Question

Use the Lewis acid-base model to explain the following reaction. $$ \mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{H}_{2} \mathrm{CO}_{3}(a q) $$

Short answer

In the given reaction, carbon dioxide (CO2) acts as a Lewis acid because its carbon atom has electron-deficient sites and can accept an electron pair. On the other hand, water (H2O) acts as a Lewis base due to the presence of two lone pairs of electrons on its oxygen atom, which can donate an electron pair. The reaction can be summarized using the Lewis acid-base model as: \(CO_{2}(g) + H_{2}O(l) \rightarrow H_{2}CO_{3}(aq)\), with CO2 (Lewis acid) accepting an electron pair from H2O (Lewis base) and forming carbonic acid (H2CO3) as a result.

Step-by-step solution

Identify the Lewis acid and base

Let's recall the definitions of a Lewis acid and a Lewis base. A Lewis acid is a species that can accept an electron pair, while a Lewis base is a species that can donate an electron pair. In the given reaction, CO2 and H2O are the reactants, so one of them must act as a Lewis acid and the other as a Lewis base.

Determine the electron pair acceptor

To find the Lewis acid, we need to identify which reactant can accept an electron pair. In the carbon dioxide molecule (CO2), the carbon atom has two double bonds with oxygen atoms, which create electron-deficient sites on the carbon. This makes carbon dioxide capable of accepting electron pairs and acting as a Lewis acid.

Determine the electron pair donor

Now, let's determine the Lewis base in the reaction. Water (H2O) has two lone pairs of electrons on the oxygen atom, which makes it capable of donating an electron pair. So, water acts as the Lewis base in this reaction.

Explain the reaction using the Lewis acid-base model

The given reaction can be explained using the Lewis acid-base model as follows: 1. Water (H2O), which acts as a Lewis base, donates an electron pair to carbon dioxide (CO2), which acts as a Lewis acid. 2. The electron pair from water's oxygen forms a bond with carbon of the carbon dioxide molecule. 3. This process results in the formation of a new compound called carbonic acid (H2CO3). The Lewis acid-base reaction can be summarized as: \(CO_{2}(g) + H_{2}O(l) \rightarrow H_{2}CO_{3}(aq)\) - CO2 (Lewis acid) accepts an electron pair from H2O (Lewis base). - Formation of carbonic acid (H2CO3) as a result of the reaction.

Key concepts

Understanding Lewis Acids and Bases

The Lewis acid-base model is a powerful tool for understanding chemical reactions, and it diverges from the traditional definitions that focus solely on hydrogen ions. In the Lewis model, an acid is not limited to substances that liberate H+ ions in solution; instead, it is defined by its ability to accept an electron pair. Similarly, a base is characterized by its readiness to donate an electron pair, not just by its capacity to produce OH- ions in solution.

In this framework, reactions can be seen as processes where acids and bases interact through the sharing of electron pairs, leading to the formation of new bonds and compounds. This broadens our perspective to include a wider variety of reactions, including some that do not involve protons at all. By focusing on electron pairs rather than ion formation, the Lewis model provides deeper insights into the behavior of substances in a range of chemical processes.

Roles of Electron Pair Donors and Acceptors

A closer look at electron pair donors and acceptors reveals how substances play roles in chemical reactions. An electron pair donor, or a Lewis base, is like a generous individual, equipped with one or more pairs of electrons that it is willing to share. In contrast, an electron pair acceptor, or a Lewis acid, can be thought of as an electron pair seeker, possessing regions of electron deficiency that are ready to accommodate new electrons.

When a base donates an electron pair to an acid, a coordinate covalent bond is formed. This type of bond is unique because both of the electrons in the bond originate from the same atom, the Lewis base. This interaction results in a more stable arrangement where the electron-deficient Lewis acid balances its charge by accommodatively hosting the electrons from the Lewis base.

Carbon Dioxide's Reaction with Water

Let’s examine a specific reaction, where carbon dioxide (CO2) interacts with water (H2O) to form carbonic acid (H2CO3). Carbon dioxide serves as a Lewis acid because it's starved for electrons, having a carbon atom doubly bonded to two oxygen atoms, creating regions of positive charge that are open to receiving more electrons. On the other hand, water plays the role of a Lewis base. The oxygen atom in water has lone pairs of electrons, making it an excellent donor in the Lewis acid-base context.

The reaction unfolds as water donates one of its electron pairs to carbon dioxide, resulting in the formation of carbonic acid. This interaction can be visualized as a dance between the molecules: water reaches out with its electron pair, and carbon dioxide graciously accepts, leading to the bonding that holds the new molecule together. As such, understanding this reaction through the lens of the Lewis acid-base model provides clarity on how molecular interactions drive the formation of new substances.