Problem 74 - General Chemistry: Principles and Modern Applications

Question

Each of the following is a Lewis acid-base reaction. Which reactant is the acid, and which is the base? Explain. (a) \(\mathrm{SO}_{3}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{H}_{2} \mathrm{SO}_{4}\) (b) \(\operatorname{Zn}(\mathrm{OH})_{2}(\mathrm{s})+2 \mathrm{OH}^{-}(\mathrm{aq}) \longrightarrow\left[\mathrm{Zn}(\mathrm{OH})_{4}\right]^{2-}(\mathrm{aq})\)

Step-by-Step Solution

Answer

In the first reaction, \( \mathrm{SO}_{3}\) is the Lewis acid and \(\mathrm{H}_{2} \mathrm{O}\) is the Lewis base. For the second, \( \operatorname{Zn}(\mathrm{OH})_{2}\) is the Lewis acid and \(\mathrm{OH}^{-}\) is the Lewis base.

Step 1: Identify the Lewis acid and base in first reaction

In the reaction \( \mathrm{SO}_{3}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{H}_{2} \mathrm{SO}_{4} \), we observe that \(\mathrm{SO}_3\) accepts a pair of electrons from \(\mathrm{H}_2\mathrm{O}\) to form a bond. Thus, \(\mathrm{SO}_3\) is the Lewis acid (electron pair acceptor) and \(\mathrm{H}_2\mathrm{O}\) is the Lewis base (electron pair donor).

Step 2: Identify the Lewis acid and base in second reaction

In the reaction \(\operatorname{Zn}(\mathrm{OH})_{2}(\mathrm{s})+2 \mathrm{OH}^{-}(\mathrm{aq}) \longrightarrow\left[\mathrm{Zn}(\mathrm{OH})_{4}\right]^{2-}(\mathrm{aq})\), it's seen that \( \operatorname{Zn}(\mathrm{OH})_{2}\) accepts a pair of electrons from each \(\mathrm{OH}^{-}\) ion. This means that \(\operatorname{Zn}(\mathrm{OH})_{2}\) is the Lewis acid and \(\mathrm{OH}^{-}\) is the Lewis base in this reaction.

Key Concepts

Lewis Acid

A Lewis acid is a chemical species that accepts an electron pair. This is a key concept in the Lewis acid-base theory, named after the chemist Gilbert N. Lewis. Lewis acids play an important role in various chemical reactions, especially those that involve the transfer of electron pairs.

In general, Lewis acids have one or more of the following properties:

• A positively charged ion, such as \( ext{H}^+\), that desires electrons to reach a neutral or more stable state.
• Atoms or molecules with incomplete octet of electrons, leading them to seek out extra electron pairs.
• Species with a central atom that is bonded to a highly electronegative element, creating a polarized bond.

For instance, in the reaction of \(\text{SO}_3\) with water, \(\text{SO}_3\) acts as a Lewis acid because it accepts an electron pair from water to form sulfuric acid, \(\text{H}_2\text{SO}_4\). Similarly, in another reaction, \(\text{Zn(OH)}_2\), acts as a Lewis acid when accepting electron pairs from hydroxide ions \(\text{OH}^-\).
Recognizing Lewis acids is vital for understanding how molecules interact in chemistry, especially in organic and inorganic reactions.

Lewis Base

A Lewis base is any species that donates an electron pair to a Lewis acid to form a covalent bond. This essential concept falls under the Lewis theory of acid-base reactions. By donating an electron pair, Lewis bases complete the electron requirement of Lewis acids.

Key characteristics of Lewis bases include:

• Availability of an electron pair, which can be found in valence shell lone pairs.
• Valence electrons that are paired and not bonded, making them available for donation.
• A general tendency to be negatively charged or neutral with lone pairs.

An example is water \(\text{H}_2\text{O}\), which acts as a Lewis base in its reaction with \(\text{SO}_3\), as it provides an electron pair. Similarly, in the reaction involving zinc hydroxide, \(\text{OH}^-\) ions donate electron pairs to \(\text{Zn(OH)}_2\), functioning as Lewis bases.

Understanding Lewis bases is crucial for grasping complex reaction mechanisms and determining how different substances combine at the molecular level.

Electron Pair Donor

Electron pair donors are substances that provide an electron pair to form a chemical bond. This ability is what makes certain molecules act as Lewis bases in chemical reactions. Understanding electron pair donors is essential to comprehend how bonds form and break in reactions.

Key points about electron pair donors:

• They often have excess electrons in lone pairs ready to be shared with an electron-deficient atom.
• They are capable of forming coordinate covalent bonds by sharing available electron pairs.
• Lewis bases are typically electron pair donors.

For instance, in the reaction between \(\text{SO}_3\) and water, the water molecule serves as an electron pair donor by contributing its lone pair to bond with \(\text{SO}_3\). Similarly, in the reaction involving \(\text{Zn(OH)}_2\), the \(\text{OH}^-\) ions act as electron pair donors.

Conceptualizing how molecules act as electron pair donors helps in predicting reactivity and the outcomes of chemical reactions, which is pivotal for studying and manipulating chemical processes.