Sulfur Trioxide

Introduction

“Sulfur Trioxide” is also presented by “$\mathrm{SO_{3}}$" which is mainly a chemical compound and presents various kinds of modifications. It is mainly available as molecules or crystals in its chemical form. It has no colour as a liquid and vaporises in air, only under certain conditions. It is mainly known for its strong oxidising powers and highly reactive nature. According to thermodynamics, it is unstable, as it can cause a fire.

What is “Sulfur Trioxide”?

Figure 1 − Structure of Sulfur Trioxide

“Sulfur Trioxide” or “$\mathrm{SO_{3}}$” is mainly manufactured in large amounts owing to its similar nature to “sulfuric acid”. “Sulfur Trioxide” is mainly present in three forms- firstly in its gaseous state as a monomer, secondly in its crystalline state as a trimmer and thirdly in its solid state as a polymer. “Sulfur Trioxide” exists in its solid state at a temperature lower than normal room temperature. In its gaseous form “Sulfur Trioxide” is even responsible for “acid rain”. “Sulfur trioxide” acts like a reagent for “sulfonation reactions” or reactions responsible for the production of “sulfonic acid”.

Properties of “Sulfur Trioxide”

The different physical and chemical properties of “Sulfur Trioxide” are as follows −

Physical properties

• It practically has no odour and no colour when in a liquid state.

• It has a density of "1.92 g/cm³" and it easily dissolves in water and thus is soluble.

• It boils at an approximate “44.9 °C” and melts at an approximate “16.9 °C”. And the “Molar mass” of the compound is 80.066 g/mol.

• The “Covalently-Bonded Unit” present in the compound is 1 and the number of “Hydrogen Bond Acceptors” is 3. Also, the complexity of the compound is approximately “61.8”.

Chemical properties

• “Sulfur Trioxide” reacts at a rapid rate and because of its great oxidising properties, also works like an “oxidising agent”.

• When reacted with water “Sulfur Trioxide” produces “Sulfuric acid”. The reaction happens in the following way:

$$\mathrm{SO_{3}\:+\:H_{2}O\:\rightarrow\:H_{2}SO_{4}}$$

• When reacted with “sodium hydroxide (NaOH)” a base, “Sulfur Trioxide” produces “sodium hydrogen phosphate $\mathrm{(NaOHSO_{4})}$”. Which is a commonly used industrial chemical? The reaction happens in the following manner:

$$\mathrm{SO_{3}\:+\:NaOH\:\rightarrow\:NaHSO_{4}}$$

“Molecular bonding” and “Structural Bonding” of $\mathrm{SO_{3}}$

• The symmetry of “$\mathrm{SO_{3}}$” is “D3h” as shown in geometrical models. It is present in a “trigonal planar” structure, when in a gaseous state as proved by the “VSEPR” theory. This means there is one atom obtaining the central position with 3 atoms obtaining corner positions as in an “equilateral triangle”.

• While in its gaseous form “Sulfur Trioxide” possesses an “electrical dipole moment” that is zero. This happens due to the “S-O” bonds being present at a “120° angle”.

Figure 2 − Lewis structure of “$\mathrm{SO_{3}}$”

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• The atom of “sulfur” presents an “oxidation” state of “+6” and its “formal charge” is zero, depending on the “electron-counting formalism”.

• According to the “Lewis structure” of “$\mathrm{SO_{3}}$”, a single “S=O double bond” and a couple of “S–O dative bonds” are present without considering the “d-orbitals”.

Preparation

“Sulfur Trioxide” is prepared under different conditions by applying different methods. They are as follows −

Conditions for Atmosphere

“Sulfur Trioxide” is produced from “sulphur dioxide” in the air as it directly oxidises and the reaction supposedly takes place in the following manner −

$$\mathrm{SO_{2}\:+\:\frac{1}{2}O_{2}\:=\:SO_{3}\:\Delta\:H\:=\:-198.4}$$

Conditions for Laboratory

In the Laboratory “Sulfur Trioxide” is produced by synthesizing it, and involves “sodium bisulfate” and the process of its going through a two-step “pyrolysis”. “Sodium pyrosulfate” is provided in place of an “intermediate product”. The process involves “dehydration” at approximately “315 °C” and takes place in the following manner −

$$\mathrm{2NaHSO_{4}\:\rightarrow\:Na_{2}S_{2}O_{7}\:+\:H_{2}O}$$

On the other hand, there is also a reaction that takes place at an approximate “460 °C” and occurs as follows:

$$\mathrm{Na_{2}S_{2}O_{7}\:\rightarrow\:Na_{2}SO_{4}\:+\:H_{2}O}$$

Conditions for Industry

The production of "$\mathrm{SO_{3}}$” is usually done by the “contact process” for industrial surroundings. A specific “iron sulfide” ore, “iron pyrite” is burnt to produce “Sulfur Trioxide”. After going through the process of “electrostatic precipitation”, “$\mathrm{SO_{2}}$” is oxidised in presence of a catalyst at an approximate “400 to 600 °C”.

Applications for “Sulfur Trioxide”

• “Sulfur Trioxide” is a swift reagent for “sulfonation” reactions. Which are usually used for making dyeing products, medicines, and detergents? “Sulfur Trioxide” is also used for the production of “sulfuric acid”.

• “$\mathrm{SO_{3}}$” also possess bleaching properties which become useful to discard the residue of “hydrogen peroxide”. It can also be used for its digestive properties to remove the mush away from “lignin”.

• It is useful in the making of “photoelectric cells” and devices that work on “solar energy”.

Precautions

On top of having powerful oxidising properties, “Sulfur Trioxide” can lead to significant burns on both ingesting it and inhaling it, because of its highly corrosive qualities. Thus it demands utmost care and caution while handling “Sulfur Trioxide” as it also produces “sulfuric acid” which is highly corrosive in nature.

Conclusion

“Sulfur Trioxide” or “$\mathrm{SO_{3}}$” is a compound which is chemical in nature and possesses powerful oxidising and reactive properties. It is basically odourless and colourless in its liquid state and has a density of “1.92 g/cm³”, and it is perfectly soluble in nature. It is prepared using different methods in different surroundings mainly in the atmosphere, in laboratories and in industries. Due to its involvement in “sulfonation reactions” as a reagent, it is used in the production of dye, different pharmaceutical products and detergents.

FAQs

1. How do "Sulfur Trioxide" and H2O react?

The two react mainly to produce “sulfuric acid” and the process requires a lot of heat. The reaction is mainly performed while burning “metal sulfides”, and then the conversion of “SO3” under a catalyst before reacting it with water.

2. How is “Sulfur Trioxide” prepared under atmospheric conditions?

“Sulfur Trioxide” is obtained from “sulphur dioxide” while in the air as it readily oxidises and the reaction supposedly takes place in the following manner −

$\mathrm{"SO_{2}\:+\:\frac{1}{2}O_{2}\:=\:SO_{3}\:\Delta\:H\:=\:-198.4"}$

3. Comment on the harmful nature of “Sulfur Trioxide”?

Uncareful indigestion and inhalation of “Sulfur Trioxide” may lead to disastrous burns inside the body due to its highly corrosive qualities.