Question

Use Lewis structures to diagram the following reaction in the manner of reaction (16.20) $$\mathrm{H}_{2} \mathrm{O}+\mathrm{SO}_{2} \longrightarrow \mathrm{H}_{2} \mathrm{SO}_{3}$$ Identify the Lewis acid and Lewis base.

Short answer

In the given reaction \(H_2O + SO_2 \longrightarrow H_2SO_3\), \(SO_2\) is the Lewis acid because it accepts an electron pair, and \(H_2O\) is the Lewis base because it donates an electron pair.

Step-by-step solution

Draw Lewis Structures

Draw the Lewis structures for the reactants, which include water \(H_2O\) and sulfur dioxide \(SO_2\), and the product, which is sulphurous acid \(H_2SO_3\). In water, oxygen shares one electron with each hydrogen, forming 2 bonds, and has 4 non-bonding electrons. In sulfur dioxide, sulfur shares 2 electrons with each oxygen, forming 2 double bonds and has 2 non-bonding electrons. In the sulphurous acid molecule, sulfur shares 2 electrons with one oxygen (double bond) and one with each other atom (single bonds), and has 2 non-bonding electrons.

Identify the Lewis Acid and Base

A Lewis acid is a substance that can accept an electron pair, while a Lewis base is a substance that can donate an electron pair. In this reaction, \(H_2O\) is the Lewis base as it donates an electron pair to \(SO_2\), which acts as the Lewis acid by accepting the pair of electrons, forming \(H_2SO_3\).

Finalize Reaction Diagram and Check

Finalize your Lewis structures and check to make sure that all atoms obey the octet rule (with up to 8 electrons in their valence shell), and that the overall charges of the reactants and products are balanced. In this case, all the molecules are neutral, so our reaction does not involve any ions.

Key concepts

Lewis Acid

A Lewis acid is a species that can accept a pair of electrons. In the simplified world of chemistry, imagine a Lewis acid as an electron pair seeker.
This concept comes from the Lewis theory, which broadens our understanding beyond just donating or accepting protons and focuses instead on electron pairs.

In our given reaction

• Sulfur dioxide \( \text{SO}_2 \), acting as the Lewis acid, accepts a pair of electrons from water \( \text{H}_2\text{O} \).
• This makes sense because \( \text{SO}_2 \) has empty orbital spaces ready to form bonds with electrons coming from other molecules.
• An essential characteristic of Lewis acids is partial positive charges or incomplete octets, allowing them to accept electron pairs easily.

Understanding Lewis acids helps predict interactions in chemical reactions and is foundational in explaining many chemical properties and reactivities.
Think of them as electron 'sharers' that help create strong bonds in chemical reactions.

Lewis Base

While a Lewis acid accepts an electron pair, a Lewis base is quite the opposite. It donates an electron pair to a chemical reaction.
Imagine a Lewis base as being abundant in electrons and seeking a partner to share them with, just like a generous friend.

In the context of our reaction:

• Water \( \text{H}_2\text{O} \) acts as the Lewis base.
• This is because oxygen in water has a pair of non-bonding electrons that can be donated or shared.
• When those electrons find an electron-accepting molecule such as \( \text{SO}_2 \), a beneficial reaction occurs for both participants.

Lewis bases usually possess lone pairs of electrons or might carry a negative charge.
These features make them key players in forming new compounds during chemical reactions, effectively stabilizing those reactions.

The role of a Lewis base is crucial because without them, the electron-seeking Lewis acids would not be able to complete their valency or fulfill their potential for forming new bonds.

Octet Rule

The Octet Rule is a fundamental concept in chemistry that helps us understand how atoms bond. It states that atoms tend to prioritize having eight electrons in their valence shell.
This atomic behavior is akin to seeking stability, much like how noble gases are stable due to their complete electron shells.

In our example reaction:

• You’ll notice that around each atom, we typically try to ensure that they follow this Octet Rule.
• For instance, the oxygen and sulfur complete their valence shells through sharing electrons among themselves to achieve stability.

However, not all situations strictly obey the octet rule. For example, in some molecules, like those with sulfur (which can have expanded valence shells), we find exceptions.
While the configuration of eight electrons provides a clearer image of atomic stability and interactions, understanding when deviations occur is pivotal in understanding advanced chemistry.
By visualizing and predicting how molecules like water and sulfur dioxide form bonds under the influence of the octet rule, students can better grasp molecular architecture and reactions.