Question

Identify the Lewis acid and Lewis base among the reactants in each of the following reactions: (a) \(\mathrm{Fe}\left(\mathrm{ClO}_{4}\right)_{3}(s)+6 \mathrm{H}_{2} \mathrm{O}(I) \rightleftharpoons\) \(\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}(a q)+3 \mathrm{ClO}_{4}^{-}(a q)\) (b) \(\mathrm{CN}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{HCN}(a q)+\mathrm{OH}^{-}(a q)\) (c) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}(\mathrm{~g})+\mathrm{BF}_{3}(g) \rightleftharpoons\left(\mathrm{CH}_{3}\right)_{3} \mathrm{NBF}_{3}(\mathrm{~s})\) (d) \(\mathrm{HIO}(\mathrm{lq})+\mathrm{NH}_{2}^{-(l q)} \rightleftharpoons \mathrm{NH}_{3}(l q)+\mathrm{IO}^{-}(l q)\) (Iq denotes liquid ammonia as solvent)

Short answer

(a) \(\mathrm{H}_{2}\mathrm{O}\) is the Lewis base and \(\mathrm{Fe}\left(\mathrm{ClO}_{4}\right)_{3}\) is the Lewis acid. (b) \(\mathrm{CN}^-\) is the Lewis base and \(\mathrm{H}_{2}\mathrm{O}\) is the Lewis acid. (c) \(\left(\mathrm{CH}_{3}\right)_{3}\mathrm{N}\) is the Lewis base and \(\mathrm{BF}_{3}\) is the Lewis acid. (d) \(\mathrm{NH}_{2}^-\) is the Lewis base and \(\mathrm{HIO}\) is the Lewis acid.

Step-by-step solution

Identify electron donors and acceptors

In this reaction, \(\mathrm{Fe}\left(\mathrm{ClO}_{4}\right)_{3}\) binds with water molecules. Water molecules, having a lone pair of electrons, can donate these electrons to other molecules, while \(\mathrm{Fe}\) has vacant orbitals for electron acceptance.

Conclusion

In this reaction, \(\mathrm{H}_{2}\mathrm{O}\) is the Lewis base (electron donor) and \(\mathrm{Fe}\left(\mathrm{ClO}_{4}\right)_{3}\) is the Lewis acid (electron acceptor). (b) $\mathrm{CN}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{HCN}(a q)+\mathrm{OH}^{-}(a q)$

Identify electron donors and acceptors

In this reaction, the cyanide ion \(\mathrm{CN}^-\) donates a pair of electrons to water, forming HCN and hydroxide ions.

Conclusion

In this reaction, \(\mathrm{CN}^-\) is the Lewis base (electron donor) and \(\mathrm{H}_{2}\mathrm{O}\) is the Lewis acid (electron acceptor). (c) $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}(\mathrm{~g})+\mathrm{BF}_{3}(g) \rightleftharpoons\left(\mathrm{CH}_{3}\right)_{3} \mathrm{NBF}_{3}(\mathrm{~s})$

Identify electron donors and acceptors

In this reaction, the nitrogen atom in \(\left(\mathrm{CH}_{3}\right)_{3}\mathrm{N}\) has a lone pair of electrons and can donate these electrons to \(\mathrm{BF}_{3}\), resulting in the formation of \(\left(\mathrm{CH}_{3}\right)_{3}\mathrm{NBF}_{3}\).

Conclusion

In this reaction, \(\left(\mathrm{CH}_{3}\right)_{3}\mathrm{N}\) is the Lewis base (electron donor) and \(\mathrm{BF}_{3}\) is the Lewis acid (electron acceptor). (d) $\mathrm{HIO}(\mathrm{lq})+\mathrm{NH}_{2}^{-(l q)} \rightleftharpoons \mathrm{NH}_{3}(l q)+\mathrm{IO}^{-}(l q)$

Identify electron donors and acceptors

In this reaction, the amide ion \(\mathrm{NH}_{2}^-\) donates a pair of electrons to \(\mathrm{HIO}\), resulting in the formation of \(\mathrm{NH}_{3}\) and \(\mathrm{IO}^{-}\).

Conclusion

In this reaction, \(\mathrm{NH}_{2}^-\) is the Lewis base (electron donor) and \(\mathrm{HIO}\) is the Lewis acid (electron acceptor).

Key concepts

Electron Donors and Acceptors

Understanding Lewis Acid-Base reactions hinges on the identification of electron donors and acceptors, key players in chemical processes. In an electron-donor-acceptor reaction, the species that donates an electron pair is called the Lewis base, while the species that accepts it is known as the Lewis acid.

Electrons are housed within orbitals, and molecules or ions with available lone pairs of electrons are prime candidates for electron donation. Take, for example, water (\rH\(_2\)O), which in the presence of iron(III) perchlorate (\rFe(ClO\(_4\))_3), acts as a Lewis base by donating its lone pair of electrons to the metal ion. The Lewis acid, in this case Fe(ClO\(_4\))_3, eagerly accepts these electrons into its vacant orbitals, fulfilling its electron octet.

Such interactions are pivotal in forming coordination compounds where the central metal ion acts as a hub, gathering electron-donating ligands around it to form a stable complex. Exploring a range of examples, from simple inorganic ions to more complex organic structures, highlights the versatility of electron donors and acceptors in various chemical contexts.

Chemical Equilibrium

Chemical equilibrium occurs when a chemical reaction and its reverse are happening at equal rates, leading to no net change in the amounts of reactants and products. This balance is not static but dynamic; reactions continue to occur, but the system's observable properties remain consistent.

For instance, when a metal ion in solution is surrounded by water and changes to a complex with hydrated ligands, as in the reaction \rFe(ClO\(_4\))_3(s) + 6 H\(_2\)O(l) \rightleftharpoons \rFe(H\(_2\)O)\(_6^{3+}\)(aq) + 3 ClO\(_4^−\)(aq), equilibrium is established between the solid salt dissolving and forming the hydrated metal complex, and the complex dissociating back to release the metal salt.

This concept extends to many areas of chemistry and is influenced by various factors such as concentration, temperature, and pressure, in accordance with Le Chatelier's principle. In the classroom and lab, understanding chemical equilibrium is fundamental for predicting reaction behavior and manipulating conditions to favor either products or reactants.

Acid-Base Chemistry

Acid-Base chemistry is a foundational aspect of chemical understanding, framing how acids and bases react together. It's defined traditionally by the Brønsted-Lowry theory, where acids are proton donors, and bases are proton acceptors. However, the Lewis theory broadens this definition: acids are electron pair acceptors, and bases are electron pair donors.

In the reaction between cyanide ions (\rCN\(^−\)) and water (\rH\(_2\)O), the cyanide ions act as Lewis bases because they have a lone electron pair to donate, resulting in the creation of hydrocyanic acid (\rHCN) and hydroxide ions (\rOH\(^−\)). Here, water undergoes a role reversal, acting as a Lewis acid despite often being considered a base in other contexts, exemplifying the Lewis Acid-Base theory's versatility.

Understanding how to identify and predict the behavior of acids and bases in such reactions is essential for students, not just for examination but also for real-world applications like drug formulation and environmental science.