Conversion of Sulphur Dioxide

Sulphur trioxide is a chemical compound having formula SO3. It is also popularly called nisso sulfan.

Sulphur trioxide exists in many forms. It is present as a gaseous monomer, crystalline trimer and solid polymer. At temperatures just below room temperature, it exists as a solid.

Properties of Sulphur Trioxide

Chemical Reactions of Sulphur Trioxide

• Hydration

Sulphur trioxide (SO3) is the anhydride of sulphuric acid (H2SO4). Therefore, it is very vulnerable to hydration:

SO3 + H2O → H2SO4     (ΔHf = −200 kJ/mol)

In a gaseous state, sulphur trioxide fumes vigorously even in a relatively dry atmosphere. This is due to the formation of a sulphuric acid mist.  

SO3 is an aggressively hygroscopic compound. The heat generated during hydration is enough to ignite wood or cotton when mixed with SO3. In this instance, the SO3 dehydrates these carbohydrates themselves.

• Reaction with a base

Sodium hydrogen sulphate is formed by the reaction between sulphur trioxide with sodium hydroxide (a base). 

SO3 + NaOH → NaHSO4

Conversion of Sulphur Dioxide to Sulphur Trioxide

Sulphur trioxide is produced when SO2 undergoes oxidation. The conversion of sulphur dioxide to sulphur trioxide occurs naturally in the atmosphere, and it can be undertaken in the laboratory for experimentation and commercially for various industrial uses.

In the Atmosphere as Acid Rain

Sulphur dioxide is released in the atmosphere when fuels are burnt. It undergoes oxidation in the presence of oxygen in the atmosphere to form SO3. Sulphur trioxide, when exposed to moisture, forms dilute sulfuric acid, also known as ‘acid rain’. This reaction occurs at a very slow pace in nature.

SO2 + ​12O2 = SO3 (ΔH = -198.4 kJ/mol)

In Laboratory for Experimentation

Sulphur trioxide is prepared in the laboratories through the 2-stage pyrolysis of the compound sodium bisulfate. 

Step 1:

When we initiate dehydration of sodium bisulphate at 315 °C, the following chemical reaction occurs:

2 NaHSO4 → Na2S2O7 + H2O

Sodium pyrosulfate (Na2S2O7) is produced as an intermediate product.

Step 2:

Sodium pyrosulfate is made to undergo cracking at temperature of 460 °C, and the following reaction occurs:

Na2S2O7 → Na2SO4 + SO3

Industrial Preparation of Sulphur Trioxide By Contact Process

Sulphuric acid is commercially manufactured by a process known as contact process. The conversion of sulphur dioxide to sulphur trioxide forms a critical step in this process. It involves the following 3 steps:

Step 1: Make sulphur dioxide

This is done by burning sulphur in abundance of air,

S (Sulphur) + O2 (Oxygen) + Δ (Heating)  →  SO2 (Sulphur dioxide)

Step 2: Conversion of sulphur dioxide to sulphur trioxide

This step is the most critical in the entire process. In this step, the conversion of sulphur dioxide to sulphur trioxide is done by mixing SO2 with atmospheric oxygen to oxidise it to form SO3 in the presence of V2O5 as a catalyst. 

2SO2 (sulphur dioxide) + O2 (oxygen) + [V2O5 (catalyst)] → 2SO3 (sulphur trioxide)

(ΔH = −196 kJ/mol)

This reaction occurs at the following conditions:

1. Sulphur dioxide and oxygen (from air) are mixed in the ratio 1:1 by volume
2. Temperature maintained at 400-450 oC
3. Pressure maintained at 1-2 atmosphere
4. Catalyst V2O5 

Step 3: Conversion of sulphur trioxide to sulfuric acid

In the last step, sulphur trioxide is absorbed in concentrated sulphuric acid to give pyrosulphuric acid or oleum. Oleum is diluted with controlled amounts of water to obtain sulphuric acid of the required concentration.

Conversion of Sulphur Dioxide to Sulphur Trioxide Questions

Study the equation of conversion of sulphur dioxide to sulphur trioxide

2SO2 + O2 + [V2O5 (catalyst)]  →  2SO3 (ΔH = −196 kJ/mol)

When we consider the conditions at which this reaction occurs, it raises a few questions.

• Why is SO2 and oxygen mixed in the ratio 1:1 when clearly the equation shows that only half the volume of oxygen is needed for the reaction to occur?

As per the Le Chatelier’s Principle, when we increase the concentration of oxygen in the mixture, we cause the equilibrium point to shift towards the right. That means that it enhances the production of SO3. Since the oxygen comes from the air, which is relatively freely available, this is an inexpensive way of enhancing the conversion of sulphur dioxide into sulphur trioxide.

• The reaction is exothermic and hence a lower temperature should favour the reaction. Then why do we not drop the temperature to below 400 oC?

Lowering the temperature will indeed help to shift the equilibrium towards the right. However, chemical kinetics tells us that at lower temperatures, the rate of a reaction slows down. Since the contact process is used for the commercial production of sulphuric acid, hence time also becomes a factor for consideration. Thus, 400-450 oC is the optimal temperature at which we get a fairly high production of SO3 in a shorter period.

• Since the number of molecules of SO3 produced are lower than the number of molecules on the left-hand side of the equation, the reaction should be favoured by increasing the pressure. However, the reaction is made to occur at nearly the atmospheric pressure. Why?

Even at the low pressures of 1-2 atmospheres, we get a 99.5% conversion of sulphur dioxide to sulphur trioxide. Therefore, the incremental yield we will get by increasing the pressure will not be able to justify the higher costs that will be required for maintaining the higher pressure.

Conclusion

In this article we learnt about sulphur trioxide and its properties. We understood how the conversion of sulphur dioxide to sulphur trioxide occurs naturally in the atmosphere, in controlled laboratory conditions, and at a commercial scale for industrial applications. If you want to go deeper into the concepts covered above, you can go through the links mentioned in the reference section.