.

Sulphur dioxide is much more common than the higher oxide and can easily be obtained by the combustion of sulphur in air. It has several uses. Its main use is in the manufacture of Sulphuric acid. Other than that it is used as a bleach, especially in bleaching wood pulp for making paper, and as a food preservative.

Sulphur trioxide is the higher oxide of sulphur and is more correctly called sulphur (VI) oxide. It is much more difficult to prepare as it involves the catalytic oxidation of sulphur dioxide. Industrially, it has great significance, and is used in the manufacture of Sulphuric acid.


The Contact Process:

The sulphuric acid has great industrial importance, therefore it has to be prepared on large-scale. It is manufactured industrially by the contact process. Let us now learn this process.

In the first step, we burn sulphur in air (oxygen). (Sulphur and air (oxygen) are our raw materials). This way, sulphur dioxide SO2 is formed.

In step two, we purify the sulphur dioxide, by removing impurities like arsenic compounds which would otherwise poison the catalyst. It is then passed through an electrostatic dust precipitator, which charges dust particles that are then removed by being attracted to oppositely charged plates.

The third step is then performed. In this step, sulphur dioxide and air and washed and passed over the catalyst I mentioned earlier: vanadium (V) oxide. This is done at a temperature of 450C and at pressures 2-3 atm. This reaction is reversible and exothermic. The sulphur dioxide and oxygen in air react to produce sulphur trioxide.

The next step is to dissolve the sulphur trioxide produced in concentrated sulphuric acid, to form oleum, or fuming sulphuric acid.

In step five, we dilute this oleum with water to the required strength of acid.

Although we can achieve sulphuric acid directly by dissolving sulphur trioxide in water, we cannot practically dissolve these two together, as the resulting reaction is too violent.


Uses of Sulphuric Acid:

There are many uses of sulphuric acid industrially. Some of them are as follows:

  • Sulphuric acid is used in the production of fertilizers, e.g., ammonium sulphate, potassium sulphate, etc.
  • It is used in the manufacture of non-soapy, organic detergents.
  • It is used as an electrolyte inside car batteries.
  • It is used in the cleaning of metals by removing the surface oxide coating. This is called pickling.
  • It is used in the making of artificial silks.


These are enough for you to learn at this stage. Thank you students.