Question

Identify the Lewis acid and Lewis base in each reaction: (a) \(\mathrm{Fe}^{3+}+2 \mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{FeOH}^{2+}+\mathrm{H}_{3} \mathrm{O}^{+}\) (b) \(\mathrm{H}_{2} \mathrm{O}+\mathrm{H}^{-} \rightleftharpoons \mathrm{OH}^{-}+\mathrm{H}_{2}\) (c) \(4 \mathrm{CO}+\mathrm{Ni} \rightleftharpoons \mathrm{Ni}(\mathrm{CO})_{4}\)

Short answer

(a) Lewis acid: Fe3+, Lewis base: H2O; (b) Lewis acid: H2O, Lewis base: H^-; (c) Lewis acid: Ni, Lewis base: CO.

Step-by-step solution

- Understanding Lewis Acid and Base

A Lewis acid is a species that can accept an electron pair, whereas a Lewis base is a species that can donate an electron pair.

- Identify in Reaction (a)

In the reaction \(\text{Fe}^{3+} + 2 \text{H}_2\text{O} \rightleftharpoons \text{FeOH}^{2+} + \text{H}_3\text{O}^+\), \(\text{Fe}^{3+}\) is the Lewis acid because it accepts electron pairs from water molecules. \(\text{H}_2\text{O}\) is the Lewis base as it donates electron pairs to \(\text{Fe}^{3+}\).

- Identify in Reaction (b)

In the reaction \(\text{H}_2\text{O} + \text{H}^- \rightleftharpoons \text{OH}^- + \text{H}_2\), \(\text{H}_2\text{O}\) is the Lewis acid because it accepts an electron pair from \(\text{H}^-\). \(\text{H}^-\) is the Lewis base as it donates an electron pair to \(\text{H}_2\text{O}\).

- Identify in Reaction (c)

In the reaction \(\text{CO} + \text{Ni} \rightleftharpoons \text{Ni}(\text{CO})_4\), \(\text{CO}\) is the Lewis base because it donates electron pairs to \(\text{Ni}\). \(\text{Ni}\) is the Lewis acid as it accepts electron pairs from \(\text{CO}\).

Key concepts

Lewis acid

A Lewis acid is a chemical species that accepts an electron pair. This concept helps explain numerous chemical reactions beyond those explained by traditional acid-base theories. For instance, when discussing reaction (a) from the exercise, \(\text{Fe}^{3+}\) is identified as a Lewis acid. It accepts electron pairs from water molecules, highlighting its role in the reaction. Recognizing a Lewis acid involves finding the species lacking electron pairs and eager to accept them.

Lewis base

A Lewis base is a species that donates an electron pair to form a bond with a Lewis acid. For example, in the reaction involving iron and water, \(\text{H}_2\text{O}\) acts as the Lewis base. It donates electron pairs to \(\text{Fe}^{3+}\), facilitating the reaction. Understanding Lewis bases helps in identifying substances with lone pairs ready to be shared.

Electron pair donation

Electron pair donation is a crucial concept in understanding Lewis bases. When a species donates its electron pair, it helps form a bond with the Lewis acid. In chemical reactions like the ones given, the donated electron pair usually comes from lone pairs present on the Lewis base. For example, in reaction (b), the \(\text{H}^-\) ion donates its electron pair to water, demonstrating electron pair donation clearly.

Electron pair acceptance

Electron pair acceptance pertains to Lewis acids. These species are hungry for electron pairs and accept them to complete their configurations. Identifying a Lewis acid often involves pinpointing the species that accepts these electron pairs. In the third reaction of our exercise, nickel \(\text{Ni}\) accepts electron pairs from carbon monoxide \(\text{CO}\), making it a Lewis acid by definition.

Chemical reactions

Chemical reactions occur through the interactions of Lewis acids and bases. By donating and accepting electron pairs, these reactions progress, forming new products. Each reaction discussed in our exercise highlights these interactions. For instance, in reaction (c), \(\text{CO}\) donates electron pairs to \(\text{Ni}\), resulting in the formation of \(\text{Ni}(\text{CO})_4\). Such insights into chemical reactions emphasize the importance of understanding Lewis acids and bases in predicting and explaining the behavior of substances.