Question

Describe the hydration of \(\mathrm{SO}_{2}\) as a Lewis acid-base reaction.

Short answer

\(\mathrm{SO}_2\) (Lewis acid) reacts with \(\mathrm{H}_2\mathrm{O}\) (Lewis base) to form \(\mathrm{H}_2\mathrm{SO}_3\).

Step-by-step solution

Understanding Lewis Acid-Base Theory

In Lewis theory, an acid is a substance that accepts an electron pair, while a base is a substance that donates an electron pair. Let's apply this concept to the reaction between sulfur dioxide (\(\mathrm{SO}_2\)) and water, where \(\mathrm{H}_2\mathrm{O}\) acts as the Lewis base (donating an electron pair) and \(\mathrm{SO}_2\) acts as the Lewis acid (accepting an electron pair).

Lewis Structure of Reactants

Draw the Lewis structures of \(\mathrm{SO}_2\) and \(\mathrm{H}_2\mathrm{O}\). \(\mathrm{SO}_2\) has a bent shape with sulfur as the central atom, double-bonded to two oxygens. \(\mathrm{H}_2\mathrm{O}\) is bent too, with oxygen having two lone pairs ready to donate.

Electron Pair Donation

Identify where the lone pair from \(\mathrm{H}_2\mathrm{O}\) can be donated. The oxygen atom in water donates a pair of electrons from its lone pairs to the sulfur atom in \(\mathrm{SO}_2\).

Formation of Sulfurous Acid

Once the electron pair is donated, \(\mathrm{SO}_2\) forms a coordinate covalent bond with \(\mathrm{H}_2\mathrm{O}\), creating an intermediate. Rearrangement of atoms and bonding results in the formation of sulfurous acid (\(\mathrm{H}_2\mathrm{SO}_3\)).

Complete Reaction Equation

Based on the previous steps, write the balanced chemical equation of the hydration reaction: \(\mathrm{SO}_2 + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{H}_2\mathrm{SO}_3\).

Key concepts

Sulfur Dioxide

Sulfur dioxide (SO₂) is a colorless gas with a sharp, irritating odor that you might recognize from the smell of a just-struck match. It's a commonly found compound in the atmosphere, predominantly released by volcanic eruptions, burning of fossil fuels, and industrial processes. In terms of chemistry, sulfur dioxide is a significant player.
It acts mainly as a Lewis acid in reactions because it can accept an electron pair. This comes down to its molecular structure, which includes sulfur double-bonded to two oxygen atoms. However, the sulfur atom has an affinity for additional electrons, making it capable of accepting lone pairs from other atoms. This capability plays a crucial role in its interactions, such as the hydration reaction with water to form sulfurous acid.

Hydration Reaction

A hydration reaction involves the addition of a water molecule to a substance. In the case of sulfur dioxide's interaction with water, it is characterized as a classic Lewis acid-base reaction, where bonding occurs due to the transfer of an electron pair.
The hydration of SO₂ is an interesting example as it results in the formation of sulfurous acid. During this reaction, the water molecule donates a pair of electrons from one of its oxygen's lone pairs to the sulfur dioxide. This step marks the initial part of the process leading to coordinate covalent bonding. Such interactions are pivotal in understanding how gases like SO₂ can be transformed into acids through simple hydration.

Electron Pair Donation

Electron pair donation is a core concept in Lewis acid-base chemistry. When looking at the hydration of sulfur dioxide, the crucial action is how water ( H₂O) acts as a Lewis base. Here's how it happens:

• The oxygen atom in the water molecule has two lone pairs of electrons.
• One lone pair is donated to the sulfur atom in SO₂, which acts as the Lewis acid by accepting the electrons.
• This donation forms a coordinate covalent bond between the oxygen from water and the sulfur from sulfur dioxide.

This movement of electrons is vital because it drives the transformation process, allowing SO₂ to eventually become part of the sulfurous acid compound.

Sulfurous Acid

Sulfurous acid ( H₂SO₃) is the product of the hydration of sulfur dioxide. It's less stable than its more well-known cousin, sulfuric acid ( H₂SO₄), and is often more transient in nature. In its formation:

• The molecule forms when water's lone electron pairs are donated to sulfur dioxide.
• This creates an intermediate state where sulfur is bound to both oxygen atoms from its original structure and an additional oxygen from water.
• The reaction rearranges the atoms and bonds, culminating in the production of sulfurous acid.

Understanding sulfurous acid's formation aids in grasping how even simple molecular interactions can trigger significant chemical transformations. This reaction is paramount for processes involving pollutants and industrial chemistry.