Methods of Preparation of Sulphur Dioxide

Gases play an important role in various chemical interactions that happen in nature. The earth’s atmosphere is a mixture of various gases that play crucial roles to ensure life on earth exists. 

One of the gases that are present in the atmosphere is sulphur dioxide. Sulphur dioxide is used in various laboratory experiments and is the starting point and facilitator to many chemical reactions. 

The preparation of sulphur dioxide is critical as it can help synthesise other chemicals and materials of importance. Sulphur dioxide is also an essential element in industries like paper pulping, food, metal, textile etc.  

What is Sulphur Dioxide?

Chemically, sulphur dioxide is one of the two oxides that sulphur forms with oxygen, the other being sulphur trioxide. 

Sulphur dioxide is a colourless gas that has a very pungent smell that can induce feelings of choking and suffocation. The direct inhalation of sulphur dioxide is hazardous, and exposure to it could corrode the mucus membrane and damage the eyes.

Natural sources of sulphur dioxide include volcanic eruptions. But most of the sulphur dioxide in the atmosphere is generated by human activities like the extraction of copper and burning of fossil fuels. 

Even with a hazardous nature, sulphur dioxide has wide applications in paper pulping, food, and metal industries. Sulphur dioxide is often used as a preservative and antioxidant in the food industry.

The structure of sulphur dioxide contains a central sulphur atom and two oxygen atoms, each on one side of the sulphur atom. Sulphur forms three bonds, two with one oxygen atom and one with the other oxygen atom. It means that the central sulphur atom has a positive charge, whereas one of the two oxygen has a negative charge.  

The sulphur molecule is bent in a V-shape, and the bond angle between the two sulphur oxygen bonds is 119 degrees. The bond order of the sulphur dioxide molecule is 1.5. Sulphur dioxide forms two resonance structures, in which the formal negative charge alternates between the two oxygen atoms.

Preparation of Sulphur Dioxide

The preparation of sulphur dioxide is often by burning pure sulphur in the presence of oxygen to form oxides. 

But finding pure sulphur in nature is very rare. Therefore, we use other economical methods such as roasting sulphide ores and using copper turnings to produce sulphur dioxide. 

Let us go into detail about each of these methods of preparation of sulphur dioxide.

Burning sulphur in the air

Burning any substance in the air in the presence of oxygen leads to the formation of its oxides. This process is combustion. Similarly, burning sulphur in the presence of oxygen results in the formation of sulphur dioxide. 

This reaction is why burning fossil fuels in the engines of vehicles leads to the formation of sulphur dioxide.

We can give the reaction as

S + O2 + Heat → SO2

Roasting of metal sulphides

Due to the reactivity of sulphur, many ores of metals form in their sulphide forms. That means we have to purify the ores to extract the sulphur in them. We can do the purification by roasting the metal sulphides. 

Roasting involves heating the ore in the presence of an excess of air. It causes the sulphur present in them to form oxides and get released in the gas form.  

The reaction for roasting zinc sulphide (a sulphide ore of zinc) is as follows:

2ZnS + 3O2 → 2ZnO + 3SO2 

Laboratory method

In the laboratory, sulphur dioxide is prepared with the help of copper turnings and concentrated sulphuric acid or H2SO4. 

The process involves placing the copper turnings in a thistle flask and adding a delivery tube to the funnel filled with concentrated H2SO4 inside the flask. Then we place a burner below the flask and add another tube leading to an empty flask to the thistle flask containing the copper turnings.

The copper turnings are heated while adding the concentrated sulphuric acid to them. A reaction occurs that releases sulphur dioxide as a byproduct. The sulphur dioxide gets collected by passing it through the empty tube via displacement of air upwards. 

If we must dehydrate the sulphur dioxide, we should pass it through concentrated sulphuric acid, a dehydrating agent.

The reaction is as below.

Cu + 2H2SO4 → CuO + 2H2O + SO2

Chemical properties of Sulphur Dioxide

Sulphur dioxide has a weak acidic nature and is used to create a weak acid called sulphurous acid. Sulphur dioxide also acts as a reducing agent in many reactions and gets oxidised to give sulphuric acid. In the presence of H2S, HI and other powerful reducing agents, SO2 acts as an oxidising agent and itself gets reduced to sulphur or forms sulphides. We also use SO2 as a bleaching agent; the bleaching action of sulphur dioxide is temporary and reversible.

Conclusion

Sulphur dioxide is a colourless, hazardous gas with a very pungent smell that can induce feelings of choking and suffocation. Direct exposure to this gas can cause damage to the eyes and internal organs. 

Sulphur Dioxide is a crucial chemical agent, as it initiates and facilitates many chemical reactions. It has several industrial uses. The preparation of sulphur dioxide is of great importance as it can help synthesise other chemicals and materials of importance.  

There are three methods for the preparation of sulphur dioxide. These techniques include burning sulphur in the presence of air, roasting metal sulphides, and heating copper turnings with concentrated sulphuric acid.