Chemistry of Oxoacids of Sulfur

Oxoacids of Sulfur

Oxoacids are a group of acids that contain oxygen. In oxoacids of sulfur, it contains sulfur, oxygen, and hydrogen. The most common oxoacids of sulfur include sulfurous acid (H₂SO₃), sulfuric acid (H₂SO₄), and thiosulfuric acid (H₂S₂O₃) which are widely used in manufacturing, agriculture, and chemical industries.

Other notable oxoacids of sulfur are peroxomonosulfuric acid (H₂SO₅), pyrosulfuric acid (H₂S₂O₇), peroxodisulfuric acid (H₂S₂O₈). In each acid, the oxidation number of the sulfur is different. Therefore, they show different chemical properties. In this article, we mainly discuss the chemistry of sulfuric acid, sulfurous acid, and thiosulfuric acid.

Sulfuric acid / Sulphuric acid

Sulfuric acid is a strong, diprotic acid with oxidative properties. In an aqueous medium, it results in two H⁺ ions and sulfate ions in the water. The oxidation state of sulfur in sulfuric acid is +6. Industrially sulfuric acid is prepared by dissolving sulfur trioxide (SO₃) in water.

Since the hydration enthalpy of sulfuric acid is high, it releases a significant amount of heat when it dissolves in water (Highly exothermic). The concentrated sulfuric acid is highly corrosive and has oxidative properties.  

When concentrated sulfuric acid contact with wood or most other organic matter, it will be dehydrated and result in carbon (char).

Dilute sulfuric acid acts as an acid. That means that will only give protons to the medium. However concentrated Sulfuric acid shows acidic oxidative properties as well as acidic properties.

Oxidative properties of conc. sulfuric acid

Conc sulfuric acid reacts with metals and some non-metals releasing sulfur dioxide gas. Here, the oxidative state of sulfur changes from +6 to +4.

01. Reactions of conc. Sulfuric acid with metals

02. Reactions of conc. Sulfuric acid with non-metals

Sulfurous acid/ Sulphurous acid

Sulfurous acid is a weak, diprotic acid.  In an aqueous medium, it partially dissociates and results in two H⁺ ions and a sulfite ion in the water. The oxidation state of sulfur in sulfurous acid is +4.

Sulfurous acid can be prepared by dissolving sulfur dioxide (SO₂) gas in water.

In the presence of water sulfurous acid can act as a bleach. Sulfurous acid reacts with water to form sulfuric acid and results in atomic hydrogen. These atomic hydrogen can turn the pigments into their respective hydride. Thus, the color is removed.

Thiosulfuric acid /Thiosulphuric acid

Thiosulfuric acid is a weak, diprotic acid. It has two sulfur atoms with different oxidation states. The oxidation state of the central sulfur atom is +4 and the other sulfur atom has 0 oxidation state.

Thiosulfuric acid will readily decompose in aqueous solution above -5 ⁰C.