Question

21\. For an acid-base reaction, what is the reacting species (the ion or molecule that appears in the chemical equation) in the following bases? (a) barium hydroxide (b) trimethylamine \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\) (c) aniline, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) (d) sodium hydroxide

Short answer

Based on the given solutions, identify the reacting species for each of the following bases in an acid-base reaction: (a) Barium hydroxide: The reacting species is the hydroxide ion, \(\mathrm{OH^-}\). (b) Trimethylamine: The reacting species is the nitrogen atom, \(\mathrm{N}\), which donates a pair of electrons to form a bond with the proton (\(\mathrm{H^+}\)). (c) Aniline: The reacting species is the nitrogen atom, \(\mathrm{N}\), in the amine group (\(\mathrm{-NH}_{2}\)), which donates a pair of electrons to form a bond with the proton (\(\mathrm{H^+}\)). (d) Sodium hydroxide: The reacting species is the hydroxide ion, \(\mathrm{OH^-}\).

Step-by-step solution

(a) Barium Hydroxide Reacting Species

Barium hydroxide has the chemical formula \(\mathrm{Ba(OH)_2}\). In an acid-base reaction, it will dissociate into its ions in water, which are \(\mathrm{Ba^{2+}}\) and \(\mathrm{OH^-}\). The reacting species, responsible for the base behavior, is the hydroxide ion, \(\mathrm{OH^-}\).

(b) Trimethylamine Reacting Species

Trimethylamine has the chemical formula \(\left(\mathrm{CH}_{3}\right)_{3}\mathrm{N}\). Upon reaction with an acid, it can accept a proton (\(\mathrm{H^+}\)) and form the ion \(\left(\mathrm{CH}_{3}\right)_{3}\mathrm{NH^+}\). So, the reacting species for trimethylamine is the nitrogen atom, \(\mathrm{N}\), which donates a pair of electrons to form a bond with the proton (\(\mathrm{H^+}\)).

(c) Aniline Reacting Species

Aniline has the chemical formula \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{2}\). In an acid-base reaction, the nitrogen in the amine group (\(\mathrm{-NH}_{2}\)) can accept a proton (\(\mathrm{H^+}\)) and form the ion \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{3}^+\). The reacting species for aniline is the nitrogen atom, \(\mathrm{N}\), in the amine group (\(\mathrm{-NH}_{2}\)), which donates a pair of electrons to form a bond with the proton (\(\mathrm{H^+}\)).

(d) Sodium Hydroxide Reacting Species

Sodium hydroxide has the chemical formula \(\mathrm{NaOH}\). In an acid-base reaction, it will dissociate into its ions in water, which are \(\mathrm{Na^+}\) and \(\mathrm{OH^-}\). The reacting species, responsible for the base behavior, is the hydroxide ion, \(\mathrm{OH^-}\).

Key concepts

Reacting Species in Bases

Understanding the reacting species in bases is crucial for grasping how these substances behave in acid-base reactions. For instance, barium hydroxide (\( \text{Ba(OH)}_2 \) ) and sodium hydroxide (\( \text{NaOH} \) ) are both strong bases that dissociate completely in water. The dissociation releases hydroxide ions (\( \text{OH}^- \) ), which are the actual reacting species in any subsequent acid-base reaction.

Conversely, organic compounds such as trimethylamine (\( (\text{CH}_3)_3\text{N} \) ) and aniline (\( \text{C}_6\text{H}_5\text{NH}_2 \) ) act as bases through a different mechanism. Instead of releasing hydroxide ions, these compounds have a nitrogen atom that possesses a lone pair of electrons. It is this lone pair that makes them capable of accepting a proton (\( \text{H}^+ \) ), thus serving as the reacting species in acid-base reactions. It's crucial to identify these species because they determine how the molecule will interact with acids and participate in chemical reactions.

Dissociation in Water

Dissociation in water is a fundamental concept in understanding acid-base chemistry. When certain substances dissolve in water, they separate into their constituent ions. This phenomenon is particularly significant for bases, as the extent of their dissociation affects their strength and reactivity.

In strong bases such as barium hydroxide and sodium hydroxide, this process results in free hydroxide ions (\( \text{OH}^- \) ) that readily react with hydrogen ions (\( \text{H}^+ \) ). The complete dissociation of these bases means a higher concentration of hydroxide ions, which corresponds to a greater ability to neutralize acids. In essence, the degree of dissociation in water not only defines the nature of the reacting species but also provides insight into the strength of the base.

Proton Acceptance

As the cornerstone of base reactivity, proton acceptance can be deemed the inverse process of acid dissociation. In essence, while acids donate protons, bases accept them. In the context of our basic substances, different bases accept protons through different active sites. For example, hydroxide ions (\( \text{OH}^- \) ), readily available in the solutions of barium hydroxide and sodium hydroxide, can accept protons to form water (\( \text{H}_2\text{O} \) ).

On the other hand, organic bases such as trimethylamine and aniline utilize the nitrogen atom, capable of bonding with an additional proton due to its electron pair. This results in the formation of their respective ammonium ions (\( (\text{CH}_3)_3\text{NH}^+ \) and \( \text{C}_6\text{H}_5\text{NH}_3^+ \) ). This acceptance of protons is key to all acid-base reactions and is fundamental in the creation of salts and water when acids and bases react with each other.