Difference Between Sulphuric Acid and Sulphurous Acid

Main Difference – Sulphuric Acid vs Sulphurous Acid

Sulphur is a chemical element having the chemical symbol “S” and atomic number 16. The electron configuration of sulphur is [Ne] 3s²3p⁴. Therefore, it has 6 electrons in the outermost electron shell that can participate in chemical bonding. Sulphur can make a maximum of 6 covalent bonds. It forms a wide variety of chemical compounds. Sulphuric acid and Sulphurous acid are two such compounds which are acids. The main difference between sulphuric acid and sulphurous acid is that sulphuric acid has the chemical formula H₂so₄ and is a good oxidizing agent whereas sulphurous acid has the chemical formula H₂SO₃ and is a good reducing agent.

Key Areas Covered

1. What is Sulphuric Acid
     – Definition, Chemical Properties, Reactions
2. What is Sulphurous Acid
     – Definition, Chemical Properties
3. What is the Difference Between Sulphuric Acid and Sulphurous Acid
     – Comparison of Key Differences

Key Terms: Atomic Number, Covalent Bonds, Oxidation State, Reducing Agent, Sulphur, Sulphuric Acid, Sulphurous Acid, Tetrahedral, Trigonal Pyramidal

What is Sulphuric Acid

Sulphuric acid is an acid having the chemical formula H₂SO₄. At room temperature, it is a colorless and odorless, syrupy liquid. The molar mass of sulphuric acid is 98.079 g/mol. It is a strong acid and is corrosive. Therefore, one should take care when handling this acid.

The melting point of sulphuric acid is 10^oC, and the boiling point is 337 °C. When considering the chemical structure of Sulphuric acid, the sulphur atom is in the middle of the molecule. Two –OH groups are bonded to the sulphur atom via single covalent bonds. Two oxygen atoms are bonded to the sulphur atoms via double bonds. The molecule has a tetrahedral structure.

Figure 1: Chemical Structure of Sulphuric Acid

The sulfuric molecule has two –O-H bonds which are very polar. Due to this polarity, the H atoms can easily be lost. The ionization of sulfuric acid occurs in two steps.

H₂SO_4(aq)    +       H₂O_(l)       →     HSO₄^–_(aq)   +       H₃O⁺_(aq)

HSO₄^–_(aq)    +       H₂O_(l)       →    SO₄^-2_(aq)     +       H₃O⁺_(aq)

Sulfuric acid is a strong oxidizing agent in concentrated conditions. But when it is diluted, the oxidizing strength is reduced. For example, at higher temperatures, concentrated sulfuric acid can oxidize Br– into Br₂. 

2HBr_(aq)     +       H₂SO_{4(aq)    →}     Br_2(aq)           +         SO_2(aq)       +       2 H₂O_(l)        

The oxidation state of the sulphur atom in sulphuric acid is +6. It is the maximum oxidation state sulphur atom can hold. Therefore, sulphuric acid can be reduced to lower oxidation states, but it cannot act as a reducing agent.

What is Sulphurous Acid

Sulphurous acid is an acid having the chemical formula H₂SO₃. It is a covalent compound containing only covalent chemical bonds. The molar mass of sulphurous acid is 82.07 g/mol. Sulphurous acid is a weak acid and unstable. It is formed when sulphur dioxide (SO₂) is dissolved in water.

Figure 2: Chemical Structure of Sulphurous Acid

The chemical geometry of sulphurous acid is trigonal pyramidal. Although the electron geometry of the molecule is tetrahedral, the molecular geometry is trigonal pyramidal due to the presence of a lone electron pair.

Sulphurous acid cannot be isolated as the pure compound because it is formed only in the aqueous solution. Sulphurous acid (aqueous solution) is a reducing agent. That is because the sulphur atom in this compound is in the oxidation state +4 and it can be further oxidized to the highest oxidation state of sulphur; +6. Sulfurous acid also acts as a bleaching agent.

Difference Between Sulphuric Acid and Sulphurous Acid

Definition

Sulphuric Acid: Sulphuric acid is an acid having the chemical formula H₂SO₄.

Sulphurous Acid: Sulphurous acid is an acid having the chemical formula H₂SO₃.

Molar Mass

Sulphuric Acid: The molar mass of Sulphuric acid is 98.079 g/mol.

Sulphurous Acid: The molar mass of Sulphurous acid is 82.07 g/mol.

Oxidation State of Sulphur

Sulphuric Acid: The oxidation state of sulphur atom in Sulphuric acid is +6.

Sulphurous Acid: The oxidation state of sulphur atom in Sulphurous acid is +4.

Geometry

Sulphuric Acid: The Sulphuric acid molecule has a tetrahedral structure.

Sulphurous Acid: The Sulphurous acid molecule has a trigonal pyramidal structure.

Nature

Sulphuric Acid: Sulphuric acid is a good oxidizing agent.

Sulphurous Acid: Sulphurous acid is a good reducing agent.

Conclusion

Sulphuric acid and sulphurous acid are acidic covalent compounds containing S, H and O atoms. The main difference between sulphuric acid and sulphurous acid is that sulphuric acid has the chemical formula h₂so₄ whereas sulphurous acid has the chemical formulah₂so₃.

References:

1. “Sulfuric Acid Formula – Sulfuric Acid Uses, Properties, Structure and Formula.” Math, Available here.
2. “Sulfurous acid.” Chemistry Learner, Available here.
3. “Sulfurous acid.” Wikipedia, Wikimedia Foundation, 17 Dec. 2017, Available here.

Image Courtesy:

1. “Sulfuric acid chemical structure” By DMacks (talk) – Own work (Public Domain) via Commons Wikimedia
2. “Sulfurous-acid-2D-pyramidal” By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia