Sulfurous acid is a weak inorganic acid with the chemical formula \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3}.\) It is formed when sulfur dioxide is dissolved in water. Therefore, it is also referred to as the aqueous solution of sulfur dioxide. Sulfurous acid is found as an intermediate formed during the formation of acid rain (by the reaction of sulfur dioxide with the moisture present in the atmosphere). In this article, we are going to discuss more about sulfurous acid.

Sulfurous Acid Formula:

Sulfurous Acid \({\rm{(}}{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3})\)

The chemical formula of sulfurous acid is \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3}.\) It contains two atoms of hydrogen, one atom of sulfur and three atoms of oxygen. Sulfurous acid can be represented as,

OR

In the structure of sulfurous acid, the sulfur atom is in the centre, with the three oxygen atoms bonded to it. Out of the three oxygen atoms, two are single-bonded and one is double-bonded. The two hydrogen atoms are attached to the single-bonded oxygen atoms.

The molar mass of sulfurous acid is \(82.07\,{\rm{g/mol}}\).

Preparation of Sulfurous Acid \(({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3})\)

Sulfurous acid can be prepared using the following methods:

1. Sulfurous acid is referred to as the aqueous solution of sulfur dioxide. Therefore, it can be prepared by dissolving sulfur dioxide in water. The chemical equation for the reaction can be given as,

\({\rm{S}}{{\rm{O}}_2} + {{\rm{H}}_2}{\rm{O}} \leftrightarrow {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3}\)

However, the above reaction is reversible, and the sulfurous acid formed decomposes readily to give back the reactants. Thus, sulfurous acid cannot be isolated in acid form. More commonly, it is produced in the form of sodium or potassium salts. For example,

When sodium sulfate is added to an aqueous solution of sulfur dioxide, sodium bisulfite salt and sulfur dioxide are formed. The chemical equation for the reaction can be given as,

\({\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_3} + {{\rm{H}}_2}{\rm{O + S}}{{\rm{O}}_2} \to 2{\rm{NaHS}}{{\rm{O}}_3}\)

2. Sulfurous acid is produced naturally as an intermediate during the formation of acid rain. That is, it is produced when the sulfur dioxide reacts with the moisture present in the atmosphere.

Note: There is no clear evidence that sulfurous acid exists in the solution, but the molecule has been detected in the gas phase. Hence, it is called an elusive acid.

Properties of Sulfurous Acid\({\rm{(}}{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3})\)

Let us discuss the physical and chemical properties of sulfurous acid \({\rm{(}}{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3}).\)

Physical Properties of Sulfurous Acid\({\rm{(}}{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3})\)

1. Sulfurous acid appears as a colorless liquid with a strong pungent smell.
2. The boiling point of sulfurous acid is approximately equal to \( – 60{\,^ \circ }{\rm{C}}.\)
3. The density of sulfurous acid is approximately equal to \(1.03\,{\rm{g/c}}{{\rm{m}}^3}.\)
4. Sulfurous acid is a weak dibasic acid.

Chemical Properties of Sulfurous Acid \(({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3})\)

1. The conjugate bases of sulfurous acid are bisulfite (or hydrogen sulfite) and sulfite ions. That is, the common anions such as \({\rm{HSO}}_3^ -\) and \({\rm{SO}}_3^{2 – }.\)
2. Sulfurous acid has not been detected or isolated as a separate compound. Also, it does not appear to exist in solution either. However, the spectroscopic evidence provides the following information: In the solution of sulfur dioxide in water, the spectroscopic evidence provided only the signals of \({\rm{S}}{{\rm{O}}_2}\) molecule and the bisulfite \({\rm{(HS}}{{\rm{O}}_{{3^ – }}})\) ion. The intensities of such signals were consistent with the following equilibrium.

\({\rm{S}}{{\rm{O}}_2} + {{\rm{H}}_2}{\rm{O}} \Leftrightarrow {\rm{HS}}{{\rm{O}}_3}^ –  + {{\rm{H}}^ + }\)

3. Sulfurous acid forms both acid sulfites (also called bisulfites) and normal sulfites. For example, it forms acid sulfites such as sodium hydrogen sulfite \(({\rm{NaHS}}{{\rm{O}}_3})\) and normal sulfites such as sodium sulfite \(({\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_3}).\) The most soluble and the best known salts of sulfurous acid are those composed of alkali metals. Such salts are formed by a reaction between sulfur dioxide and a solution of the alkali metal hydroxide. This reaction involves the neutralization of sulfurous acid.

4. The \(^{17}{\rm{O}}\,{\rm{NMR}}\) (Nuclear Magnetic Resonance) spectroscopy has given a relevant evidence that, the solutions of sulfurous acid and protonated sulfites contain a mixture of isomers that exists in dynamic equilibrium with each other. That is,

5. Sulfurous acid can be oxidized to sulfuric acid or to sulfate by accepting another oxygen atom.

6. Decomposition of sulfurous acid:
Sulfurous acid is highly unstable. When it is formed, it suddenly decomposes into its constituents. That is,
\({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3}({\rm{aq}}) \to {{\rm{H}}_2}{\rm{O}}({\rm{I}}) + {\rm{S}}{{\rm{O}}_2}({\rm{g}})\)

Uses of Sulfurous Acid

The uses of sulfurous acid include:

1. Sulfurous acid, which is also known as the aqueous solution of sulfur dioxide, is used as a disinfectant and as a reducing agent.
2. Sulfurous acid is used as a mild bleaching agent in the place of chlorine sensitive materials
3. Sulfurous acid is used in the manufacture of paper products.

Summary

Sulfurous acid is a weak inorganic acid with the chemical formula \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3}\). It is formed by when sulfur dioxide is dissolved in water. Hence, it is also called the aqueous solution of sulfur dioxide. Sulfurous acid is formed naturally in the atmosphere as an intermediate during the formation of acid rain. Sulfurous acid has not been detected or isolated as a separate compound. Also, it does not appear to exist in solution either. In this article, we have explained the formula, preparation, physical and chemical properties, and the uses of sulfurous acid in detail.

FAQs on Sulfurous Acid \({\rm{(}}{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3})\)

Q1. How do you write the formula for sulfurous acid?
Ans: Sulfurous acid is a weak inorganic acid. Its chemical formula can be written as \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3}\).

Q2. Why is \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3}\) called sulfurous acid?
Ans: \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3}\) is formed when sulfur dioxide is dissolved in water. Therefore, it is called as sulfurous acid.

Q3. What is the formula of hydroiodic acid?
Ans: The chemical formula of hydroiodic acid is HI. It is formed when hydrogen iodide is dissolved in water.

Q4. What is the formula of hydrochloric acid?
Ans: Hydrochloric acid is a strong mineral acid with the chemical formula \({\rm{HCI}}\) It is formed when hydrogen chloride gas is dissolved in water.

Q5. What type of acid is sulfurous acid?
Ans: Sulfurous acid is a weak dibasic acid. Its chemical formula is \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3}.\)

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